### Set up a New ReScript Project

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v10.0.0/installation.mdx

Clones the ReScript project template, installs dependencies, and builds the project using npm scripts. This is the recommended way to start a new ReScript project.

```bash
git clone https://github.com/rescript-lang/rescript-project-template
cd rescript-project-template
npm install
npm run res:build
node src/Demo.bs.js
```

--------------------------------

### Set Up New ReScript Project

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v8.0.0/installation.mdx

Clones the ReScript project template, installs dependencies, builds the project, and runs the compiled JavaScript output. This is a common starting point for new ReScript projects.

```shell
git clone https://github.com/rescript-lang/rescript-project-template
cd rescript-project-template
npm install
npm run build
node src/Demo.bs.js
```

--------------------------------

### Add ReScript npm Scripts

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v9.0.0/installation.mdx

Adds convenience npm scripts to package.json for building ReScript projects and starting a development watcher.

```json
"scripts": {
  "re:build": "bsb -make-world -clean-world",
  "re:start": "bsb -make-world -clean-world -w"
}
```

--------------------------------

### ReScript Build Configuration (rescript.json)

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v12.0.0/installation.mdx

Example of a ReScript build configuration file. This JSON file specifies project settings like source directories, module formats, and file suffixes for compiled JavaScript.

```json
{
  "name": "your-project-name",
  "sources": [
    {
      "dir": "src", // update this to wherever you're putting ReScript files
      "subdirs": true
    }
  ],
  "package-specs": [
    {
      "module": "esmodule",
      "in-source": true
    }
  ],
  "suffix": ".res.js"
}
```

--------------------------------

### Start ReScript Watcher

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v12.0.0/installation.mdx

Starts ReScript in development mode with a watcher that automatically recompiles ReScript files upon detecting changes. This streamlines the development workflow.

```shell
npm run res:dev
```

--------------------------------

### Start ReScript Project Watcher

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v8.0.0/installation.mdx

Starts a development watcher for a ReScript project. This command automatically recompiles ReScript files whenever changes are detected, streamlining the development process.

```shell
npm run start
```

--------------------------------

### Create a basic ReScript React component

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/react/latest/installation.mdx

A simple ReScript component that renders 'Hello World' using React. This code is used to test the installation and setup.

```rescript
// src/Test.res
@react.component
let make = () => {
  <div> {React.string("Hello World")} </div>
}
```

--------------------------------

### Install ReScript Globally

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v8.0.0/installation.mdx

Installs the ReScript compiler ('bs-platform') globally on your system using npm. This makes the ReScript command-line tools available from any directory.

```shell
npm install -g bs-platform
```

--------------------------------

### Add ReScript npm Scripts

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v10.0.0/installation.mdx

Adds convenience scripts to the package.json file for building and watching ReScript files during development. `res:build` compiles the project, while `res:dev` starts a watcher for automatic recompilation.

```json
{
  "scripts": {
    "res:build": "rescript",
    "res:dev": "rescript build -w"
  }
}
```

--------------------------------

### Blog Post Frontmatter Example

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/blogpost-guide.mdx

Provides a complete example of the frontmatter required for a blog post, including author, date, title, and description. Frontmatter is essential for the blog post to be displayed correctly.

```yaml
---
author: hongbo
co-authors:
  - chenglou
  - ryyppy
date: "2017-10-01"
previewImg:
badge: release
title: Bloomberg announces BuckleScript 1.0
description: |
  some multiline
  description
---

```

--------------------------------

### Install ReScript Locally (npm, yarn, pnpm, bun, deno)

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v12.0.0/installation.mdx

Installs ReScript as a local dependency within a JavaScript project. This is the first step in manually integrating ReScript into an existing project.

```shell
npm install rescript
```

```shell
yarn add rescript
```

```shell
pnpm install rescript
```

```shell
bun install rescript
```

```shell
// you will need deno configured to have a node_modules folder
deno install npm:rescript --allow-scripts
```

--------------------------------

### Create New ReScript Project (npm)

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v11.0.0/installation.mdx

Initiates a new ReScript project using the create-rescript-app generator with npm. This is the fastest way to start a new project.

```shell
npm create rescript-app@latest
```

--------------------------------

### Setup and Build Commands for ReScript

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/README.md

Commands to set up the ReScript project, install dependencies, perform initial builds, and run the development server. Also includes instructions for running ReScript in watch mode.

```shell
# For first time clone / build (install dependencies)
npm i

# Initial build
npx rescript

# Build the index data. Only needed for initial clone (or content H2 changes)
npm run update-index

# In a new tab
npm run dev

open localhost:3000

# In case you want to run ReScript in watchmode:
npx rescript -w
```

--------------------------------

### Install ReScript Locally (bun)

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v11.0.0/installation.mdx

Installs ReScript and its core library as local dependencies in a project using bun. This is part of the manual setup for integrating ReScript into an existing project.

```shell
bun install rescript @rescript/core
```

--------------------------------

### Install ReScript Locally for Existing JS Project

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v8.0.0/installation.mdx

Installs the ReScript compiler ('bs-platform') as a development dependency in an existing JavaScript project. This allows you to use ReScript within your current project structure.

```shell
npm install --save-dev bs-platform
```

--------------------------------

### Create New ReScript Project (npx)

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v11.0.0/installation.mdx

Initiates a new ReScript project using the create-rescript-app generator with npx. This is an alternative method to start a new project.

```shell
npx create-rescript-app
```

--------------------------------

### Create New ReScript Project (npm, npx, yarn, pnpm, bun)

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v12.0.0/installation.mdx

Initiates a new ReScript project using the create-rescript-app generator. This command is compatible with various package managers and sets up a project with frameworks like Next.js or Vite, along with React and Tailwind CSS.

```shell
npm create rescript-app@latest
```

```shell
npx create-rescript-app
```

```shell
yarn create rescript-app
```

```shell
pnpm create rescript-app
```

```shell
bun create rescript-app
```

--------------------------------

### Install ReScript Locally in Existing JS Project

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v10.0.0/installation.mdx

Installs the ReScript package as a local dependency in an existing JavaScript project using npm.

```bash
npm install rescript
```

--------------------------------

### ReScript Build Configuration (rescript.json)

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v11.0.0/installation.mdx

Example ReScript build configuration file (rescript.json). It defines project settings like sources, module format, output suffix, and dependencies.

```json
{
  "name": "your-project-name",
  "sources": [
    {
      "dir": "src", 
      "subdirs": true
    }
  ],
  "package-specs": [
    {
      "module": "esmodule",
      "in-source": true
    }
  ],
  "suffix": ".res.js",
  "bs-dependencies": ["@rescript/core"],
  "bsc-flags": ["-open RescriptCore"]
}
```

--------------------------------

### Install ReScript Locally (pnpm)

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v11.0.0/installation.mdx

Installs ReScript and its core library as local dependencies in a project using pnpm. This is part of the manual setup for integrating ReScript into an existing project.

```shell
pnpm install rescript @rescript/core
```

--------------------------------

### ReScript Build Configuration (bsconfig.json)

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v10.0.0/installation.mdx

Defines the build settings for ReScript projects, including source directories, output module format, and file suffixes. This JSON file configures how ReScript is compiled.

```json
{
  "name": "your-project-name",
  "sources": [
    {
      "dir": "src",
      "subdirs": true
    }
  ],
  "package-specs": [
    {
      "module": "es6",
      "in-source": true
    }
  ],
  "suffix": ".bs.js",
  "bs-dependencies": []
}
```

--------------------------------

### Install ReScript Locally (yarn)

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v11.0.0/installation.mdx

Installs ReScript and its core library as local dependencies in a project using yarn. This is part of the manual setup for integrating ReScript into an existing project.

```shell
yarn add rescript @rescript/core
```

--------------------------------

### Install ReScript Locally (npm)

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v11.0.0/installation.mdx

Installs ReScript and its core library as local dependencies in a project using npm. This is part of the manual setup for integrating ReScript into an existing project.

```shell
npm install rescript @rescript/core
```

--------------------------------

### Add npm Scripts to package.json

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v12.0.0/installation.mdx

Defines convenience scripts in a project's `package.json` file for common ReScript build and development tasks. These scripts abstract the underlying ReScript commands.

```json
"scripts": {
  "res:build": "rescript",
  "res:dev": "rescript -w"
}
```

--------------------------------

### ReScript Codeblock (Default)

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/markdown-guide.mdx

A standard ReScript codeblock example without any specific line highlighting.

```res
let a = "This is a Reason codeblock"
```

--------------------------------

### Create New Blog Post File

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/blogpost-guide.mdx

Demonstrates how to create a new markdown file for a blog post using the 'touch' command. The filename must start with the date in YYYY-MM-DD format.

```shell
touch _blogposts/2020-01-30-my-new-blog-post.mdx
```

--------------------------------

### Install ReScript Core Package

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v11.0.0/migrate-to-v11.mdx

Installs the ReScript Core package using npm. This package is a new standard library for ReScript and needs to be manually installed in ReScript 11.

```console
$ npm install @rescript/core
```

--------------------------------

### Install @rescript/react using npm

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/react/latest/installation.mdx

Installs the @rescript/react package using npm. This is a fundamental step for integrating ReScript with React projects.

```shell
npm install @rescript/react
```

--------------------------------

### Initialize ReScript Project

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v8.0.0/build-overview.mdx

Initializes a new ReScript project in a specified directory using the 'bsb -init' command. This sets up the basic project structure and configuration.

```sh
bsb -init my-directory-name
```

--------------------------------

### ReScript CLI Help

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v11.0.0/build-overview.mdx

Displays the usage instructions and available subcommands for the ReScript command-line interface. It lists options like version and help, and subcommands such as build, clean, format, convert, dump, and help.

```sh
❯ rescript help
Usage: rescript <options> <subcommand>

`rescript` is equivalent to `rescript build`

Options:
  -v, -version  display version number
  -h, -help     display help

Subcommands:
  build
  clean
  format
  convert
  dump
  help

Run `rescript <subcommand> -h` for subcommand help. Examples:
  rescript build -h
  rescript format -h
```

--------------------------------

### Install bs-platform and @rescript/std

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v9.0.0/build-external-stdlib.mdx

Installs the necessary ReScript platform and the standalone standard library package for external use. Ensure versions match.

```shell
npm install bs-platform@9.0.0 --save-dev
npm install @rescript/std@9.0.0 --save
```

--------------------------------

### Example: Find First Binding

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v10.0.0/api/belt/map-string.mdx

An example demonstrating the use of `findFirstBy` to find a key-value pair in a map.

```rescript
let s0 = Belt.Map.String.fromArray([("4", 4), ("1", 1), ("2", 2), ("3", 3)])

Belt.Map.String.findFirstBy(s0, (k, _) => k == "4") == Some(("4", 4))
```

--------------------------------

### Install ReScript 10.0

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/_blogposts/2022-08-25-release-10-0-0.mdx

Command to install the latest ReScript version using npm. This is a foundational step for using the new features.

```bash
npm install rescript@10
```

--------------------------------

### View Available ReScript Templates

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v8.0.0/build-overview.mdx

Displays all available project templates for the ReScript build system using the 'bsb -themes' command. This helps users choose a starting point for their projects.

```sh
bsb -themes
```

--------------------------------

### ReScript Intro Component Usage

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/markdown-guide.mdx

Demonstrates the ReScript Intro component, used for introductory text or chapter heroes, providing a quick overview of a document's topic.

```re
<Intro>

Your hero text

</Intro>
```

--------------------------------

### Install ReScript 9.0.1

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/_blogposts/2021-02-09-release-9-0.mdx

Command to install the specified version of ReScript using npm. This is a basic installation command with no specific dependencies or inputs beyond having npm installed.

```sh
npm install bs-platform@9.0.1
```

--------------------------------

### View ReScript Build System Help

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v8.0.0/build-overview.mdx

Displays all available options and commands for the ReScript build system (bsb). This is a general help command for understanding the tool's capabilities.

```sh
bsb -help
```

--------------------------------

### Get List Length

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v9.0.0/api/belt/list.mdx

Returns the number of elements in a list. Example shows getting the length of a list containing integers.

```rescript
let length: t<'a> => int

Belt.List.length(list{1, 2, 3}) // 3
```

--------------------------------

### Create ReScript Records for JavaScript Objects

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v8.0.0/newcomer-examples.mdx

Demonstrates creating JavaScript objects using ReScript record syntax, which provides type safety and structure. The example defines a `payload` type and then creates a `student1` object, showing the ReScript, ML, and resulting JavaScript code.

```re
type payload = {
  name: string,
  age: int,
};

let student1 = {
  name: "John",
  age: 30,
};

```

```ml
type payload = {
  name: string;
  age: int;
}

let student1 = {
  name = "John";
  age = 30;
}

```

```js
var student1 = {
  name: "John",
  age: 30,
};

```

--------------------------------

### Rescript substringToEnd: Get substring from start to end

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v10.0.0/api/js/string.mdx

The substringToEnd function extracts a substring starting from a given position to the end of the string. It returns the entire string if the start position is zero or negative, and an empty string if the start position is beyond the string's length.

```rescript
let substringToEnd: (~from: int, t) => t
```

```rescript
Js.String.substringToEnd(~from=4, "playground") == "ground"
Js.String.substringToEnd(~from=-3, "playground") == "playground"
Js.String.substringToEnd(~from=12, "playground") == ""
```

--------------------------------

### Example: Find First Entry

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v8.0.0/api/belt/map-dict.mdx

An example demonstrating how to use findFirstBy to retrieve a specific key-value pair from a MapDict.

```rescript
module IntCmp = Belt.Id.MakeComparable({
    type t = int;
    let cmp = Pervasives.compare;
  });

let s0 = Belt.Map.Dict.fromArray([|(4, "4"), (1, "1"), (2, "2"), (3, "3")|], ~cmp=IntCmp.cmp);

Belt.Map.Dict.findFirstBy(s0, (k, _) => k == 4) == Some((4, "4"));
```

--------------------------------

### Add Element to Beginning of Array Example

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v10.0.0/api/js/array.mdx

Shows an example of using `unshift` to add an element to the start of an array and observes the change in array length and content.

```rescript
let arr = ["b", "c", "d"]
Js.Array.unshift("a", arr) == 4
arr == ["a", "b", "c", "d"]
```

--------------------------------

### substr - Get Substring

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v10.0.0/api/js/string-2.mdx

Retrieves a portion of a string from a specified starting position to the end. Handles negative start positions and positions beyond string length.

```APIDOC
## substr

### Description
Returns the substring of `str` from position `n` to the end of the string. If `n` is negative, the start position is calculated from the end of the string. If `n` exceeds the string length, an empty string is returned.

### Method
N/A (This is a function signature, not an HTTP endpoint)

### Endpoint
N/A

### Parameters
#### Path Parameters
N/A

#### Query Parameters
N/A

#### Request Body
N/A

### Request Example
N/A

### Response
#### Success Response (N/A)
N/A

#### Response Example
```res
Js.String2.substr("abcdefghij", ~from=3) == "defghij"
Js.String2.substr("abcdefghij", ~from=-3) == "hij"
Js.String2.substr("abcdefghij", ~from=12) == ""
```
```

--------------------------------

### Example: Get Map Keys as Array (ReScript)

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v10.0.0/api/belt/map.mdx

Demonstrates extracting keys from a map into a sorted array using `Belt.Map.keysToArray`. The example uses `Belt.Map.fromArray` to initialize the map.

```res
module IntCmp = Belt.Id.MakeComparable({
  type t = int
  let cmp = (a, b) => Pervasives.compare(a, b)
})

Belt.Map.keysToArray(Belt.Map.fromArray([(2, "2"), (1, "1"), (3, "3")], ~id=module(IntCmp))) == [
    1,
    2,
    3,
  ]
```

--------------------------------

### Basic Let Binding Syntax

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v8.0.0/let-binding.mdx

Demonstrates the fundamental 'let' binding syntax in ReScript and its ML-like predecessor, showing how values are assigned to names and the resulting JavaScript output. Illustrates simple variable declaration and reassignment.

```re
let greeting = "hello!";
let score = 10;
let newScore = 10 + score;
```

```ml
let greeting = "hello!"
let score = 10
let newScore = 10 + score
```

```js
var greeting = "hello!";
var score = 10;
var newScore = 20;
```

--------------------------------

### Re.lastIndex

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v8.0.0/api/js/re.mdx

Gets the index where the next match will start searching.

```APIDOC
## Re.lastIndex

### Description

Returns the index where the next match will start its search.
This property will be modified when the RegExp object is used, if the global ("g") flag is set.

### Method

`Re.lastIndex

### Endpoint

N/A (Function within a module)

### Parameters

- **t** (RegExp object) - The RegExp object.

### Request Example

```re
let re = [%re "/ab*/g"];
let str = "abbcdefabh";

let break = ref(false);
while (! break^) {
  switch (Js.Re.exec_(re, str)) {
  | Some(result) =>
    Js.Nullable.iter(
      Js.Re.captures(result)[0],
      [@bs] match => {
        let next = Belt.Int.toString(Js.Re.lastIndex(re));
        Js.log("Found " ++ match ++ ". Next match starts at " ++ next);
      },
    )
  | None => break := true
  };
};
```

### Response

- **int**: The index for the next search.
```

--------------------------------

### substringToEnd - Get Substring to End

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v10.0.0/api/js/string-2.mdx

Retrieves the portion of a string from a specified starting position to the end. Handles zero or negative start positions, and positions beyond the string length.

```APIDOC
## substringToEnd

### Description
Returns the substring of `str` from position `start` to the end. If `start` is zero or negative, the entire string is returned. If `start` is greater than or equal to the string length, an empty string is returned.

### Method
N/A (This is a function signature, not an HTTP endpoint)

### Endpoint
N/A

### Parameters
#### Path Parameters
N/A

#### Query Parameters
N/A

#### Request Body
N/A

### Request Example
N/A

### Response
#### Success Response (N/A)
N/A

#### Response Example
```res
Js.String2.substringToEnd("playground", ~from=4) == "ground"
Js.String2.substringToEnd("playground", ~from=-3) == "playground"
Js.String2.substringToEnd("playground", ~from=12) == ""
```
```

--------------------------------

### substrAtMost - Get Substring with Length

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v10.0.0/api/js/string-2.mdx

Extracts a substring of a specified length starting from a given position. Supports negative start positions and handles invalid length or position inputs.

```APIDOC
## substrAtMost

### Description
Returns the substring of `str` of length `n` starting at position `pos`. Handles negative `pos` values and cases where `pos` or `n` are out of bounds, returning an empty string as appropriate.

### Method
N/A (This is a function signature, not an HTTP endpoint)

### Endpoint
N/A

### Parameters
#### Path Parameters
N/A

#### Query Parameters
N/A

#### Request Body
N/A

### Request Example
N/A

### Response
#### Success Response (N/A)
N/A

#### Response Example
```res
Js.String2.substrAtMost("abcdefghij", ~from=3, ~length=4) == "defg"
Js.String2.substrAtMost("abcdefghij", ~from=-3, ~length=4) == "hij"
Js.String2.substrAtMost("abcdefghij", ~from=12, ~length=2) == ""
```
```

--------------------------------

### Basic Source Directory Configuration

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v11.0.0/build-configuration.mdx

Specifies directories to include as source files for the ReScript build. The 'src' and 'examples' directories are explicitly included.

```json
{
  "sources": ["src", "examples"]
}
```

--------------------------------

### Create New ReScript Project (bun)

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v11.0.0/installation.mdx

Initiates a new ReScript project using the create-rescript-app generator with bun. This option is for developers using the bun runtime.

```shell
bun create rescript-app
```

--------------------------------

### Install ReScript Compiler using npm

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/_blogposts/2024-01-11-release-11-0-0.mdx

Command to install the ReScript compiler version 11.0 using npm. This is the recommended way to get the latest version for your project.

```sh
npm install rescript@11
```

--------------------------------

### Simplify Module Access with 'open' Keyword

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v8.0.0/module.mdx

Demonstrates how to use the 'open' keyword in ReScript to bring module contents into the current scope, avoiding repetitive module name prepending. Shows examples before and after using 'open', including ML and JS outputs.

```rescript
let p = School.getProfession(School.person1);
```

```ml
let p = School.getProfession School.person1
```

```javascript
var p = School.getProfession(School.person1);
```

--------------------------------

### Install ReScript 8.3

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/_blogposts/2020-09-25-release-8-3.mdx

Command to install the specific version of ReScript's compiler package (bs-platform) using npm.

```bash
npm i bs-platform@8.3.0
```

--------------------------------

### Get Value from HashMap

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v8.0.0/api/belt/hash-map.mdx

Retrieves the value associated with a given key from the hash map. Returns Some(value) if the key exists, otherwise returns None. The example shows getting an existing and a non-existent key.

```rescript
let get: (t('key, 'value, 'id), 'key) => option('value);
```

```rescript
module IntHash = Belt.Id.MakeHashable({
    type t = int;
    let hash = a => a;
    let eq = (a, b) => a == b;
  });

let s0 = Belt.HashMap.make(~hintSize=10, ~id=(module IntHash));
Belt.HashMap.set(s0, 1, "value1");

Belt.HashMap.get(s0, 1) == Some("value1");
Belt.HashMap.get(s0, 2) == None;
```

--------------------------------

### Rescript substrAtMost: Get substring of specific length

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v10.0.0/api/js/string.mdx

The substrAtMost function extracts a substring of a given length starting from a specified position. It adjusts negative start positions based on string length and returns an empty string for out-of-bounds start positions or non-positive lengths. Similar to `substr`, `substring` is preferred.

```rescript
let substrAtMost: (~from: int, ~length: int, t) => t
```

```rescript
Js.String.substrAtMost(~from=3, ~length=4, "abcdefghij") == "defg"
Js.String.substrAtMost(~from=-3, ~length=4, "abcdefghij") == "hij"
Js.String.substrAtMost(~from=12, ~length=2, "abcdefghij") == ""
```

--------------------------------

### Build ReScript Project

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v8.0.0/build-overview.mdx

Builds the entire ReScript project. The '-make-world' flag triggers a full build of all project targets. Adding '-w' enables a watcher that automatically rebuilds on file changes.

```sh
bsb -make-world
```

```sh
bsb -make-world -w
```

--------------------------------

### Create JavaScript Objects in ReScript

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v8.0.0/newcomer-examples.mdx

Shows two methods for creating JavaScript objects in ReScript: directly using object literal syntax and using ReScript records which are then compiled to JS objects. Both methods are demonstrated with ReScript, ML, and JS output.

```re
let student1 = {
  "name": "John",
  "age": 30,
};

```

```ml
let student1 = [%bs.obj {
  name = "John";
  age = 30
}]

```

```js
var student1 = {
  name: "John",
  age: 30,
};

```

--------------------------------

### Install and Run ReScript Language Server

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/_blogposts/2024-01-11-release-11-0-0.mdx

Installs the ReScript Language Server globally and provides the command to run it for standard input/output communication, enabling editor tooling integration.

```sh
npm install -g @rescript/language-server
npx rescript-language-server --stdio
```

--------------------------------

### Get Element by Index (Exception)

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v9.0.0/api/belt/list.mdx

Retrieves the element at a specific index, similar to `get`, but raises an exception if the index is out of bounds. Use with caution. Examples show successful access and an out-of-bounds access that would raise an error.

```rescript
let getExn: (t<'a>, int) => 'a

let abc = list{"A", "B", "C"}

abc->Belt.List.getExn(1) // "B"

abc->Belt.List.getExn(4) // Raises an Error
```

--------------------------------

### ReScript Tagged Variant Example

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/_blogposts/2023-04-17-improving-interop.mdx

Demonstrates a basic ReScript tagged variant and its JavaScript runtime representation. This example shows the default behavior before customization.

```rescript
type entity = User({"name": string}) | Group({"workingName": string})

let user = User({"name": "Hello"})
```

```javascript
var user = {
  TAG: /* User */ 0,
  name: "Hello"
};
```

--------------------------------

### Counter Example with useReducer - ReScript/JS

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/react/latest/hooks-reducer.mdx

An example demonstrating a counter component using `React.useReducer`. It defines actions (Inc, Dec), state structure, and a reducer function to handle state transitions, along with React component setup.

```rescript
// Counter.res

type action = Inc | Dec
type state = {count: int}

let reducer = (state, action) => {
  switch action {
  | Inc => {count: state.count + 1}
  | Dec => {count: state.count - 1}
  }
}

@react.component
let make = () => {
  let (state, dispatch) = React.useReducer(reducer, {count: 0})

  <>
    {React.string("Count:" ++ Belt.Int.toString(state.count))}
    <button onClick={(_) => dispatch(Dec)}> {React.string("-")} </button>
    <button onClick={(_) => dispatch(Inc)}> {React.string("+")} </button>
  </>
}
```

```javascript
function reducer(state, action) {
  if (action) {
    return {
      count: (state.count - 1) | 0,
    };
  } else {
    return {
      count: (state.count + 1) | 0,
    };
  }
}

function Counter(Props) {
  var match = React.useReducer(reducer, {
    count: 0,
  });
  var dispatch = match[1];
  return React.createElement(
    React.Fragment,
    undefined,
    "Count:" + String(match[0].count),
    React.createElement(
      "button",
      {
        onClick: function (param) {
          return Curry._1(dispatch, /* Dec */ 1);
        },
      },
      "-",
    ),
    React.createElement(
      "button",
      {
        onClick: function (param) {
          return Curry._1(dispatch, /* Inc */ 0);
        },
      },
      "+",
    ),
  );
}
```

--------------------------------

### ReScript Codeblock (Default Language)

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/markdown-guide.mdx

An example of a codeblock using the default language ('text'), which does not provide syntax highlighting.

```text
The default language is set to `text` and will not provide any syntax
highlighting
```

--------------------------------

### Install and Use ReScript Documentation Tool

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/_blogposts/2024-01-11-release-11-0-0.mdx

Installs the experimental ReScript documentation generation tool globally and demonstrates its usage to extract documentation from ReScript files into a JSON format.

```sh
npm install --save-dev @rescript/tools
npx rescript-tools doc src/MyFile.res > doc.json
```

--------------------------------

### Copy Within Array with Start Index in ReScript

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v10.0.0/api/js/array.mdx

Copies a section of an array to another location within the same array starting from a specified index using `Js.Array.copyWithinFrom` in ReScript. This operation modifies the original array. The example copies from index 2 to index 0.

```rescript
let arr = [100, 101, 102, 103, 104]
Js.Array.copyWithinFrom(~from=2, ~to_=0, arr) == [102, 103, 104, 103, 104]
arr == [102, 103, 104, 103, 104]
```

--------------------------------

### Get List Head

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v9.0.0/api/belt/list.mdx

Retrieves the first element of a list, returning Some(value) if the list is not empty, and None if it is. Examples demonstrate both cases.

```rescript
let head: t<'a> => option<'a>

Belt.List.head(list{}) // None
Belt.List.head(list{1, 2, 3}) // Some(1)
```

--------------------------------

### Example: Get Map Values as Array (ReScript)

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v10.0.0/api/belt/map.mdx

Shows how to retrieve all values from a map as an array in key-sorted order using `Belt.Map.valuesToArray`.

```res
module IntCmp = Belt.Id.MakeComparable({
  type t = int
  let cmp = (a, b) => Pervasives.compare(a, b)
})

Belt.Map.valuesToArray(
  Belt.Map.fromArray([(2, "2"), (1, "1"), (3, "3")], ~id=module(IntCmp)),
) == ["1", "2", "3"]
```

--------------------------------

### Simplified Module Access with `open` in ReScript

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v11.0.0/module.mdx

Shows the effect of using `open School` in ReScript to access `getProfession` and `person1` directly, contrasting with the previous example. Includes the JavaScript output.

```rescript
open School
let p = getProfession(person1)
```

```javascript
var p = School.getProfession(School.person1);
```

--------------------------------

### Extend Module with 'include' (ReScript, ML, JS)

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v8.0.0/module.mdx

Demonstrates extending a BaseComponent module with ActualComponent using 'include'. The 'include' keyword copies the BaseComponent's content into ActualComponent, allowing for overrides and new definitions. Examples are provided in ReScript, ML, and the resulting JavaScript output.

```rescript
module BaseComponent = {
  let defaultGreeting = "Hello";
  let getAudience = (~excited) => excited ? "world!" : "world";
};

module ActualComponent = {
  /* the content is copied over */
  include BaseComponent;
  /* overrides BaseComponent.defaultGreeting */
  let defaultGreeting = "Hey";
  let render = () => defaultGreeting ++ " " ++ getAudience(~excited=true);
};
```

```ml
module BaseComponent = struct
  let defaultGreeting = "Hello"
  let getAudience ~excited = if excited then "world!" else "world"
end

module ActualComponent = struct
  (* the content is copied over *)
  include BaseComponent
  (* overrides BaseComponent.defaultGreeting *)
  let defaultGreeting = "Hey"
  let render () = defaultGreeting ^ " " ^ (getAudience ~excited:true)
end
```

```javascript
function getAudience(excited) {
  if (excited) {
    return "world!";
  } else {
    return "world";
  }
}

var BaseComponent = {
  defaultGreeting: "Hello",
  getAudience: getAudience,
};

var defaultGreeting = "Hey";

function render(param) {
  return "Hey world!";
}

var ActualComponent = {
  getAudience: getAudience,
  defaultGreeting: defaultGreeting,
  render: render,
};
```

--------------------------------

### ReScript Code Example

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v12.0.0/build-external-stdlib.mdx

A simple ReScript code snippet demonstrating the usage of `Array.forEach` and `Console.log`.

```rescript
Array.forEach([1, 2, 3], num => Console.log(num))
```

--------------------------------

### Get Byte Offset of Uint16Array

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v10.0.0/api/js/typed-array-2_uint-16-array.mdx

Retrieves the byte offset of the Uint16Array within its underlying ArrayBuffer. This indicates the starting position of the array's data.

```res
let byteOffset: t => int
```

--------------------------------

### Create New ReScript Project (yarn)

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v11.0.0/installation.mdx

Initiates a new ReScript project using the create-rescript-app generator with yarn. This method is suitable for projects using yarn as their package manager.

```shell
yarn create rescript-app
```

--------------------------------

### Rescript substr: Get substring from position to end

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v10.0.0/api/js/string.mdx

The substr function returns a portion of a string starting from a specified position to the end. It handles negative positions by calculating from the string's length and returns an empty string if the start position is out of bounds. It is a legacy function, with `substring` recommended for modern use.

```rescript
let substr: (~from: int, t) => t
```

```rescript
Js.String.substr(~from=3, "abcdefghij") == "defghij"
Js.String.substr(~from=-3, "abcdefghij") == "hij"
Js.String.substr(~from=12, "abcdefghij") == ""
```

--------------------------------

### Create New ReScript Project (pnpm)

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v11.0.0/installation.mdx

Initiates a new ReScript project using the create-rescript-app generator with pnpm. This is for users who prefer pnpm for package management.

```shell
pnpm create rescript-app
```

--------------------------------

### Remove Elements From Position to End In-Place Example

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v10.0.0/api/js/array.mdx

Shows how to use `removeFromInPlace` to remove elements from an array starting at a given index until the end, modifying the array.

```rescript
let arr = ["a", "b", "c", "d", "e", "f"]
Js.Array.removeFromInPlace(~pos=4, arr) == ["e", "f"]
arr == ["a", "b", "c", "d"]
```

--------------------------------

### Build ReScript project with dependencies

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/_blogposts/2021-05-07-release-9-1.mdx

Shows how to explicitly build a ReScript project with all its dependencies using the `rescript build -with-deps` command, replacing the old `bsb -make-world` command.  However, the example also clarifies that this is often unnecessary as `rescript build` now handles this automatically.

```sh
rescript build -with-deps
```

--------------------------------

### ReScript CLI Usage

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/_blogposts/2021-05-07-release-9-1.mdx

Demonstrates the usage of the unified `rescript` command-line interface, replacing the separate `bsc` and `bsb` commands. This includes examples of formatting files, building projects, and watching for changes.

```sh
❯ rescript -help
Available flags
-v, -version  display version number
-h, -help     display help
Subcommands:
    build
    clean
    format
    convert
    help
Run rescript subcommand -h for more details,
For example:
    rescript build -h
    rescript format -h
The default `rescript` is equivalent to `rescript build` subcommand
```

--------------------------------

### substring - Get Substring Between Positions

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v10.0.0/api/js/string-2.mdx

Extracts characters from a string between two specified positions (start inclusive, end exclusive). Handles inverted or out-of-bounds positions.

```APIDOC
## substring

### Description
Returns characters from `str` starting at `start` up to, but not including, `finish`. If `start` is negative, it's treated as 0. If `finish` is zero or negative, an empty string is returned. If `start` is greater than `finish`, the positions are swapped.

### Method
N/A (This is a function signature, not an HTTP endpoint)

### Endpoint
N/A

### Parameters
#### Path Parameters
N/A

#### Query Parameters
N/A

#### Request Body
N/A

### Request Example
N/A

### Response
#### Success Response (N/A)
N/A

#### Response Example
```res
Js.String2.substring("playground", ~from=3, ~to_=6) == "ygr"
Js.String2.substring("playground", ~from=6, ~to_=3) == "ygr"
Js.String2.substring("playground", ~from=4, ~to_=12) == "ground"
```
```

--------------------------------

### Example: Get Value by Key (ReScript)

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v10.0.0/api/belt/map.mdx

Illustrates retrieving a value associated with a specific key from a map using `Belt.Map.get`. It shows cases where the key exists and where it does not.

```res
module IntCmp = Belt.Id.MakeComparable({
  type t = int
  let cmp = (a, b) => Pervasives.compare(a, b)
})

Belt.Map.get(Belt.Map.fromArray([(2, "2"), (1, "1"), (3, "3")], ~id=module(IntCmp)), 2) ==
  Some("2")

Belt.Map.get(Belt.Map.fromArray([(2, "2"), (1, "1"), (3, "3")], ~id=module(IntCmp)), 2) == None
```

--------------------------------

### Build ReScript Project

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v10.0.0/build-overview.mdx

Initiates the build process for the current ReScript project, including its own source files and any pinned dependencies. This is the primary command for compiling ReScript code.

```shell
rescript
```

--------------------------------

### Define and Use a ReScript Module Signature and Implementation

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v10.0.0/module.mdx

Illustrates the relationship between a ReScript implementation file (.res) and its corresponding interface file (.resi). The .res file defines the module's implementation, while the .resi file defines its public signature. Includes example ReScript code and its JavaScript output.

```res
/* file React.res (implementation. Compiles to module React) */
type state = int
let render = (str) => str
```

```js
function render(str) {
  return str;
}
```

```res
/* file React.resi (interface. Compiles to the signature of React.res) */
type state = int
let render: string => string
```

--------------------------------

### Get Element by Index

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v9.0.0/api/belt/list.mdx

Retrieves the element at a specific index in a list, returning Some(value) or None if the index is out of bounds. Examples show accessing an element and an out-of-bounds access.

```rescript
let get: (t<'a>, int) => option<'a>

let abc = list{"A", "B", "C"}

abc->Belt.List.get(1) // Some("B")

abc->Belt.List.get(4) // None
```

--------------------------------

### Create Parametrized Types in ReScript

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v8.0.0/newcomer-examples.mdx

Illustrates how to define parametrized types in ReScript, allowing for generic type definitions. This example defines a `universityStudent` record and a `response` type that can hold any student type, showing both ReScript and ML syntaxes.

```re
type universityStudent = {gpa: float};

type response('studentType) = {
  status: int,
  student: 'studentType,
};

```

```ml
type universityStudent = {gpa: float}

type 'studentType response = {
  status: int;
  student: 'studentType;
}

```

```js
// Empty output

```

--------------------------------

### Get List Tail

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v9.0.0/api/belt/list.mdx

Returns a new list containing all elements except the first. Returns None if the list is empty, otherwise Some(tail). Examples show both scenarios.

```rescript
let tail: t<'a> => option<t<'a>>

Belt.List.tail(list{1, 2, 3}) // Some(list{2, 3})

Belt.List.tail(list{}) // None
```

--------------------------------

### ReScript For Loop (Descending)

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v9.0.0/control-flow.mdx

Demonstrates the ReScript 'for' loop with the 'downto' keyword for iterating in reverse order, from a start value down to an end value, inclusive. Includes a practical example.

```rescript
for i in startValueInclusive downto endValueInclusive {
  Js.log(i)
}
```

```rescript
// prints: 3 2 1, one per line
for x in 3 downto 1 {
  Js.log(x)
}
```

--------------------------------

### Basic ReScript Console Output

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/_blogposts/2025-03-05-what-can-i-do-with-rescript.mdx

A simple ReScript code snippet that prints 'Hello' to the console. This demonstrates the basic syntax and how to compile and run ReScript code in a Node.js environment.

```rescript
// index.res
Console.log("Hello")
```

--------------------------------

### ReScript For Loop (Ascending)

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v9.0.0/control-flow.mdx

Illustrates the ReScript 'for' loop for iterating from a start value to an end value, inclusive. It also shows an example of counting upwards and how it translates to JavaScript.

```rescript
for i in startValueInclusive to endValueInclusive {
  Js.log(i)
}
```

```rescript
// prints: 1 2 3, one per line
for x in 1 to 3 {
  Js.log(x)
}
```

--------------------------------

### Multi-Syntax CodeTab with Highlighting

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/markdown-guide.mdx

Demonstrates the CodeTab component for displaying multiple syntax versions of the same code snippet, including line highlighting support.

```res
let a = "Some ReScript code"
switch myValue {

}
```

```js
var a = "Some JavaScript code";

var highlighted = "Also supports highlighting ranges";
```

--------------------------------

### Get a value from HashMap.Int by key

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v8.0.0/api/belt/hash-map-int.mdx

Retrieves the value associated with a given key. Returns `Some(value)` if the key exists, otherwise `None`. The example shows retrieving existing and non-existing keys.

```rescript
let hMap = Belt.HashMap.Int.make(~hintSize=10);
Belt.HashMap.Int.set(hMap, 1, "value1");

Belt.HashMap.Int.get(hMap, 1) == Some("value1");
Belt.HashMap.Int.get(hMap, 2) == None;
```

--------------------------------

### Use `open` to Simplify Module Access (ReScript) and JS Output

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v12.0.0/module.mdx

Demonstrates how to use the `open` keyword in ReScript to make module members directly accessible without prefixing. This example shows the ReScript code and its JavaScript equivalent.

```rescript
let p = {
  open School
  getProfession(person1)
}
```

```javascript
var p = School.getProfession(School.person1);

```

--------------------------------

### Add Multiple Elements to Beginning of Array Example

Source: https://github.com/rescript-lang/rescript-lang.org/blob/master/pages/docs/manual/v10.0.0/api/js/array.mdx

Demonstrates adding multiple elements from one array to the start of another using `unshiftMany`, showing the resulting array and its new length.

```rescript
let arr = ["d", "e"]
Js.Array.unshiftMany(["a", "b", "c"], arr) == 5
arr == ["a", "b", "c", "d", "e"]
```