newcomer-examples.mdx

title	description	canonical
Newcomer Examples	Quick examples for users new to ReScript	/docs/manual/latest/newcomer-examples

Newcomer Examples

An example is worth a thousand words.

This section is dedicated to newcomers trying to figure out general idioms & conventions. If you're a beginner who's got a good idea for an example, please suggest an edit!

Use the option type

<CodeTab labels={["ReScript", "JS Output"]}>

let possiblyNullValue1 = None
let possiblyNullValue2 = Some("Hello")

switch possiblyNullValue2 {
| None => Console.log("Nothing to see here.")
| Some(message) => Console.log(message)
}

var possiblyNullValue1;
var possiblyNullValue2 = "Hello";

if (possiblyNullValue2 !== undefined) {
  console.log(possiblyNullValue2);
} else {
  console.log("Nothing to see here.");
}

Create a Parametrized Type

<CodeTab labels={["ReScript", "JS Output"]}>

type universityStudent = {gpa: float}

type response<'studentType> = {
  status: int,
  student: 'studentType,
}

// Empty output

Creating a JS Object

<CodeTab labels={["ReScript", "JS Output"]}>

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

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

Or using record:

<CodeTab labels={["ReScript", "JS Output"]}>

type payload = {
  name: string,
  age: int,
}

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

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

Modeling a JS Module with Default Export

See here.

Checking for JS nullable types using the option type

For a function whose argument is passed a JavaScript value that's potentially null or undefined, it's idiomatic to convert it to an option. The conversion is done through the helper functions in ReScript's Nullable module. In this case, toOption:

<CodeTab labels={["ReScript", "JS Output"]}>

let greetByName = (possiblyNullName) => {
  let optionName = Nullable.toOption(possiblyNullName)
  switch optionName {
  | None => "Hi"
  | Some(name) => "Hello " ++ name
  }
}

function greetByName(possiblyNullName) {
  if (possiblyNullName == null) {
    return "Hi";
  } else {
    return "Hello " + possiblyNullName;
  }
}

This check compiles to possiblyNullName == null in JS, so checks for the presence of null or undefined.