Overview

Comparison to JS

Note: the comparison is against our Reason syntax, not our ML syntax.

Semicolon

Comments

Variable

String & Character

Boolean

Number

* JS has no distinction between integer and float.

Object/Record

Array

* Heterogenous arrays in JS are disallowed for us. Use tuple instead.

Null

* Again, only a spiritual equivalent; we don't have nulls, nor null bugs! But we do have an option type for when you actually need nullability.

Function

Blocks

const myFun = (x, y) => {
  const doubleX = x + x;
  const doubleY = y + y;
  return doubleX + doubleY;
};

let myFun = (x, y) => {
  let doubleX = x + x
  let doubleY = y + y
  doubleX + doubleY
}

If-else

* Our conditionals are always expressions! You can write let result = if (a) {"hello"} else {"bye"}

Destructuring

* Gives good compiler warning that data might not be of length 2.

Loop

JSX

* Argument punning!

Exception

* No finally.

Blocks

The last expression of a block delimited by {} implicitly returns (including function body). In JavaScript, this can only be simulated via an immediately-invoked function expression (since function bodies have their own local scope).

let result = (function() {
  const x = 23;
  const y = 34;
  return x + y;
})();

let result = {
  let x = 23
  let y = 34
  x + y
}

Common Features' JS Output