Functions and closures in JavaScript

Declarations, arrows, defaults, rest, and closures.

function greet(name, greeting = "Hello") {
  return `${greeting}, ${name}!`;
}

const square = (x) => x * x;
const sum = (...nums) => nums.reduce((a, b) => a + b, 0);

const makeCounter = () => {
  let count = 0;
  return () => ++count;
};

const counter = makeCounter();
counter();
const value = counter();

How it works

1. Default parameters fill in missing arguments.
2. Arrow functions are concise and capture this lexically.
3. makeCounter returns a closure over its own state.

Keywords and builtins used here

• const
• function
• let
• return

The run, in numbers

Lines

15

Characters to type

301

Tokens

98

Three-star pace

90 tpm

At the three-star pace of 90 tokens a minute, this run takes about 65 seconds.

Type this snippet

Step 1 of 3 in Functions, step 14 of 43 in Language basics.

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