JavaScript Functions — The Basics

Functions are the unit of work in JavaScript

In JavaScript, functions are first-class — they are values you can store in variables, pass to other functions, return from functions, and attach as properties of objects. Everything from a single utility to an entire framework is built out of functions composing together.

This is the first of three posts on functions. It covers the foundations — how to define them and how to call them. The next two go deeper into closures, this, and higher-order patterns and modern function features such as arrow functions, destructured parameters, and async/await.

Three ways to create a function

1. Function declaration

The classic form. Hoisted in full, so it can be called above its definition.

1greet('Ali')
2
3function greet(name) {
4  return `Hello, ${name}`
5}

2. Function expression

Assigns an anonymous (or named) function to a variable. Behaves like any other value — not hoisted.

1const greet = function (name) {
2  return `Hello, ${name}`
3}

A named function expression is useful for stack traces and self-reference:

1const factorial = function fact(n) {
2  return n <= 1 ? 1 : n * fact(n - 1)
3}

3. Arrow function (ES2015)

The modern default for anonymous functions. Shorter syntax, no own this, no arguments object.

1const greet = name => `Hello, ${name}`
2
3// Multi-statement body needs braces and an explicit return
4const shippingFee = weight => {
5  if (weight < 1) return 5
6  if (weight < 10) return 10
7  return 25
8}

Note

Arrow functions are not just shorter syntax — they inherit this from the enclosing scope. That makes them ideal for callbacks but unsuitable as object methods or constructors. The advanced post covers this in depth.

Declaration vs expression at a glance

Calling a function

A function value and a function call are not the same thing.

1function speak() {
2  return 'hi'
3}
4
5speak // [Function: speak]   ← the function itself
6speak() // "hi"                ← the result of calling it

This trips up developers when passing callbacks:

1button.addEventListener('click', handleClick) // pass the function
2button.addEventListener('click', handleClick()) // BUG: calls now, passes the result

Parameters and arguments

A parameter is the name listed in the function signature; an argument is the value supplied at the call site.

Default parameter values (ES2015)

1function fetchPage(url, { method = 'GET', retries = 3 } = {}) {
2  // ...
3}
4
5fetchPage('/users') // method=GET, retries=3
6fetchPage('/users', { retries: 1 }) // method=GET, retries=1

A default expression is evaluated every time the function is called, so you can use it for non-constant defaults:

1function log(msg, time = new Date()) {
2  console.log(`[${time.toISOString()}] ${msg}`)
3}

Rest parameters

Collect any number of trailing arguments into a real array:

1function sum(...numbers) {
2  return numbers.reduce((a, b) => a + b, 0)
3}
4
5sum(1, 2, 3, 4) // 10

Rest must be the last parameter. There can be only one.

Spread at the call site

The same ... syntax, used as an argument, expands an iterable into individual arguments:

1const points = [3, 7, 2]
2Math.max(...points) // 7
3
4const all = [1, 2, ...points, 9] // [1, 2, 3, 7, 2, 9]

Return values

Every function returns something. If you omit return, the function returns undefined:

1function noop() {}
2noop() // undefined

Warning

JavaScript inserts a semicolon after a bare return keyword on its own line. The expression below returns undefined, not the object:

1function make() {
2  return
3  {
4    value: 1
5  }
6}

Always keep return and the value it returns on the same line, or wrap the expression in parentheses.

Mental model: functions as values

Because functions are values, every pattern that works for values works for functions:

 1// Store in an array
 2const ops = [x => x + 1, x => x * 2]
 3
 4// Pass as an argument
 5;[1, 2, 3].map(x => x * x)
 6
 7// Return from another function
 8function adder(by) {
 9  return x => x + by
10}
11const add5 = adder(5)
12add5(10) // 15

That last example — a function returning a function that “remembers” the outer variable — is a closure. Closures, plus higher-order functions, plus the this binding, are the subject of the next post.

Summary

• Three forms: function declaration (hoisted), function expression (not hoisted), arrow function (no own this).
• A bare function name is the function value; only () calls it.
• Use default parameters, rest parameters, and spread for clean, modern signatures.
• An omitted return produces undefined. Beware automatic semicolon insertion after return.
• Functions are values — they fit into every collection, callback, and pipeline.

Next: Functions in depth — closures, this, and higher-order patterns.