Function Parameters and Arguments in JavaScript

Function Parameters and Arguments in JavaScript

Functions receive values through parameters. Modern JavaScript has several ways to handle them: defaults, rest, and spread.

Parameters vs Arguments

• Parameters: the names listed in the function definition.
• Arguments: the actual values passed when calling.

function greet(name, greeting) {  // parameters
  console.log(`${greeting}, ${name}`);
}
greet("Kunal", "Hello");          // arguments

Extra and Missing Arguments

• If you pass more arguments than parameters, extras are ignored (but available via arguments or rest).
• If you pass fewer, missing parameters are undefined.

function foo(a, b) {
  console.log(a, b);
}
foo(1, 2, 3); // 1 2 (3 ignored)
foo(1);       // 1 undefined

Default Parameters

function greet(name = "friend", greeting = "Hi") {
  console.log(`${greeting}, ${name}`);
}
greet();              // "Hi, friend"
greet("Asha");        // "Hi, Asha"
greet(undefined, "Yo"); // "Yo, friend"

Defaults apply when the argument is undefined or omitted. Other falsy values (0, "", null) do not trigger the default.

Rest Parameters

Collect remaining arguments into a real array:

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

Rest must be the last parameter. It is a true array (has map, filter, etc.), unlike arguments.

Spread in Calls

Pass array elements as individual arguments:

const nums = [1, 2, 3];
Math.max(...nums); // 3

Destructuring Parameters

function render({ name, age = 0 }) {
  console.log(name, age);
}
render({ name: "Kunal" }); // "Kunal 0"

The arguments Object

Available in regular (non-arrow) functions. Array-like, has length and indices, but no array methods:

function show() {
  console.log(arguments.length, arguments[0]);
}
show("a", "b"); // 2 "a"

Arrow functions do not have their own arguments. Use rest parameters in arrows.

The Takeaway

Parameters are names, arguments are values. Defaults fill in undefined. Rest collects extras into a real array. Spread expands an array into individual arguments. arguments is array-like and only in regular functions; prefer rest in modern code.