The arguments keyword

JavaScriptJunior~1 min read

What `arguments` is

arguments is a built-in object available inside any regular function (but not an arrow function). It contains all the arguments passed to that function on call.

Example

javascript

function showArguments() {
  console.log(arguments);
}

showArguments("apple", 42, true);

Result in the console:

javascript

[Arguments] { '0': 'apple', '1': 42, '2': true }

So arguments is an array-like object:

indices (0, 1, 2, ...)
a length property
but no array methods (map, forEach, etc.)

Accessing arguments

javascript

function sumAll() {
  let total = 0;
  for (let i = 0; i < arguments.length; i++) {
    total += arguments[i];
  }
  return total;
}

console.log(sumAll(1, 2, 3, 4)); // 10

Here we don't declare parameters, we just take everything passed via arguments.

Difference from parameters

javascript

function show(a, b) {
  console.log(a, b);
  console.log(arguments[0], arguments[1]);
}

show("x", "y");

// a,b → "x","y"
// arguments[0], arguments[1] → "x","y"

They match as long as you don't change the values.

Changes to `arguments` and parameters (in regular functions)

javascript

function demo(a) {
  console.log(a, arguments[0]); // 10, 10
  a = 20;
  console.log(a, arguments[0]); // 20, 20
}

demo(10);

In regular functions, arguments and parameters are linked - changing one changes the other (but this does not work in strict mode ('use strict')).

Arrow functions have no `arguments`

javascript

const arrow = () => {
  console.log(arguments); // ReferenceError
};

arrow(1, 2, 3);

Why:

Arrow functions do not create their own arguments object.
If you access it, it is taken from the outer function, if there is one.

The alternative - the rest (`...`) operator

A modern and safe alternative to arguments - rest parameters:

javascript

function sumAll(...args) {
  return args.reduce((sum, n) => sum + n, 0);
}

console.log(sumAll(1, 2, 3)); // 6

Advantages of ...args over arguments:

It's a real array (has .map, .filter, .reduce).
Works in arrow functions.
No confusion with "linked" values.
The code looks cleaner and clearer.

Quick summary

Property	`arguments`	`...rest`
Type	Array-like object	Array
Array methods (`map`, `forEach`)	No	Yes
Available in arrow functions	No	Yes
Linked to parameters	Yes (in non-strict)	No
Modern way	Deprecated	Recommended

Example to remember

javascript

// Old way
function oldSum() {
  return Array.from(arguments).reduce((a, b) => a + b);
}

// New way
const newSum = (...nums) => nums.reduce((a, b) => a + b);

console.log(oldSum(1, 2, 3)); // 6
console.log(newSum(1, 2, 3)); // 6

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