What is an array-like object?

JavaScriptJunior~1 min read

An array-like object

is an object that looks like an array, but is not a real array.

It has:

numeric indices (0, 1, 2, ...),
a length property,
but no array methods (push, map, forEach, and so on).

Examples of array-like objects:

1. `arguments` inside a function

javascript

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

showArgs(10, 20, 30);
// Arguments(3) [10, 20, 30, callee: ƒ, Symbol(Symbol.iterator): ƒ]

arguments looks like an array, but:

javascript

console.log(arguments[0]);  // 10
console.log(arguments.length); // 3
console.log(arguments.map); // undefined

2. DOM collections

javascript

const divs = document.querySelectorAll("div");
console.log(divs);

NodeList and HTMLCollection are also array-like objects. They contain elements by index and have length, but do not have methods like map, filter, and so on.

Why it is not an array

A real array inherits methods from Array.prototype, but an array-like object does not.

javascript

Array.isArray(arguments); // false

How to turn an array-like object into a real array

1. Via `Array.from()`

javascript

function demo() {
  const arr = Array.from(arguments);
  console.log(Array.isArray(arr)); // true
}
demo(1, 2, 3);

2. Via the spread operator (`...`)

javascript

function demo() {
  const arr = [...arguments];
  console.log(arr); // [1, 2, 3]
}
demo(1, 2, 3);

3. Via `slice.call()`

javascript

function demo() {
  const arr = Array.prototype.slice.call(arguments);
  console.log(arr); // [1, 2, 3]
}
demo(1, 2, 3);

Short version:

Property	Array-like object	Real array
Has numeric indices	Yes	Yes
Has `length`	Yes	Yes
Has methods (`map`, `push`, `filter`)	No	Yes
`Array.isArray()` returns `true`	No	Yes

Examples of array-like objects in JavaScript:

Source	Type
`arguments`	array-like object of function arguments
`document.querySelectorAll()`	`NodeList`
`document.getElementsByClassName()`	`HTMLCollection`
`FileList`	list of files from `<input type="file">`

Conclusion: An array-like object is an object similar to an array in structure (indices + length), but without array methods.

To work with it like a regular array, you can convert it using Array.from() or [...spread].

Short Answer

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