JavaScript – Difference between Array and Array-like object

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What is it?

An Object which has a length property of a non-negative Integer, and usually some indexed properties. For example

var ao1 = {length: 0},                     // like []
    ao2 = {0: 'foo', 5: 'bar', length: 6}; // like ["foo", undefined × 4, "bar"]

You can convert Array-like Objects to their Array counterparts using Array.prototype.slice

var arr = Array.prototype.slice.call(ao1); // []

Whats the difference between it and a normal array?

It’s not constructed by Array or with an Array literal [], and so (usually) won’t inherit from Array.prototype. The length property will not usually automatically update either.

ao1 instanceof Array; // false
ao1[0] = 'foo';
ao1.length; // 0, did not update automatically

Whats the difference between an array-like object and a normal object?

There is no difference. Even normal Arrays are Objects in JavaScript

ao1 instanceof Object; // true
[] instanceof Object; // true

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