JavaScript: Array-Like Object

What is a array-like object?

A object such that:

1. has property names of consecutive non-negative integers {"0", "1", "2", …}.
2. has a property "length" with a value that's the number of these properties.
3. is not a true array. That is, Array.isArray(obj) returns false.

〔►see JavaScript: Understand JS Array〕

What is the difference between array and array-like object?

For a true array, value of length will automatically change.

// create a array-like object
var a1 = {0:"a", 1:"b" , length:2};
a1[2] = 99;
console.log(a1.length) // 2
// length would be 3 if it's a true array

// real array
var a2 = ["a", "b"];
a2[2] = 99;
console.log(a2.length) // 3

Real array object's parent is Array.prototype.

〔►see JavaScript: Prototype and Inheritance〕

〔►see JavaScript: Array.prototype〕

Array-like object's parent (if it has a parent) is usually not Array.prototype.

How to check if a object is true array?

Array.isArray(obj)

Where does array-like object came from?

In DOM, method such as document.getElementsByClassName returns the object NodeList or HTMLCollection, and they are array-like objects.

• JavaScript: Get Elements by ID, Tag, Name, Class, CSS Selector
• JavaScript: Array vs NodeList vs HTMLCollection

JavaScript's arguments object is a array-like object.

〔►see JavaScript: Function's “arguments” Object〕

How to convert array-like object to array?

JavaScript: How to Convert a Array-Like Object to Array?