JS: Array (overview)

Array syntax

• JS: Array (basics)

Array is object type with special purpose

JavaScript array is a Object, with a magic property key "length", and special treatement of non-negative integer string property keys "0", "1", "2", etc.

console.assert(typeof [3, 4] === "object");

Since JS Array is a Object, we can add property to array that's not an integer.

const xx = [3, 4];

// array is a object, you can add properties to it
xx.yy = 7;

console.assert(Object.hasOwn( xx, "yy" ));

console.assert(xx.yy === 7);

Array also has the attribute “extensible”, just like other objects. [see Prevent Adding Property]

console.assert(Object.isExtensible([3, 4]));

Index vs property key

The index of array is the same as string property key.

console.assert(Object.hasOwn([3, 4], "0"));

Reading Non-Existent Index Return Undefined

// accessing array with non-existent index results undefined
const xx = [3];
console.assert(xx[200] === undefined);

Check if a object is true array

• JS: Array.isArray

Length special property

Every array object has a special property key "length". It is the object's own property, and is a key of type string. The “length” property is automatically updated when array elements are added or removed using Array.prototype methods.

Array length can be set. If you set it, the array will be lengthened or shortened.

• When lengthened, it creates a Sparse Array.
• When shortened, extra elements are removed.

// creating a sparse array by setting the length property

const xx = ["a", "b"];

// set the length property beyond the last index
xx.length = 5;

console.log(Object.getOwnPropertyNames(xx));
// [ "0", "1", "length" ]

console.log(xx);
// [ "a", "b", <3 empty items> ]

// truncating a array by setting the length

const xx = [1, 2, 3, 4, 5];

console.log(Object.getOwnPropertyNames(xx));
// [ "0", "1", "2", "3", "4", "length" ]

xx.length = 1;

// index 1 doesnt exist
console.log(xx[1]);

console.log(Object.getOwnPropertyNames(xx));
// [ "0", "length" ]

[see Array.prototype.length]

Array methods

• JS: Array.prototype

String to Array

• JS: String.prototype.split

Array is iterable

That means, you can use for-of Loop and Spread Operator on them.