JS: Array (basics)

Create array (array literal expression)

[value1, value2, etc]

console.log([1, 2, 3]);
// [ 1, 2, 3 ]

// elements with mixed types
console.log(["one", 3]);
// [ "one", 3 ]

• JS: Create Array

Length

xArray.length return the count of elements.

console.log([7, 8, 2].length);
// 3

Get an element

via at (method)

// get a element
console.log([3, 4].at(0));
// 3

console.log([3, 4].at(-1));
// 4

• JS: Array.prototype.at

via bracket operator

xArray[index]

index cannot be negative.

This is the bracket notation for property access. [see JS: Property Dot Notation vs Bracket Notation]

// get a element
console.log([17, 4, 1][0]);
// 17

Modify array element

const xx = [2, 4, 1];
xx[0] = 99;
console.log(xx);
// [ 99, 4, 1 ]

🟢 tip: you should not modify array element like this. because it is easy to go out of bound and accidentally create Sparse Array.

Out of bound

if index is out of bound, result is a Sparse Array.

const xx = [0, 1];
xx[4] = 99;
console.log(xx);
// [ 0, 1, <2 empty items>, 99 ]

Change array element, return new copy

• JS: Array.prototype.with

Nested array

Array can be nested.

const xx = [3, [4, 5]];
console.log(xx[1][1]);
// 5

Loop over array

for (let x of [3, 4, 5]) {
  console.log(x);
}
/*
3
4
5
*/

• JS: for-of Loop
• JS: Array.prototype.map
• JS: Array.prototype.forEach

Clear all array elements

• JS: Array.prototype.splice (remove and add at index)

const xx = [3, 4, 5];
xx.splice(0);
console.log(xx.length);
// 0

Note, it's different from simply assign it a new empty array such as

xArray = []

Because that gives the variable xArray a new value.