JS: Array Tutorial

Create Array

The basic syntax to create a array is:

[value1, value2, etc]

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

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

Create Array with n Elements

// create array with 4 elements, all 0
console.log(Array(4).fill(0));
// [ 0, 0, 0, 0 ]

// create array with element 0 to 3
console.log(Array(4).fill(0).map((x,i) => x+i));
// [ 0, 1, 2, 3 ]

Length

xArray.length return the count of elements.

[7, 8, 2].length === 3

Get an Element

via at (method)

• xArray.at(index)
• index can be negative. means count from right.
• JS: Array.prototype.at

// get a element
console.log([2, 4, 1].at(0) === 2);
// true

console.log([2, 4, 1].at(-1) === 1);
// true

via bracket operator

xArray[index]

index cannot be negative.

// get a element
console.log([2, 4, 1][0] === 2);
// true

Modify Array Element

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

Nested Array

Array can be nested.

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

Loop Over Array

for-of Loop

• JS: for-of Loop

// for-of loop on array
let xx = [3, 4, 5];
for (let x of xx) {
 console.log(x);
}

/*
3
4
5
*/

// for-of loop on array. with index and value
let xx = ["a", "b", "c"];
for (let [i, x] of xx.entries()) {
 console.log(i, x);
}

/*
0 a
1 b
2 c
*/

Clear All Array Elements

• JS: Array.prototype.splice

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.