JavaScript: Array Basics

By Xah Lee. Date: . Last updated: .

Array is a data structure that holds a sequence of values, in order.

Each value is “indexed” by a integer, starting from 0.

Creating Array

The basic syntax to create a array is: [expr_0, expr_1, expr_2, …].

const aa = ["one", "two", 3];
console.log(aa); // [ 'one', 'two', 3 ]

Example of assigning array items one at a time:

const aa = [];  // define a empty array
aa[0] = "zero"; // assign a value to a element
aa[1] = "one";
aa[3] = "more";

console.log(aa); // [ 'zero', 'one', , 'more' ]

// value of aa[2] is “undefined”

Creating a Array with n Elements

// creating array with n elements, 1 2 3 ... n

const arr = [];
for (let i = 1; i <= 5; i++) { arr.push(i); }

console.log(arr);
// [ 1, 2, 3, 4, 5 ]

Array Length

arrayX.length return the number of elements in arrayX.

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

Access Array Element

Access a element. arrayX[index]

const aa = [2, 4, 1];

// access a element
console.log(aa[0]);     // 2

Modify Array Element

Modify a element.

const aa = [2, 4, 1];

aa[0]= "no";

console.log(aa);     // [ 'no', 4, 1 ]

Nested Array

Array can be nested.

const aa = ["pa", ["deep", [4,5]], 3];
console.log(aa[1][1][0]); // prints 4

Loop thru Array

Best way to loop thru array is by using “for of” loop.

[see for-of Loop]

// for-of loop on array
let aa = [3,4,5];

for (let x of aa) {
    console.log(x);
} // prints 3 4 5

Here is “for of” loop with index and value:

// for-of loop on array
let aa = ["a", "b", "c"];

for (let [i,x] of aa.entries()) {
    console.log(i,x);
}

// prints
// 0 'a'
// 1 'b'
// 2 'c'

The most general purpose way to go thru array is using “for” loop.

// loop thru array
const aa = [3,7,4];

for (let i = 0; i < aa.length; i++) {
    console.log( aa[i]);
}

Other common way of going thru a array is array methods forEach and map.

JavaScript Array

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