JavaScript: Array Basics

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: [expr0, expr1, expr2, etc].

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

Example of assigning array items one at a time:

// define a empty array
const aa = [];

// assign a value to a slot
aa[0] = 82;
aa[1] = 34;
aa[3] = 96;

console.log(aa);
// [ 82, 34, <1 empty item>, 96 ]

// value of aa[2] do not exist

Creating a Array with n Elements

There are many ways to create a array with 1 to n elements. Most common way is by for Loop

// create array with elements 1 2 3 ... n

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

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

Array Length

arrayX.length return the count of elements.

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

Loop thru Array

Best way to loop thru array is by using for-of Loop 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.

• Array.prototype.map
• Array.prototype.forEach