Curriculum

JavaScript Basic

JSHome

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JS Versions

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JS Objects

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JS Funtions

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JS Classes

0/3

JS Async

0/10

JS HTML DOM

0/25

JS Browser BOM

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JS Web APIs

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JS AJAX

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JS JSON

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JS vs jQuery

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JS Graphics

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Text lesson

JavaScript new Array()

JavaScript features a built-in array constructor, new Array().

However, using []is considered safer and more concise.

Both statements below achieve the same outcome, generating a new empty array named “points”:

const points = new Array();
const points = [];

Both of these distinct statements generate a new array consisting of 6 numbers:

const points = new Array(40, 100, 1, 5, 25, 10);
const points = [40, 100, 1, 5, 25, 10];

The use of the new keyword can lead to unexpected outcomes:

// Create an array with three elements:
const points = new Array(40, 100, 1);

// Create an array with two elements:
const points = new Array(40, 100);

// Create an array with one element ???
const points = new Array(40);

A Common Error

const points = [40];

differs from:

const points = new Array(40);

// Create an array with one element:
const points = [40];

// Create an array with 40 undefined elements:
const points = new Array(40);

How to Recognize an Array

A frequent query arises: How can I determine if a variable is an array?

The challenge lies in the fact that the JavaScript operator typeof yields “object“:

const fruits = [“Banana”, “Orange”, “Apple”];
let type = typeof fruits;

The reason the typeof operator returns “object” is because a JavaScript array is classified as an object.

Solution 1:

To address this issue, ECMAScript 5 (JavaScript 2009) introduced a new method called Array.isArray():

Array.isArray(fruits);

Solution 2:

The instanceof operator yields true if an object is instantiated by a specified constructor:

const fruits = [“Banana”, “Orange”, “Apple”];

fruits instanceof Array;

Nested Arrays and Objects

Values in objects can be arrays, and values in arrays can be objects.

Example

const myObj = {
  name: “John”,
age: 30,
  cars: [
    {name:“Ford”, models:[“Fiesta”, “Focus”, “Mustang”]},
    {name:“BMW”, models:[“320”, “X3”, “X5”]},
    {name:“Fiat”, models:[“500”, “Panda”]}
]
}

To access arrays within arrays, use a for-in loop for each nested array:

Example

for (let i in myObj.cars) {
x += “<h1>” + myObj.cars[i].name + “</h1>”;
for (let j in myObj.cars[i].models) {
x += myObj.cars[i].models[j];
}
}