JavaScript >> Define function, array, object, variable, class and property in JavaScript

2022-05-22 JavaScript

Table of Contents

This tutorial will explain how to define function, array, object, variable, class and property in JavaScript.

JavaScript

JavaScript define function

Define using function keyword

function add(a, b) {
  return a + b;
}

Use arrow syntax to define a function

let add = (a, b) => {
  return a + b;
}

JavaScript define array

Define an empty array and add/remove elements

let arr = [];
arr.push(1);
arr.push(2);
arr.push(3);
console.log(arr);
arr.pop(); // remove the last element
console.log(arr);

[1, 2, 3]
[1, 2]

Define an array in ES6.

let array = [1, 2, 3];
console.log(array)

[1, 2, 3]

Use Array object to define an array.

let a = new Array(10);
a[0] = "abc"
a[9] = "aaa"
console.log(a)

(10) ['abc', empty × 8, 'aaa']

JavaScript define object

Define object in Literal Syntax.

let obj = {
    name: "Kevin",
    age: 30,

    sayHello: function() {
        console.log("Hello, " + this.name);
    }
};
obj.sayHello();

Hello, Kevin

Define object using constructor.

function Person(name, age) {
    this.name = name;
    this.age = age;
}


let person = new Person("Kevin", 30);
console.log(person.name);

Kevin

JavaScript define varaible

Define variables using var keyword in ES5

var age = 20;
var name = "Kevin";
console.log(name + "'s age is " + age);

Kevin's age is 20

Define variables using let keyword in ES6

ES6 introduced the let keyword to define variables.

let age = 20;
let name = "Kevin";
console.log(name + "'s age is " + age);

Define local and global variables

var global = "global";

function test() {
    var local = "local";
    console.log("Reference global variable in local scope: " + global);
    console.log("Reference local variable in local scope: " + local);
}

test();

console.log("Reference global variable in global scope: " + global);
console.log("Reference local variable in global scope: " + local); // Error will occur

Reference global variable in local scope: global
Reference local variable in local scope: local
Reference global variable in global scope: global
Uncaught ReferenceError: local is not defined

Define constant using const keyword in ES6

The constant can’t be changed after it is defined.

const PI = 3.14;
console.log(PI);

3.14

If we try to change it, we will get an error.

PI = 2.0;

TypeError: Assignment to constant variable.

Define mutiple variables in one line.

var name = "Kevin", age = 30;
let a = 20, b = "test";

In ES6, we can define multiple variables in one line as follows.

let [a,b,c,d] = [0,1,2,3];

JavaScript define Class

class Student {
    constructor(name, age) {
        this.name = name;
        this.age = age;
    }

    sayHello() {
        console.log("Hello, " + this.name);
    }
}

let s = new Student("Kevin", 16);
s.sayHello();
console.log(s.name + "'s age is " + s.age);

Hello, Kevin
Kevin's age is 16

JavaScript define property

const student = {
    name: "Kevin",
    age: 16,

    sayHello() {
        console.log("Hello, " + this.name);
    }
};

Object.defineProperty(student, "score", {
    writable: true,
    value: 0
});

student.score = 88;

console.log(student.name + "'s score is " + student.score);

Kevin's score is 88

Subscribe and be the FIRST reader of our latest articles

* indicates required

Contact us