How to check the equality of 2 values in Javascript? (If they are the same)

About

This page discusses Equality between 2 Javascript values. Because every value in javascript is an object, it applies also to the object.

Method

Object.is

Object.is verifies the value.

• in case of the primitive, this is the value
• in other cases, this is the reference

This is the function used by libraries to check if 2 values received are the same. (React uses it intensively)

Syntax:

Object.is(value1, value2);

let foo = { a: 1 };
let bar = foo;
console.log(Object.is(foo, foo));              // true
console.log(Object.is(foo, bar));              // true
console.log(Object.is(foo, { a: 1 }));              // false

• Special Values

Object.is(null, null);            // true
Object.is(undefined, undefined);  // true
Object.is(window, window);        // true

Operator

For a comparison:

• between object, both == and === object equality operator will simply check whether the references match.
• if one side of the equality comparison is not an object, valueOf is called before any comparison

For objects, they will not compare the content (ie not anything about the underlying values).

Equality Operator Example

Object

• A object constructor function

function Employee(name) {
    this.name = name;
    this.dept = 'general';
    this.valueOf = function(){
        return this.name;
    };
}

• Two objects (with JavaScript - new)

let employee1 = new Employee('foo');
let employee2 = new Employee('foo');

• Test

console.log("The variable employee1 has the type "+(typeof employee1 ));
console.log("The variable employee1 has the valueOf "+employee1.valueOf());
console.log("They are not equal "+(employee1 == employee2));
console.log("But their value is "+(employee1.valueOf() == employee2.valueOf()));

Array

• An array is an object

var a = [1,2,3];  
var b = [1,2,3]; 
console.log("The array a has the type "+(typeof a));
console.log("The array a in a string format is "+a.toString()+ "( valueOf() )" );

• The string representation of an array is element1, …, elementn

var c = "1,2,3"; 
console.log("The variable c has the type "+(typeof c));

• Equality

console.log("\nEquality");
console.log( a == c ); // true - same value as string
console.log( b == c ); // true - same value as string
console.log( a == b ); // false (not the same reference)