Strict and Loose Equality

A comparison operator compares the value of the left operand to the value of the right operand and returns a boolean value indicating whether they are equal or not.

Strict Equality Operators

The “===” operator compares two values and if they are exactly equal (type and value) it returns true, otherwise false.

console.log(3===3);    // true
console.log(true === true); // true
console.log(undefined === undefined); // true

console.log('1'===1);       // false
console.log(0 === false);   // false

Loose Equality Operators

The “==” operator compares two values and if they are loosely equal (coercion and value) it returns true, otherwise false.

Any two values that pass the strict equality test will pass the loose equality test. If they are not the same type, then an attempt will be made to coerce the type of one value to the type of the other and then the two values will be compared.

console.log(1==1);     // true
console.log("hello" == "hello"); // true

// successful after coercion
console.log('1'==1);   // true
console.log(true==1);  // true

// unsuccessful after coercion
console.log('10'==true);

The term falsy is used in JavaScript to describe a value that evaluates to false when encountered in a boolean context. Both "" and 0 are falsy values and will evaluate to false.

console.log("" == false);  // true
console.log(0  == false);  // true

Two different objects, even if they have the same data within them, are not equal.

let p1 = {firstName: 'Joe', lastName: 'Jones' };
let p2 = {firstName: 'Joe', lastName: 'Jones' };

console.log(p1 == p2); // false

let a1 = [1,2,3];
let a2 = [1,2,3];

console.log(a1 == a2);  // false

Summary

The rules for determining whether two values will be loosely equal after coercion are complex. They are available in the ECMAScript specification below.

To avoid unintentional errors, it is recommend to use the strict equality, "===" operator.

ECMAScript Language Specification - ECMA-262 Edition 5.1