JavaScript has two equality/inequality operators: the strict and the abstract equality operators.
The operator looks similar to that used in languages like Java and C++:
console.log(2 == 2); // true
console.log(1 == 2); // false
console.log(3 != 2); // trueThe problem is, when the operands are of different types, the abstract equality operator converts the types first!
console.log("2" == 2); // true
console.log(1 == true); // true
console.log(0 == false); // true
console.log("" == false); // true
console.log("0" == false); // true (both are converted to 0)Type conversion before checking for equality is more error prune than it can be desired.
Do not use the “abstract equality” operators == and !=.
The strict equality uses an extra equal sign. It performs the equality comparison in a more exact identity sort of matching.
When the operands have the same type, their value is compared:
console.log(2 === 2); // true
console.log(1 === 2); // false
console.log(3 !== 2); // trueIf the operands don’t have the same type, then they are considered unequal:
console.log("2" === 2); // false
console.log(1 === true); // false
console.log(0 === false); // false
console.log("" === false); // false
console.log("0" === false); // falseWhile numbers, boolean values and strings compared by value, object/array references are strictly equal if they refer to the same object in memory:
const student = { name: "John Doe" };
const courseAssistant = { name: "John Doe" };
const headAssistant = courseAssistant;
console.log(student === courseAssistant); // false
console.log(headAssistant === courseAssistant); // trueA good read: Object equality in Javascript.
Note that undefined and null are only equal to themselves (unless compared using abstract equality).
console.log(undefined === undefined); // true
console.log(null === null); // true
console.log(undefined === null); // false
console.log(undefined == null); // trueHere is a bit of madness, NaN is not equal to NaN no matter what equality operator you use
console.log(NaN == NaN); // false
console.log(NaN === NaN); // falseSo JavaScript’s equality operator is not an equivalence relation (since it is not reflexive).
If you want to check if a variable is NaN, you should try one of the following static methods:
let num = NaN;
console.log(Number.isNaN(num)); // true
console.log(Object.is(num, NaN)); // trueObject.is is a new addition to JavaScript which works similar to strict equality except for NaN and +0/-0.
console.log(+0 === -0); // true
console.log(Object.is(-0, +0)); // falseJavaScript Object.is is designed to have the properties of an equivalence relation (it is reflexive, symmetric, and transitive).
MDN Web Docs has a great article on JavaScripts “Equality comparisons and sameness” available at this link.