Strict Equality in JavaScript

You can check strict equality in JavaScript using the operator ===. This operator does strict comparison and considers the operands equal if they’re of the same type, and their values are equal.

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

There one particular corner case with the strict equality operator in JS. It’s related to NaN. NaN is a special numeric value that’s not equal to itself.

console.log(NaN === NaN) // false

NaN appears when JavaScript can’t make sense of the provided arithmetic expression and can’t calculate it.

For example, you can try to subtract a string from a number and get NaN.

const x = 10 - 's';

console.log(x); // NaN

console.log(typeof x);   // number
console.log(typeof NaN); // number

console.log(x === NaN);  // false

To fix the issue, you can use the function Object.is to check if the numbers are equal. Contrary to the strict equality operator ===, the Object.is method returns true even if both numbers are NaN.

const x = 5;
const y = NaN;

console.log(Object.is(x, 5));   // true
console.log(Object.is(y, NaN)); // true