== versus === and truthy values in JavaScript

=== (strict equality) compares type and value. == (loose equality) first converts the two sides to a common type, and the rules for that are easy to forget.

console.log(0 == '', 0 === '', '1' == 1, null == undefined, null === undefined, NaN === NaN);

true false true true false false

Use === and !== everywhere. The only common exception is x == null, which is true for both null and undefined.

The falsy values

Only these are falsy; everything else, including '0', 'false', [] and {}, is truthy: false, 0, -0, 0n, '' (empty string), null, undefined and NaN.

?? and ?. instead of ||

|| treats 0 and '' as missing, which is often wrong. ?? only replaces null and undefined. ?. stops safely when something is missing.

const student = { name: 'Asha', marks: 0, address: null };
console.log(student.marks || 'no marks', student.marks ?? 'no marks');
console.log(student.address?.city ?? 'city not given');
console.log(Number.isNaN(Number('abc')));

no marks 0
city not given
true

To check for NaN, use Number.isNaN(x), because NaN is the one value not equal to itself.