There are a few nuances with checking whether a variable or object property is undefined in JavaScript. Here’s what you need to know.

In JavaScript, checking if a variable is undefined can be a bit tricky since a null variable can pass a check for undefined if not written properly. As a result, this allows for undefined values to slip through and vice versa. Make sure you use strict equality === to check if a value is equal to undefined.

let x;
const y = null;

x == null && y == undefined; // true
x === null || y === undefined; // false
x == null && typeof x == 'object'; // false
x === undefined && typeof x == 'undefined'; // true

Another alternative is checking if typeof x === 'undefined'. The biggest difference between these two approaches is that, if x has not been declared, x === undefined throws a ReferenceError, but typeof does not.

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How to Check if a JavaScript Variable is Undefined
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