What is the most appropriate way to test if a variable is undefined in JavaScript? I've seen several possible ways: if (window.myVariable) Or. if (typeof(myVariable) != "undefined") Or. if (myVariable) // This throws an error if undefined. Should this be in Try/Catch?

Mar 13, 2025 · A variable that has not been assigned a value is of type undefined. A method or statement also returns undefined if the variable that is being evaluated does not have an assigned value. A function returns undefined if a value was not returned .

The undefined property indicates that a variable has not been assigned a value, or not declared at all. undefined() is an ECMAScript1 (JavaScript 1997) feature. It is supported in all browsers:

Mar 13, 2024 · The undefined property is used to check if a value is assigned to a variable or not. Syntax: var x; if (typeof x === "undefined") { txt = "x is undefined"; } else { txt = "x is defined"; } Return Value: It returns 'defined' if the variable is assigned …

Nowadays most browsers support the Nullish coalescing operator (??) and the Logical nullish assignment (??=), which allows a more concise way to assign a default value if a variable is null or undefined, for example:

Undefined. In JavaScript, a variable without a value, has the value undefined. The type is also undefined.

Apr 3, 2025 · Symbol is a built-in object whose constructor returns a symbol primitive — also called a Symbol value or just a Symbol — that's guaranteed to be unique.

Feb 12, 2024 · In JavaScript,!== is a strict inequality operator, and undefined is a special value representing the absence of a value or the lack of an assigned value to a variable. The !== operator checks for both value and type equality, and !== undefined is used to verify if a variable is not equal to the undefined value.

Jul 11, 2022 · In this article, you will learn the various methods and approaches you can use to know if a variable is undefined in JavaScript. This is necessary if you want to avoid your code throwing errors when performing an operation with an undefined variable.

May 28, 2024 · There are a few ways to check for ‘undefined’. 1. Using the ‘typeof’ operator: Here, ‘typeof‘ operator returns the string ‘undefined’ if the variable has not been assigned a value. Here, the ‘===’ operator checks if the value of the variable is exactly equal to ‘undefined’. Example 1: Example 2: Example 3: Example 4: