Aug 14, 2019 · if(lastName != undefined) This didn't work? Are you getting a ReferenceError in the console? If so, instead of avoiding the error by using typeof (as Crockford followers will …

If it is undefined, it will not be equal to a string that contains the characters "undefined", as the string is not undefined. You can check the type of the variable: if (typeof(something) != …

In general it is recommended to use === instead of ==. The proposed solution is an exception to this rule. The JSHint syntax checker even provides the eqnull option for this reason. From the …

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. …

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 …

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 …

Feb 25, 2021 · Make sure you use strict equality === to check if a value is equal to undefined. const y = null; Another alternative is checking if typeof x === 'undefined'. The biggest …

Feb 12, 2024 · 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. Example: To demonstrate the …

The undefined property indicates that a variable has not been assigned a value, or not declared at all.

Feb 18, 2025 · In JavaScript, undefined is the default value for variables that have been declared but not initialized. On the other hand, null is an intentional assignment that explicitly indicates …