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The most reliable way I know of checking for undefined is to use void 0. This is compatible with newer and older browsers, alike, and cannot be overwritten like window.undefined can in some cases. if( myVar === void 0){ //yup it's undefined }
While literally using the keyword undefined, Boolean(undefined) works, trying that with an undefined variable doesn't work, and that is the whole point of doing the check for null or undefined. This: if (Boolean(undeclareVarName)) { console.log('yes'); } else { console.log('no'); } throws a ReferenceError saying "ReferenceError ...
11 lip 2022 · In case you are in a rush, here are the three standard methods that can help you check if a variable is undefined in JavaScript: if(myStr === undefined){} if(typeof myArr[7] === "undefined"){} if(user.hobby === void 0){} Let’s now explain each of these methods in more detail.
31 gru 2009 · Assuming that the variable jsVar is a boolean and that we want to call the proceed() method when jsVar is true, we can do the following check. if (jsVar && jsVar == true) proceed(); The above code snippet first check that jsVar has been defined and then checks that its value is true.
28 maj 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 any value and 'undefined' if the variable is not assigned any value. More example code for the abo
29 sie 2023 · Use typeof to check if a variable is undefined. This is the safest way as it does not throw an error even if the variable has not been declared. Use strict equality operator ( ===) to check for null or undefined. Loose equality operator ( ==) can lead to unexpected results due to type coercion.
9 mar 2024 · Way 1 — Strict equality with undefined. This is the most common and recommended way. Use the strict equality operator ( ===) to compare the variable with the primitive value undefined.
