https://stackoverflow.com › questions › 3390396

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

https://developer.mozilla.org › en-US › docs › Web › JavaScript › Reference › Global_Objects › undefined

Examples. Strict equality and undefined. You can use undefined and the strict equality and inequality operators to determine whether a variable has a value. In the following code, the variable x is not initialized, and the if statement evaluates to true. js.

https://www.w3schools.com › jsref › jsref_undefined.asp

The undefined property indicates that a variable has not been assigned a value, or not declared at all. Browser Support. undefined() is an ECMAScript1 (ES1) feature. ES1 (JavaScript 1997) is fully supported in all browsers: More Examples. Example. Variable not declared: if (typeof y === "undefined") { txt = "y is undefined"; } else {

https://www.javascripttutorial.net › javascript-undefined

JavaScript uses the undefined value in the following situations. 1) Accessing an uninitialized variable. When you declare a variable and don’t initialize it to a value, the variable will have a value of undefined. For example: let counter; console.log(counter); // undefined Code language: JavaScript (javascript)

https://developer.mozilla.org › fr › docs › Web › JavaScript › Reference › Global_Objects › undefined

Exemples. L'égalité stricte et undefined. Il est possible d'utiliser undefined et les opérateurs stricts pour l'égalité et l'inégalité strictes afin de déterminer si une variable a une valeur affectée. Dans le code qui suit, la variable x n'est pas initialisée et la première instruction if sera évaluée à true (vrai). js.

https://www.geeksforgeeks.org › undefined-in-javascript

In JavaScript, undefined is a primitive value that represents the absence of a value or the uninitialized state of a variable. It's typically used to denote the absence of a meaningful value, such as when a variable has been declared but not assigned a value. It can also indicate the absence of a return value in a function or the ...

https://www.programiz.com › javascript › null-undefined

JavaScript undefined. If a variable is declared but the value is not assigned, then the value of that variable will be undefined. For example, let name; console.log(name); // undefined. It is also possible to explicitly assign undefined to a variable. For example,

https://www.tutorialsteacher.com › javascript › javascript-null-and-undefined

What is undefined? A variable is undefined when you haven't assigned any value yet, not even a null. Example: undefined Variable. let num; . console.log(num);//"undefined" Generally, variables are undefined when you forgot to assign values or change existing code. For example, consider the following Greet() function that returns a string.