Une variable pour laquelle aucune valeur n'a été assignée sera de type undefined. Une méthode ou instruction renvoie également undefined si la variable à évaluer n'a pas de valeur assignée. Une fonction renvoie undefined si aucune valeur n'a été renvoyée.
https://stackoverflow.com › questions › 3390396
How can I check for "undefined" in JavaScript? - Stack OverflowIf you are interested in knowing whether the variable hasn't been declared or has the value undefined, then use the typeof operator, which is guaranteed to return a string: if (typeof myVar !== 'undefined') Direct comparisons against undefined are troublesome as undefined can be overwritten. window.undefined = "foo";
Learn what undefined means in JavaScript and how to use it to check if a variable has a value. See examples, specifications, and browser compatibility for undefined.
https://www.w3schools.com › jsref › jsref_undefined.asp
JavaScript undefined Property - W3SchoolsLearn how to use the undefined property to check if a variable has no value or has not been declared. See examples, browser support and related tutorials.
https://www.javascripttutorial.net › javascript-undefined
JavaScript undefinedLearn what undefined is in JavaScript and how to handle it effectively. See situations where undefined occurs and how to avoid it with default values, nullish coalescing operator, and in operator.
https://www.delftstack.com › fr › howto › javascript › javascript-undefined
Vérifier si une variable n'est pas définie dans JavaScriptCe didacticiel explique comment vérifier si une variable n’est pas définie dans JavaScript. Une variable est dite undefined si elle est déclarée sans se voir attribuer une valeur initiale. Vous trouverez ci-dessous plusieurs façons de le faire en JavaScript.
https://stackoverflow.com › questions › 1984721
How to handle 'undefined' in JavaScript - Stack OverflowAs is often the case with JavaScript, there are multiple ways to do this: typeof foo !== 'undefined' window.foo !== undefined 'foo' in window
https://devdoc.net › ... › en-US › JavaScript › Reference › Global_Objects › undefined.html
undefined - JavaScript | MDNLearn about the global undefined property and value in JavaScript, which represents the primitive type undefined. See how to use it in strict equality, typeof, and void operators, and its browser compatibility.
https://www.kilukru.dev › difference-de-null-et-undefined-en-javascript
Différence de Null et Undefined en JavaScriptLorsque nous vérifions null et undefined, il est assez intéressant de jeter un coup d’œil aux opérateurs d’égalité, car nous avons effectué une conversion de type (ou, dans un terme plus technique, type coercion) lorsque nous devions le faire. null == undefined // true
https://www.freecodecamp.org › news › javascript-check-if-undefined-how-to-test-for...
JavaScript Check if Undefined – How to Test for Undefined in JSIn 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.
