https://stackoverflow.com › questions › 3390396
How can I check for "undefined" in JavaScript? - Stack OverflowWhat 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://stackoverflow.com › questions › 2559318
How to check for an undefined or null variable in JavaScript?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 ...
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.
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://www.freecodecamp.org › news › javascript-check-if-undefined-how-to-test-for...
JavaScript Check if Undefined – How to Test for Undefined in JSIn 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.
https://masteringjs.io › tutorials › fundamentals › undefined-check
How to Check if a JavaScript Variable is UndefinedWhen using x === undefined, JavaScript checks if x is a declared variable that is strictly equal to undefined. If you want to check if x is strictly equal to undefined regardless of whether is has been declared or not, you should use typeof x === 'undefined'.
https://stackabuse.com › javascript-check-if-variable-is-a-undefined-or-null
JavaScript: Check if Variable is undefined or null - Stack AbuseIn this short guide, you'll learn how to check if a variable is undefined, null or nil in vanilla JavaScript and with Lodash - with practical examples and advice on when to use which approach.
https://waytolearnx.com › 2019 › 07 › comment-verifier-si-une-variable-est-undefined-ou-null...
Comment vérifier si une variable est undefined ou null en JavaScript ...Vous pouvez utiliser l’opérateur d’égalité (==) en JavaScript pour vérifier si une variable est indéfinie ou nulle. Par exemple : var str; if (str == null) { alert('La variable str est nulle!'); } Sortie: La variable str est nulle! Utiliser le type « undefined »
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.
https://www.w3schools.com › jsref › jsref_undefined.asp
JavaScript undefined Property - W3SchoolsThe undefined property indicates that a variable has not been assigned a value, or not declared at all.
