Here is an example of the code. let a; console.log(typeof a === 'undefined'); // true This method is safe because it won’t throw an error if the variable is undeclared. 2. Using Strict Equality (===) Strict equality (===) can also be used to check if a variable is ...
One of the simplest ways to check if a variable is undefined is by using the typeof operator. This operator returns a string that indicates the type of the unevaluated operand. If the variable is not defined, typeof will return the string “undefined”. Here’s how you can use it: let...
If you want to check whether a variable has been initialized or defined (i.e. test whether a variable has been declared and assigned a value) you can use the typeof operator.The most important reason of using the typeof operator is that it does not throw the ReferenceError if the ...
Variable Scope Solution 2: We can use typeofoperatorto check the variable is defined or not. if (typeof variable !== 'undefined') { // the variable is defined } Solution 3: You can use this code: if (typeof variable === 'undefined') { // variable is undefined } ...
Here's a Code Recipe to check whether a variable or value is either an array or not. You can use the Array.isArray() method. For older browser, you can use the polyfill 👍 constvariable=['🍝','🍜','🍲'];// ✅ NEWER BROWSERArray.isArray(variable);// 🕰 OLDER BROWSERObj...
Method 1: Using the OR (||) operator to check undefined and null variable As discussed above, users can check if a variable is null or undefined using theOR (||)operator. It checks if the variable satisfies either of the two conditions. When users use it with two Boolean values, the ...
variable instanceof Array 1. This method runs about1/3 the speedas the first example. Still pretty solid, looks cleaner, if you're all about pretty code and not so much on performance. Note that checking for numbers does not work as variable instanceof Number always returns...
AReferenceErroris thrown if you use (reference) a variable that has not been declared: Example letx =5; try{ x = y +1;// y cannot be used (referenced) } catch(err) { document.getElementById("demo").innerHTML= err.name; }
A function to check a cookie value A Function to Set a Cookie First, we create afunctionthat stores the name of the visitor in a cookie variable: Example functionsetCookie(cname, cvalue, exdays) { constd =newDate(); d.setTime(d.getTime() + (exdays*24*60*60*1000)); ...
The exampleButton variable is only populated after the component is rendered. If an unpopulated ElementReference is passed to JS code, the JS code receives a value of null. To manipulate element references after the component has finished rendering, use the OnAfterRenderAsync or OnAfterRender...