November 24, 2022 - It can also be checked if the variable is initialized or not. In the below code, the control will not go inside the if block. let x if (typeof x !== "undefined") { console.log(x) } You will see the value of x logged into the console, if you execute the following code: let x = 1 if (typeof x !== "undefined") { console.log(x) } If you need to check if a property exists within an object then you can use the prototype method hasOwnProperty

July 11, 2025 - To do this, we will use the typeof operator. The typeof operator will return undefined if the variable is not initialized and the operator will return null if the variable is left blank intentionally.

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April 22, 2025 - What’s the best way in JavaScript to check if a variable exists and is initialized, regardless of its type? This version keeps the intent intact, flows conversationally, and smoothly includes the keyword javascript check if variable exists.

August 15, 2023 - To check that a variable is defined, avoiding false positives and false negatives, we must check that its type is not undefined, using the typeof operator and strict inequality: ... let myVariable; if (typeof myVariable !== "undefined") { ...

January 10, 2019 - const languages = [ { title: '1970s', languages: [ { name: 'C', year: 1972 } ] }, { title: '1980s', languages: [ { name: 'C++', year: 1983 }, { name: 'Perl', year: 1987 } ] }, { title: '1990s', languages: [ { name: 'Haskell', year: 1990 }, { name: 'Python', year: 1991 }, { name: 'Java', year: 1995 }, { name: 'Javascript', year: 1995 }, { name: 'PHP', year: 1995 }, { name: 'Ruby', year: 1995 } ] }, { title: '2000s', languages: [ { name: 'C#', year: 2000 }, { name: 'Scala', year: 2003 }, { name: 'Clojure', year: 2007 }, { name: 'Go', year: 2009 } ] }, { title: '2010s', languages: [ { name: 'Elm', year: 2012 } ] } ]; When the user changes the value of a text input, I want to check if the new value (which I’m saving as var newValue) matches one of the name properties in the above, but the nested nature of the array is causing me headaches.

September 28, 2018 - Hi guys, I have a layout that sets some headers dependent on the page variables. Now, the header should be set to page.title by default, but if the page has name defined in the front matter, this should be used instead. I already found out how to check if a variable exists, but this doesn’t work for me when strict_variables ist set to true.

A common idiom in JavaScript to differentiate between two possible values is to check for the presence of a member. As we mentioned, you can only access members that are guaranteed to be in all the constituents of a union type. ... fly) {Property 'fly' does not exist on type 'Fish | Bird'.

By using the ?. operator instead of just ., JavaScript knows to implicitly check to be sure obj.first is not null or undefined before attempting to access obj.first.second. If obj.first is null or undefined, the expression automatically short-circuits, returning undefined. This is equivalent to the following, except that the temporary variable is in fact not created:

November 17, 2020 - You can use this idea to determine if the global variable myGlobalVar is defined: simply check the global object for corresponding property existence: window.hasOwnProperty('myGlobalVar').

Use .toHaveProperty to check if property at provided reference keyPath exists for an object.

February 18, 2025 - In JavaScript, undefined is the default value for variables that have been declared but not initialized. On the other hand, null is an intentional assignment that explicitly indicates the absence of a value. ... This article covers different ways to check if a variable is undefined, helping you write code that handles these situations smoothly.

<script> var x; var y = 10; if(typeof x !== 'undefined'){ // this statement will not execute alert("Variable x is defined."); } if(typeof y !== 'undefined'){ // this statement will execute alert("Variable y is defined."); } // Attempt to access an undeclared z variable if(typeof z !== 'undefined'){ // this statement will not execute alert("Variable z is defined."); } /* Throws Uncaught ReferenceError: z is not defined, and halt the execution of the script */ if(z !== 'undefined'){ // this statement will not execute alert("Variable z is defined."); } /* If the following statement runs, it will also throw the Uncaught ReferenceError: z is not defined */ if(z){ // this statement will not execute alert("Variable z is defined."); } </script>

August 26, 2025 - The nullish coalescing operator treats undefined and null as specific values. So does the optional chaining operator (?.), which is useful to access a property of an object which may be null or undefined. Combining them, you can safely access a property of an object which may be nullish and provide a default value if it is.

September 12, 2020 - Hence when if statement is executed, myObj variable does exist. This is code hoisting of var. The JavaScript interpreter will only only hoist the variable defined with var. ... window is the top-level object of JavaScript. All global variables are its properties, so to check whether myObj exists, it is the same as checking whether window has the property myObj.

December 14, 2019 - 'exists': 'nope'); // exists console.log(y !== undefined ? 'exists': 'nope'); // nope console.log(z !== undefined ? 'exists': 'nope'); // ReferenceError: z is not defined · Closer, but now we are casting types again, and still doing the comparison that breaks on undeclared variables