JavaScript ReferenceError - Assignment to undeclared variable Last Updated : 15 Jul, 2025 Suggest changes Share Like Article Like Report This JavaScript exception Assignment to undeclared variable occurs in strict-mode If the value has been assigned to an undeclared variable. Message: ReferenceError: assignment to undeclared variable "x" (Firefox) ReferenceError: "x" is not defined (Chrome) ReferenceError: Variable undefined in strict mode (Edge) Error Type: ReferenceError Cause of the error: Somewhere in the code, there is an assignment without the var, let, or const keywords. When a value is assigned to an undeclared variable this error occurs. Example 1: In this example, the const keyword is used with the variable assignment, So the error has not occurred. HTML <!DOCTYPE HTML> <html> <body style="text-align:center;"> <h1 style="color:green;"> GeeksforGeeks </h1> <p> JavaScript ReferenceError- Assignment to undeclared variable </p> <button onclick="Geeks();"> click here </button> <p id="GFG_DOWN"> </p> <script> let el_down = document.getElementById("GFG_DOWN"); function GFG() { 'use strict'; const var_1 = "Value assigned without declaration"; } function Geeks() { try { GFG(); el_down.innerHTML = "'Assignment to undeclared variable'" + "error has not occurred"; } catch (e) { el_down.innerHTML = "'Assignment to undeclared variable'" + "error has occurred"; } } </script> </body> </html> Output: Example 2: In this example, the var, let or const keyword is not used with the variable assignment, So the error has occurred. HTML <!DOCTYPE HTML> <html> <body style="text-align:center;"> <h1 style="color:green;"> GeeksforGeeks </h1> <p> JavaScript ReferenceError - Assignment to undeclared variable </p> <button onclick="Geeks();"> click here </button> <p id="GFG_DOWN"> </p> <script> let el_down = document.getElementById("GFG_DOWN"); function GFG() { 'use strict'; var_1 = true; } function Geeks() { try { GFG(); el_down.innerHTML = "'Assignment to undeclared variable'" + "error has not occurred"; } catch (e) { el_down.innerHTML = "'Assignment to undeclared variable'" + "error has occurred"; } } </script> </body> </html> Output: P PranchalKatiyar Follow Article Tags : JavaScript Web Technologies JavaScript-Errors Explore JavaScript BasicsIntroduction to JavaScript4 min readVariables and Datatypes in JavaScript6 min readJavaScript Operators5 min readControl Statements in JavaScript4 min readArray & StringJavaScript Arrays7 min readJavaScript Array Methods7 min readJavaScript Strings5 min readJavaScript String Methods9 min readFunction & ObjectFunctions in JavaScript5 min readJavaScript Function Expression3 min readFunction Overloading in JavaScript4 min readObjects in JavaScript4 min readJavaScript Object Constructors4 min readOOPObject Oriented Programming in JavaScript3 min readClasses and Objects in JavaScript4 min readWhat Are Access Modifiers In JavaScript ?5 min readJavaScript Constructor Method7 min readAsynchronous JavaScriptAsynchronous JavaScript2 min readJavaScript Callbacks4 min readJavaScript Promise4 min readEvent Loop in JavaScript4 min readAsync and Await in JavaScript2 min readException HandlingJavascript Error and Exceptional Handling6 min readJavaScript Errors Throw and Try to Catch2 min readHow to create custom errors in JavaScript ?2 min readJavaScript TypeError - Invalid Array.prototype.sort argument1 min readDOMHTML DOM (Document Object Model)9 min readHow to select DOM Elements in JavaScript ?3 min readJavaScript Custom Events4 min readJavaScript addEventListener() with Examples9 min readAdvanced TopicsClosure in JavaScript4 min readJavaScript Hoisting6 min readScope of Variables in JavaScript3 min readJavaScript Higher Order Functions7 min readDebugging in JavaScript4 min read My Profile ${profileImgHtml} My Profile Edit Profile My Courses Join Community Transactions Logout Like