What are undeclared and undefined variables in JavaScript?
Last Updated :
20 Jun, 2023
Undefined: It occurs when a variable has been declared but has not been assigned any value. Undefined is not a keyword.
Undeclared: It occurs when we try to access any variable that is not initialized or declared earlier using the var or const keyword. If we use ‘typeof’ operator to get the value of an undeclared variable, we will face the runtime error with the return value as “undefined”. The scope of the undeclared variables is always global.
For example:
Undefined:
let geek;
undefined
console.log(geek)
Undeclared:
// ReferenceError: myVariable is not defined
console.log(myVariable)
Example 1: This example illustrates a situation where an undeclared variable is used.
javascript
function GFG() {
'use strict';
x = "GeeksForGeeks";
}
GFG();
|
Output:
ReferenceError: x is not defined
Example 2: This example checks whether a given variable is undefined or not.
Javascript
function checkVar() {
let string;
if (typeof variable === "undefined") {
string = "Variable is undefined";
} else {
string = "Variable is defined";
}
console.log(string);
}
checkVar();
|
Output
Variable is undefined
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