What are undeclared and undefined variables in JavaScript?

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. 

function GFG() {

    // 'use strict' verifies that no undeclared
    // variable is present in our code
    'use strict';
    x = "GeeksForGeeks";
}

GFG(); // Accessing the above function

Output: 

ReferenceError: x is not defined

Example 2: This example checks whether a given variable is undefined or not. 

function checkVar() {
    let string;

    if (typeof variable === "undefined") {
        string = "Variable is undefined";
    } else {
        string = "Variable is defined";
    }
    
    console.log(string);
}

checkVar();

Variable is undefined