How to unset a JavaScript variable?

To unset a variable in JavaScript, you can set it to undefined or use the delete operator for object properties. However, the approach depends on how the variable was declared.

Setting Variable to undefined

The most common way to unset a variable is by assigning undefined to it:

let userName = "John Doe";
console.log("Before:", userName);

// Unset the variable
userName = undefined;
console.log("After:", userName);

Before: John Doe
After: undefined

Using delete Operator

The delete operator works only on object properties, not on variables declared with var, let, or const:

// For object properties
let user = {
    name: "Alice",
    age: 25
};

console.log("Before delete:", user.name);
delete user.name;
console.log("After delete:", user.name);

// For variables (doesn't work)
let testVar = "test";
console.log("Delete result:", delete testVar); // false
console.log("Variable still exists:", testVar);

Before delete: Alice
After delete: undefined
Delete result: false
Variable still exists: test

Comparison of Methods

Practical Example

let config = {
    apiUrl: "https://api.example.com",
    timeout: 5000,
    debug: true
};

// Unset a property
delete config.debug;
console.log("Config after delete:", config);

// Unset a variable
let tempData = "temporary";
tempData = undefined;
console.log("Temp data:", tempData);

Config after delete: { apiUrl: 'https://api.example.com', timeout: 5000 }
Temp data: undefined

Conclusion

Use variable = undefined to unset regular variables, and delete operator to remove object properties. The delete operator cannot remove variables declared with var, let, or const.