delete is used to remove a property from an object. Variables declared via var, let and const cannot be deleted (delete will return false). Setting strict mode on will raise a syntax error when attempting to delete a variable.

Invalid:

const a = 1;
let b = 2;
let c = 3;
delete a; // would return false
delete b; // would return false
delete c; // would return false

Valid:

let obj = {
  a: 1,
};
delete obj.a; // return true