JavaScript Undefined


Declared variable without a value will have the value undefined

var a;

console.log(a); // logs: undefined

Trying to retrieve the value of undeclared variables results in a ReferenceError. However, both the type of undeclared and unitialized variables is "undefined":

var a;
console.log(typeof a === "undefined"); // logs: true
console.log(typeof variableDoesNotExist === "undefined"); // logs: true