In this post, we will discuss about typeof operator in javascript. First we should know what is typeof.

What is typeof: 

typeof is an operator that is used to identify the type of any object in javascript. It returns a value that is string. So you can compare any string value from returned value of typeof.

// Examples
typeof 12;   // number";
typeof false or typeof true; // "boolean"
typeof undefined; // "undefined"
typeof {};     // "object"

// Call with parentheses
typeof(12);  // "number";

In the above code, you can see we have used typeof in two ways.

typeof is an operand and not a function and the second method it is not actually a function call. The operation in parenthesis is evaluated then passed to typeof.

When to use typeof:
typeof can be used if you want to check some variable that is undefined or not. If you are using any undefined variable into your code, you can get a ReferenceError. By using typeof into your code can prevent you from this error.

if( typeof foo !== "undefined") {
    // defined
}else {
    // undefined
}

This is how you can use typeof in your code. :)