typeof is a JavaScript keyword that will return the type of a variable when you call it. You can use this to validate function parameters or check if variables are defined. There are other uses as well.

The typeof operator is useful because it is an easy way to check the type of a variable in your code. This is important because JavaScript is a is a dynamically typed language. This means that you aren’t required to assign types to variables when you create them. Because a variable is not restricted in this way, its type can change during the runtime of a program.

For example:

var x = 12345; // number
x = 'string'; // string
x = { key: 'value' }; // object

As you can see from the above example, a variable in JavaScript can change types throughout the execution of a program. This can be hard to keep track of as a programmer, and this is where the typeof operator is useful.

The typeof operator returns a string that represents the current type of a variable. You use it by typing typeof(variable) or typeof variable. Going back to the previous example, you can use it to check the type of the variable x at each stage:

var x = 12345; 
console.log(typeof x) // number
x = 'string'; 
console.log(typeof x) // string
x = { key: 'value' };
console.log(typeof x) // object

This can be useful for checking the type of a variable in a function and continuing as appropriate.

Here’s an example of a function that can take a variable that is a string or a number:

function doSomething(x) {
  if(typeof(x) === 'string') {
    alert('x is a string')
  } else if(typeof(x) === 'number') {
    alert('x is a number')
  }
}

Another way the typeof operator can be useful is by ensuring that a variable is defined before you try to access it in your code. This can help prevent errors in a program that may occur if you try to access a variable that is not defined.

function(x){
  if (typeof(x) === 'undefined') {
    console.log('variable x is not defined');
    return;
  }
  // continue with function here...
}

The output of the typeof operator might not always be what you expect when you check for a number.
Numbers can turn in to the value NaN (Not A Number) for multiple reasons.

console.log(typeof NaN); //"number"

Maybe you tried to multiply a number with an object because you forgot to access the number inside the object.

var x = 1;
var y = { number: 2 };
console.log(x * y); // NaN
console.log(typeof (x * y)); // number

When checking for a number, it is not sufficient to check the output of typeof for a number, since NaN also
passes this test.
This function check for numbers, and also doesn’t allow the NaN value to pass.

function isNumber(data) {
  return (typeof data === 'number' && !isNan(data));
}

Even thought this is a useful validation method, we have to be careful because javascript has some weird parts and one of them is the result of typeof over particular instructions. For example, in javascript many things are just objects so you’ll find.

var x = [1,2,3,4]; 
console.log(typeof x)  // object

console.log(typeof null)  // object

More Information:

MDN Documentation for typeof