Published: 2020-08-28

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JavaScript typeof Number

Often you will need to check that you have a number before using it in your JavaScript, here's how.

typeof 77; // number
77 instanceof Number; // false
Object.prototype.toString.call(77) === "[object Number]";

First you can use the typeof operator on any number either floating point or integer and if it’s a number and either true will be returned if the value is a number.

console.log(typeof 77); // number

OK, job done! There are a couple of other ways to check for numbers though one of which should be avoided.

The Number constructor in JavaScript is used for working with numbers and the Number() function can be used to convert values to numbers. However, calling 77 instanceof of Number; returns false as 77 is a literal value not an instance of the Number constructor.

console.log(77 instanceof Number); // false

Finally, you can use the call method on Object.prototype.toString which will always return [[object Number] if you pass a number into the call method.

Object.prototype.toString.call(77); // '[object Number]'

The above can be used for type checking any primitive and is considered one of the safest ways of type checking although for numbers typeof works just fine.

Object.prototype.toString.call(77); // '[object Number]'
Object.prototype.toString.call("hi"); // '[object String]'
Object.prototype.toString.call(["hi"]); // '[object Array]'
Object.prototype.toString.call({ greet: "hi" }); // '[object Object]'