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Recipe · Numbers

How to Determine if a Value is a Number in JavaScript

Published 23 Jun 20233 methods

Quick answer

By isNaN() function
var num = "123";
var notNum = "Hello";
console.log(isNaN(num)); // Returns false
console.log(isNaN(notNum)); // Returns true

Other methods on this page: By parseFloat() and isNaN() combination, By regular expression testing

By isNaN() function

The isNaN() function in JavaScript checks if a value is not a number. It returns true if the value is NaN (not a number), and false if the value is a number. For example, isNaN(123) will return false, whereas isNaN(‘Hello’) will return true.


var num = "123";
var notNum = "Hello";
console.log(isNaN(num)); // Returns false
console.log(isNaN(notNum)); // Returns true

How it works

The isNaN() function works by attempting to convert the input into a number. If the conversion results in NaN (Not a Number), it returns true, indicating that the input is not a number. If the conversion is successful, it returns false, indicating that the input is a number.

  • First, two variables num and notNum are defined. The num variable is assigned the string “123”, and the notNum variable is assigned the string “Hello”.
  • Then, the isNaN() function is called with num as the argument. The function tries to convert “123” into a number, which is successful, so it returns false. This is logged to the console.
  • Next, the isNaN() function is called with notNum as the argument. The function tries to convert “Hello” into a number, which is not possible, so it returns true. This is logged to the console.
  • Therefore, by using the isNaN() function, we can determine whether a value is a number or not in JavaScript.

By parseFloat() and isNaN() combination

We can determine if a value is a number in JavaScript by using the parseFloat() and isNaN() functions. Firstly, the value is converted into a floating-point number with parseFloat(). Then, isNaN() is used to check if the converted value is a number or not. If isNaN() returns false, it indicates that the initial value was a number. Here is an example:


function isNumber(value) {
 return !isNaN(parseFloat(value));
}
console.log(isNumber("123")); // true
console.log(isNumber("abc")); // false

How it works

The provided value is passed to the parseFloat() function, which tries to convert this value into a floating-point number. If it can be converted, it returns the number; otherwise, it returns NaN (Not a Number). This returned value is then checked by the isNaN() function, which returns true if the argument is NaN and false if the argument is a number. In our function, we use the logical NOT operator (!) to reverse this boolean value, so that true is returned if the argument is a number and false if it’s not.

  • Step 1: The function receives a value as an argument.
  • Step 2: The parseFloat() function attempts to convert this value into a floating-point number.
  • Step 3: The isNaN() function checks if the result of the parseFloat() function is Not a Number (NaN).
  • Step 4: If the value is NaN, isNaN() will return true. However, the logical NOT operator (!) reverses this, hence it results in false. This means the initial value was not a number.
  • Step 5: If the value is a number, isNaN() will return false. The logical NOT operator (!) reverses this, hence it results in true. This means the initial value was a number.

By regular expression testing

We can check if a value is a number in JavaScript by using a regular expression that matches number patterns, and then test this against our value. For instance, if we want to check if “123” is a number, we’d use this approach.


let value = "123";
let isNumber = /^d+$/.test(value);

How it works

The regular expression /^d+$/ checks if a string only contains digits (0-9). The test method returns a boolean indicating whether or not the string passes the regular expression’s condition.

  • /^d+$/: This is the regular expression we’re testing our string against. The ^ symbol means the start of the string, d+ denotes one or more digits, and $ symbolizes the end of the string. So the entire expression checks if the string starts, continues and ends with one or more digits, meaning it’s entirely made up of numbers.
  • value: This is the string we’re checking to see if it’s purely numeric.
  • test: This is a method that tests for a match in a string. It returns true if it finds a match, else it returns false.
  • isNumber: This will be a boolean value. If the value is entirely numeric, isNumber will be true, else it will be false.