# How To Check If String Is A Valid Number In Javascript

There are already built-in functions to check if string is a valid number in javascript. We will learn how to do this using four different methods. Each method is rather straightforward, so you can choose the method you often use.

## Check if string is a valid number in javascript

### Method 1: Using isNaN()

Syntax:

isNaN(value)

Parameter:

• value: The value to be tested.

The isNaN() function determines whether a value is NaN or not (NaN means “Not-a-Number”). The isNaN() function returns a TRUE boolean value if a value is not a number. Hence, if the string is a number the method will return FALSE. For example:

let s = "123456";
console.log(isNaN(s));
let c = "abcdef";
console.log(isNaN(c));

Output:

false
true

The logic behind this method is very simple. If the string we are testing is a valid number, it will return false. Otherwise, it will return true, meaning the string is not a valid number.

### Method 2: Using ‘+’ operator

Syntax:

+x

Parameter:

• x: an operand, can be of any type such as string or number

If the string is a valid number, the unary plus operator (+ operator) returns the numeric value of the string represented. Otherwise, it will return null if the string isn’t a valid number:

let s = "123456";
console.log(+s);
let c = "abcdef";
console.log(+c);

The + operator attempts to convert the operand to a number if it is not already. If it cannot parse a particular value, it will evaluate to null.

Output:

123456
null

The + operator method can help you check if the string is a valid number in just a line of code. Moreover, this method is well-known for its effective complexity, which means it doesn’t require the usage of any built-in function. Therefore, we highly recommend you use the unary + operator to check if string is a valid number in javascript.

### Method 3: Using Number()

The Number() built-in function helps you convert any type into number type.

Syntax:

Number(value)

Parameter:

• value: The numeric value of the object being created.

If the type of parameter you are passing is a valid number, it will return a value representing the numeric value of your parameter. Otherwise, it will return null meaning the parameter does not represent a valid number:

let s = "123456";
console.log(Number(s));
let c = "abcdef";
console.log(Number(c));

Output:

123456
null

### Method 4: Using parseInt()

Syntax:

parseInt(string)

Parameter:

• string: A string starting with an integer. Leading whitespace in this argument is ignored.

Like the Number() function and + operator approach, the parseInt() function will return an integer parsed from the given string. Or returns null when the string cannot be converted into a valid number.

let s = "123456";
console.log(parseInt(s));
let c = "abcdef";
console.log(parseInt(c));

Output:

123456
null

## Summary

Using four different methods, we have learned how to check if string is a valid number in javascript. Although we have many ways to deal with this type of problem, we advise you to use the method of using unary + operator because it is the most efficient method.

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