Determine whether a variable is a number in JavaScript

TodayJavaScriptHow to determine whether a variable is a number.

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Introduction

JavaScript is a dynamically typed language, which means that the interpreter determines the type of a variable at runtime. In fact, this also allows us to use the same variable to store different types of data in the same code. Without documentation and consistency, we don't always know the type of a variable when working with code.

When we expect a variable to be a number, operating on strings or arrays can lead to strange results in the code. In this article, we will introduce some functions that determine whether a variable is a number.

Strings of numbers like "10" should not be accepted. In JavaScript, special values ​​such as NaN, Infinity, and -Infinity are also of numeric type.

Based on these requirements, the best function to use is the isFinite() function in the built-in Number object. However, developers often use other functions such as the Number.isNaN() and typeof() functions.

We first create some variables:

let intVar = 2;
let floatVar = 10.5;
let stringVar = '4';
let nanVar = NaN;
let infinityVar = Infinity;
let nullVar = null;
let undefinedVar = undefined;复制代码

Use Number.isFinite() function name

Number.isFinite()The function checks whether the variable is a number , also checks whether it is a limited value. So for numbers of NaN, Infinity, or -Infinity, it returns false.

Let’s check it using the variables defined above:

> Number.isFinite(intVar);
true
> Number.isFinite(floatVar);
true
> Number.isFinite(stringVar);
false
> Number.isFinite(nanVar);
false
> Number.isFinite(infinityVar);
false
> Number.isFinite(nullVar);
false
> Number.isFinite(undefined);
false复制代码

This is exactly what we want. Special non-finite numbers and any variable of non-numeric type are ignored. So, if you want to check if a variable is a number, the best way is to use the Number.isFinite() function.

Using the Number.isNaN() method

The standard Number object has a isNaN() method. It accepts a parameter and determines whether its value is NaN. Since we want to check if a variable is a number, we will use the not operator ! in the check.

> !Number.isNaN(intVar);
true
> !Number.isNaN(floatVar);
true
> !Number.isNaN(stringVar);
true # Wrong
> !Number.isNaN(nanVar);
false
> !Number.isNaN(infinityVar);
true # Wrong
> !Number.isNaN(nullVar);
true # Wrong
> !Number.isNaN(undefinedVar);
true # Wrong复制代码

This method is quite permissive as it accepts values ​​that are not numbers at all. This approach works best when you know you have a number and want to check if it is a NaN value, rather than a general number check.

Use the typeof() method

typeof()The function is a global function that accepts a variable or value as a parameter and returns a string representation of its type. JavaScript has a total of 9 types

• undefined
• boolean
• number
• string
• bigint
• symbol
• object
• null (typeof() displays object)
• function (a special type of object )

To verify whether the variable is a number, we only need to check whether the value returned by typeof() is `"number". Let's try it with a test variable:

> typeof(intVar) == 'number';
true
> typeof(floatVar) == 'number';
true
> typeof(stringVar) == 'number';
false
> typeof(nanVar) == 'number';
true # Wrong
> typeof(infinityVar) == 'number';
true # Wrong
> typeof(nullVar) == 'number';
false
> typeof(undefined) == 'number';
false复制代码

typeof() The performance of the function is much better than Number.isNaN(). It correctly determines that the string variables null and undefined are not numbers. However, for NaN and Infinity, it returns true.

Although this is technically the correct result, NaN and Infinity are special numeric values ​​and for most use cases we prefer to ignore them.

Summary

In this article, we learned how to check if a variable is a number in JavaScript. The Number.isNaN() function only applies when we know the variable is a number and need to verify whether it is NaN`.

If there are NaN, Infinity or -Infinity and other numbers in the code, the typeof()` function is applicable.

Number.isFinite()The method captures all finite numbers and is the most suitable for our requirements.

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