Javascript if char is number

Javascript if char is number

Last updated: Jan 2, 2023
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# Table of Contents

# Check if a Character in String is a Number with Regex

Use the RegExp.test() method to check if a character in a string is a number.

The test() method will return true if the character is a valid number and false otherwise.

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function isNumber(char)  return /^\d$/.test(char); > const str = 'a1'; console.log(isNumber(str[0])); // 👉️ false console.log(isNumber(str[1])); // 👉️ true console.log(isNumber('100')); // 👉️ false console.log(isNumber('')); // 👉️ false console.log(isNumber(undefined)); // false 

We used the RegExp.test method to test a regular expression against the character.

The forward slashes / / mark the start and end of the regular expression.

The caret ^ matches the beginning of the input and the dollar sign $ matches the end of the input.

The \d character matches any digit from 0 to 9.

If the character we pass to the function is a number, the isNumber() function returns true , otherwise, false is returned.

If you also want to return true for multiple characters if they are numbers, e.g. 100 , update the regular expression.

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function isNumber(char)  return /^\d+$/.test(char); > const str = 'a1'; console.log(isNumber(str[0])); // 👉️ false console.log(isNumber(str[1])); // 👉️ true console.log(isNumber('100')); // 👉️ true console.log(isNumber('')); // 👉️ false console.log(isNumber(undefined)); // false 

We added the plus + special character to the regular expression.

The plus + matches the preceding item (a digit) 1 or more times.

In its entirety, the regular expression matches a string that starts with and ends with a digit and contains one or more digits.

Alternatively, you can use the isNan() function.

# Check if a Character in String is a Number using isNaN()

You can also use the isNan() function to check if a character is a number.

The function checks if the provided value is NaN (not a number). If the function returns false , then the character is a valid number.

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function isNumber(char)  if (typeof char !== 'string')  return false; > if (char.trim() === '')  return false; > return !isNaN(char); > const str = 'a1'; console.log(isNumber(str[0])); // 👉️ false console.log(isNumber(str[1])); // 👉️ true console.log(isNumber('123')); // 👉️ true console.log(isNumber('')); // 👉️ false console.log(isNumber(undefined)); // false 

The isNumber() function returns true if the supplied string is a number and false otherwise.

We first check if the value passed to the function is not of type string , in which case we return false straight away.

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if (typeof char !== 'string')  return false; > 

Then we check if the provided value is an empty string or contains only whitespace, in which case we also return false .

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if (char.trim() === '')  return false; > 

Finally, we use the isNaN function to check if the provided character is not a number.

The logical NOT (!) operator is used to negate the value returned from the isNaN (is not a number) function.

The isNaN function tries to convert the string to a number and returns true if it fails.

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console.log(isNaN('a')); // 👉️ true console.log(isNaN('')); // 👉️ false console.log(isNaN(' ')); // 👉️ false 

This seems quite confusing at first, but the isNaN function converts an empty string or a string containing spaces to a number (0), so it returns false .

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console.log(Number('')); // 👉️ 0 console.log(Number(' ')); // 👉️ 0 

This is why we used the trim() method — to trim the string and verify that it’s not an empty string.

We know that if the isNaN function gets called with a string that contains at least 1 character and returns true , then the string is NOT a valid number.

Conversely, if the isNaN function gets called with a string that contains at least 1 character and returns false , then the string is a valid number.

Here are some more examples of calling isNaN() with strings.

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console.log(isNaN('123')); // 👉️ false console.log(isNaN('1.23')); // 👉️ false console.log(isNaN('1,23')); // 👉️ true console.log(isNaN('123test')); // 👉️ true 

Which approach you pick is a matter of personal preference. I’d use the RegExp.test() method because I find it more direct and intuitive.

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How To Check If A Character Is A Number Using JavaScript

We can use comparison operators to check if a character is a number using JavaScript. Follow this article to learn how to do it with the explanation and examples below. Let’s go into detail now.

Check If A Character Is A Number Using JavaScript

These are some methods to check if a character is a number using JavaScript.

Use comparison operators

You can check if a character is a number by using comparison operators. You have to compare the character with the character ‘0’ and ‘9’, not the number 0 or 9. The character satisfies the condition is not less than ‘0’ and is not greater than ‘9’.

Look at the example below to learn more about this method.

function isNumber(char) < // if the size of the char is >1, it is a String, not a Character. if (char.length > 1) return false if (char >= '0' && char return false > let char1 = 'a' let char2 = '1' let char3 = '11' let char4 = '1a' console.log(isNumber(char1)) console.log(isNumber(char2)) console.log(isNumber(char3)) console.log(isNumber(char4))

Use isNaN() method

The isNaN() method returns true if the specified value is a Number. Otherwise, it returns false.

Return Value: true or false.

Look at the example below:

function isNumber(char) < // if the size of the char is >1, it is a String, not a Character. if (char.length > 1) return false if (!isNaN(char)) < return true >return false > let char1 = 'a' let char2 = '1' let char3 = '11' let char4 = '1a' console.log(isNumber(char1)) console.log(isNumber(char2)) console.log(isNumber(char3)) console.log(isNumber(char4))

Use the parseInt() method

The parseInt() method returns an integer if the specified value is an integer. Otherwise, it returns NaN. So you can use isNaN() method with the parseInt() method inside to check if a character is a number or not.

Take a look in the example below:

function isNumber(char) < // if the size of the char is >1, it is a String, not a Character. if (char.length > 1) return false if (!isNaN(parseInt(char))) < return true >return false > let char1 = 'a' let char2 = '1' let char3 = '11' let char4 = '1a' console.log(isNumber(char1)) console.log(isNumber(char2)) console.log(isNumber(char3)) console.log(isNumber(char4))

Like the parseInt() method, you can use the number object or the ‘+’ operator and the isNaN() method to check if a character is a number or not.

You can learn the syntax of using the number object or the ‘+’ operator here.

Summary

To check if a character is a number using JavaScript, you can use the comparison operators, isNaN() method or parseInt() method. Choose the method that is suitable for you. We hope this guide is helpful to you. Thanks!

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How to Check if a Character is a Number in JavaScript: Methods and Examples

Learn how to check if a character or string is a number in JavaScript. Explore methods like regular expressions, isNaN(), typeof, and Number.isNaN() with examples and best practices.

JavaScript is a popular programming language used in web development, and one common task is to check if a character or string is a number. There are multiple methods to achieve this, including regular expressions, the isNaN() function, the typeof operator, and the Number.isNaN() method. In this blog post, we will explore these methods and provide examples and best practices for checking if a character or string is a number in JavaScript.

Regular expressions for checking if a character or string is a number

A regular expression can be used to check if a character is a number, such as /^\d$/ . The ^ and $ characters are used to denote the beginning and end of the string respectively, and the \d character class matches any digit. To check if a string contains only numbers, a regular expression can be used such as /^\d+$/ . The + quantifier matches one or more of the preceding element, so this expression will match any string that contains only digits.

The test() method can be used to check if a string contains numbers or characters. The includes() method can be used to check if a string contains a particular character. The pattern variable can be used to check if a character is a number or letter. The test() method can also be used to check if a string contains only digits.

const pattern = /^\d+$/; console.log(pattern.test("123")); // true console.log(pattern.test("abc")); // false 

Using isNaN() and typeof to check if a value is a number

The isNaN() function checks whether a value is an illegal number (Not-a-Number) and returns true if the value equates to NaN. The typeof operator can be used to check if a value is a number. Both isNaN() and typeof have their own limitations and may return unexpected results.

console.log(isNaN("abc")); // true console.log(isNaN("123")); // false console.log(typeof 123); // "number" console.log(typeof "123"); // "string" 

Using Number.isNaN() to determine if a value is Not-A-Number

The Number.isNaN() method determines if the passed value is Not-A-Number. This method is more reliable than the isNaN() function.

console.log(Number.isNaN("abc")); // false console.log(Number.isNaN("123")); // false console.log(Number.isNaN(NaN)); // true 

Other methods for checking if a character is a number

The isDigit() method of the Character class can be used to check if a given character is a digit. The charCodeAt() method can be used to get the character code of a character in a string. The isNumericChar() function and arr[char] are used to check if a character is numeric.

console.log(isDigit(5)); // true console.log("abc".charCodeAt(0)); // 97 console.log(isNumericChar("5")); // true 

Other quick examples for checking if a character is a number in JavaScript

In Javascript , for instance, javascript check if string is number code example

// native returns true if the variable does NOT contain a valid number isNaN(num)// can be wrapped for making simple and readable function isNumeric(num)< return !isNaN(num) >isNumeric(123) // true isNumeric('123') // true isNumeric('1e10000') // true (This translates to Infinity, which is a number) isNumeric('foo') // false isNumeric('10px') // false

In Javascript , for instance, check if char is number js code example

In Javascript , js check if string is number code sample

isNaN(num) // returns true if the variable does NOT contain a valid numberisNaN(123) // false isNaN('123') // false isNaN('1e10000') // false (This translates to Infinity, which is a number) isNaN('foo') // true isNaN('10px') // true

In Javascript , for instance, check if char is number js code sample

var c = justPrices[i].substr(commapos+2,1); if (c >= '0' && c else < // it isn't >// or function isCharNumber(c) < return c >= '0' && c

Conclusion

Checking if a character or string is a number in JavaScript can be achieved through various methods. Regular expressions, isNaN() , typeof , Number.isNaN() , and various functions and methods can be used. It is important to choose the most appropriate method for the specific use case and test for edge cases. By understanding and utilizing these methods, programmers can efficiently and reliably check if a character or string is a number in JavaScript.

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