Extract a Number from a String in JavaScript

Extract a Number from a String in JavaScript #

Use the String.replace() method to extract a number from a string, e.g. str.replace(/\D/g, ''). The replace() method will extract a number from the string by replacing all non-digit characters with empty strings.

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const str = 'hello 123 world';

const replaced = str.replace(/\D/g, ''); // 👉️ '123'
console.log(replaced);

let num;

if (replaced !== '') {
  num = Number(replaced); // 👉️ 123
}

console.log(num)

The replace method returns a new string containing all the numbers in the original string.

The first argument we passed to the String.replace method is a regular expression that we want to match in the string.

The \D character matches any character that is NOT a digit.

We added the g (global) flag to match all non-digit characters and replaced them with empty strings.

The if statement is there to check if the string contains any numbers.

If it doesn't, we don't want to convert an empty string to a number because that evaluates to 0.

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console.log(Number('')); // 👉️ 0

Here is a complete example of the case where the string doesn't contain numbers.

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const str = 'hello world';

const replaced = str.replace(/\D/g, ''); // 👉️ ''
console.log(replaced);

let num;

if (replaced !== '') {
  // 👇️ never runs
  num = Number(replaced);
}

console.log(num); // 👉️ undefined

We replace all non-digit characters with empty strings, and because the string contains only non-digit characters, String.replace() returns an empty string.

Another commonly used approach, which I wouldn't recommend, is to use the String.match method.

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const str = 'hello 123 world';

const replaced = str.match(/\d+/); // 👉️ ['123']

let num;

if (replaced !== null) {
  num = Number(replaced[0]); // 👉️ 123
}
console.log(num);

We passed a regular expression to the String.match method.

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

The + means match one or more of the preceding characters (the digits in the range 0-9).

If the String.match method doesn't match anything, it returns null.

The if statement is there to make sure we don't try to convert null to a number, which would return 0.

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console.log(Number(null)); // 👉️ 0

Which approach you pick is a matter of personal preference. I find the approach using String.replace more direct and intuitive.

Here is an example of the case where the string contains no digits when using String.match().

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const str = 'hello world';

const replaced = str.match(/\d+/); // 👉️ null

let num;

if (replaced !== null) {
  // 👇️ never runs
  num = replaced[0];
}
console.log(num); // 👉️ undefined

The match method returns null because the string contains no digits.

Further Reading #

• Check if Array of Strings contains a Substring in JS
• Replace a String case insensitive in JavaScript
• Check if String contains specific Character in JavaScript
• Split a String only on the First Occurrence of a Character
• Get the first N words from a String in JavaScript
• How to check if a String is Empty in JavaScript
• Get the First and Last Characters of a String in JavaScript
• Get the last Character of a String in JavaScript
• Get a Substring between 2 Characters in JavaScript
• How to Remove all Line Breaks from a String in JavaScript
• Remove all Whitespace from a String in JavaScript
• Get the Index of a Character in a String in JavaScript
• Get the Substring after a specific Character in JavaScript
• Get the Substring before a specific Character in JavaScript