Get the Index of the First Number in a String in JavaScript

Get the Index of the First Number in a String #

To get the index of the first number in a string, call the search() method passing it a regular expression that matches a digit. The search method returns the index of the first match of the regular expression in the string.

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const str = 'hello987';

const indexFirstNumber = str.search(/\d/);
console.log(indexFirstNumber); // 👉️ 5

const firstNumber = Number(str[indexFirstNumber])
console.log(firstNumber); // 👉️ 9

We used the String.search method to get the index of the first number in a string.

The only parameter the method takes is a regular expression.

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

The \d special character matches any digit in the range of 0 to 9. Specifying the \d character is the same as specifying a range of [0-9].

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const str = 'hello987';

// 👇️ using range 0-9 instead of \d
const indexFirstNumber = str.search(/[0-9]/);
console.log(indexFirstNumber); // 👉️ 5

const firstNumber = Number(str[indexFirstNumber])
console.log(firstNumber); // 👉️ 9

You can use whichever approach you find more readable.

If you ever need help reading a regular expression, check this regex cheatsheet from MDN out.

If the regular expression is not matched in the string, the search method returns -1.

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console.log('hello'.search(/[0-9]/)); // 👉️ -1

If you try to access a string at index -1, you would get undefined back.

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console.log('hello'[-1]); // 👉️ undefined

If you need to handle the scenario where the string does not contain any numbers, you can use an if statement.

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const str = 'hello987';

const indexFirstNumber = str.search(/[0-9]/);

if (indexFirstNumber !== -1) {
  console.log('✅ string contains at least 1 digit');
  const firstNumber = Number(str[indexFirstNumber]);

  // firstNumber is defined only here
} else {
  console.log('⛔️ string contains no digits');
}

In this example, we only declare the firstNumber variable if the search method matched at least 1 digit in the string.

Further Reading #

• Get the First Number in a String using JavaScript
• Count the Number of Digits in a String in JavaScript