Get Index of First, Last or All occurrences in String in JS

# Table of Contents

1. Get the Index of a Character in a String in JavaScript
2. Get the Index of a Character's Last Occurrence in a String
3. Get the index of all occurrences of characters in a String

# Get the Index of a Character in a String in JavaScript

Use the String.indexOf() method to get the index of a character in a string.

The String.indexOf() method returns the index of the first occurrence of the character or -1 if the character is not found in the string.

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const str = 'hello world';

const index = str.indexOf('l');
console.log(index); // 👉️ 2

We used the String.indexOf() method to get the index of the first occurrence of the l character in the string hello world.

If the indexOf method doesn't find the specified character in the string, it returns -1.

# Get the Index of a Character's Last Occurrence in a String

Use the String.lastIndexOf() method to get the index of the last occurrence of a character in a string.

The String.lastIndexOf() method returns the index of the last occurrence of a character in a string or -1 if the character is not found.

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const str = 'hello world';

const lastIndex = str.lastIndexOf('l');
console.log(lastIndex); // 👉️  9

We used the String.lastIndexOf() method to get the index of the last occurrence of the l character in the string hello world.

The last l is the 10th character in the string, so it has an index of 9.

The lastIndexOf() method returns -1 if the character is not found in the string.

# Get the index of all occurrences of characters in a String

To get all indexes of a specific character in a string:

1. Declare a new variable and initialize it to an empty array.
2. Use a for loop to iterate over the string.
3. Check if each character is equal to the specific character.
4. Push the index of the matching characters into the new array.

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const str = 'hello world';

const indexes = [];

for (let index = 0; index < str.length; index++) {
  if (str[index] === 'l') {
    indexes.push(index);
  }
}

console.log(indexes); // 👉️ [2, 3, 9]

We declared a new variable and initialized it to an array.

The array will store the indexes of the character l in the string hello world.

We used a simple for loop to iterate for str.length iterations.

On each iteration, we check if each character is equal to l.

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const str = 'hello world';

const indexes = [];

for (let index = 0; index < str.length; index++) {
  if (str[index] === 'l') {
    indexes.push(index);
  }
}

console.log(indexes); // 👉️ [2, 3, 9]

If the condition is met, we push the index into the indexes array.

Alternatively, you can use the String.split() method.

# Get the index of all occurrences of characters in a String using split()

This is a three-step process:

1. Use the String.split() method to split the string into an array of characters.
2. Use the map() method to iterate over the array of characters.
3. Check if each character is the specified character and return the corresponding index.

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function findIndexes(string, char) {
  return string
    .split('')
    .map((c, idx) => {
      if (c === char) {
        return idx;
      }

      return -1;
    })
    .filter(element => element !== -1);
}

const str = 'hello world';
const indexes = findIndexes(str, 'l');

console.log(indexes); // 👉️ [ 2, 3, 9 ]

We used the String.split() method to split the string into an array of characters.

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const str = 'hello world';
// [
//   'h', 'e', 'l', 'l',
//   'o', ' ', 'w', 'o',
//   'r', 'l', 'd'
// ]
console.log(str.split(''));

The next step is to use the Array.map() method to iterate over the array of characters and check if each character is equal to the specified character.

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const str = 'hello world';
const char = 'l';

// [
//   -1, -1,  2,  3, -1,
//   -1, -1, -1, -1,  9,
//   -1
// ]
console.log(
  str.split('').map((c, idx) => {
    if (c === char) {
      return idx;
    }

    return -1;
  }),
);

If the current character is equal to the specified character, we return its index, otherwise, we return -1.

The last step is to use the Array.filter() method to filter out all -1 values from the array.

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function findIndexes(string, char) {
  return string
    .split('')
    .map((c, idx) => {
      if (c === char) {
        return idx;
      }

      return -1;
    })
    .filter(element => element !== -1);
}

const str = 'hello world';
const indexes = findIndexes(str, 'l');

console.log(indexes); // 👉️ [ 2, 3, 9 ]

The array contains the all indexes of the character in the string.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

• Get the Index of the Max/Min value in Array in JavaScript
• Get an Object's Key or Value by Index in JavaScript
• Insert String at specific Index of another String in JS
• Split a String at a specific Index using JavaScript
• Replace a Character at a specific Index in JavaScript