JavaScript - How To Count String Occurrence in String?

Here are the various methods to count string occurrence in a string using JavaScript.

1. Using match() Method (Common Approach)

The match() method is a simple and effective way to count occurrences using a regular expression. It returns an array of all matches, and the length of the array gives the count.

const s1 = "hello world, hello JavaScript!";
const s2 = "hello";
const count = (s1.match(new RegExp(s2, "g")) || []).length;
console.log(count); 

2

• new RegExp(word, "g"): Creates a global regular expression for the target word.
• match(): Returns an array of matches.
• || []: Handles cases where no matches are found (returns an empty array).

2. Using while Loop

You can manually iterate through the string to count occurrences by checking for the substring at each position.

const s1 = "hello world, hello JavaScript!";
const s2 = "hello";
let count = 0;
let pos = 0;

while ((pos = s1.indexOf(s2, pos)) !== -1) {
    count++;
    // Move past the current match
    pos += s2.length; 
}
console.log(count);

2

• indexOf(s2, pos): Finds the position of the substring starting from pos.
• pos += s2.length: Advances the position to avoid counting overlapping matches.

3. Using split() Function

By splitting the string at each occurrence of the target substring, the resulting array.length-1 will give the count.

const s1 = "hello world, hello JavaScript!";
const s2 = "hello";
const count = s1.split(s2).length - 1;
console.log(count);

2

• split(word): Splits the string at every occurrence of the substring.
• length - 1: The number of occurrences equals the number of splits minus one.

4. Using indexOf() in a Loop

Another approach using indexOf() involves repeatedly searching for the substring.

const s1 = "hello world, hello JavaScript!";
const s2 = "hello";
let count = 0;
let pos = s1.indexOf(s2);

while (pos !== -1) {
    count++;
    pos = s1.indexOf(s2, pos + s2.length);
}
console.log(count); 

2

• indexOf(word, start): Finds the substring starting from the specified index.
• The loop continues until indexOf() returns -1 (no more matches).

5. Using Regular Expressions

Regular expressions offer a concise and powerful way to count substring occurrences.

const s1 = "hello world, hello JavaScript!";
const s2 = "hello";
const regex = new RegExp(s2, "g");
const count = (s1.match(regex) || []).length;
console.log(count);

2

• RegExp(word, "g"): Enables global matching of the substring.
• match(): Returns all matches as an array.

6. Using reduce() Method

You can use reduce() to iterate through an array representation of the string (e.g., split by spaces) and count matches.

const s1 = "hello world, hello JavaScript!";
const s2 = "hello";
const count = s1.split(" ")
    .reduce((acc, curr) => 
        (curr === s2 ? acc + 1 : acc), 0);
console.log(count);

2

• split(" "): Breaks the string into an array of words.
• reduce(): Accumulates the count by comparing each element with the target word.

7. Using includes() and slice() Methods

You can use includes() to check for the substring and slice() to move past each match.

const s1 = "hello world, hello JavaScript!";
const s2 = "hello";
let count = 0;
let temp = s1;

while (temp.includes(s2)) {
    count++;
    temp = temp.slice(temp.indexOf(s2) + s2.length);
}
console.log(count);

2

• includes(word): Checks if the substring exists in the string.
• slice(): Creates a new substring starting after the current match.

Which Method To Choose?

• The match() method with a global regular expression is the most efficient and widely used method for counting substring occurrences.
• For specific cases, such as overlapping matches or advanced patterns, regular expressions provide flexibility.
• Approaches like split() and loops with indexOf() are clear and concise alternatives, especially for simple substring counting. Choose the method that best fits your needs based on the input and complexity!