JavaScript - Frequency of characters in Order of Appearance

The operation involves counting how many times each character in alphabetic characters in a given string.

Using Object (Simple and Efficient for Small to Moderate Strings)

Object in JavaScript allow storing key value pairs. So we use items as keys and their frequencies as values.

function freqCount(s) {
  const freq = {};
  for (let x of s) {
    freq[x] = (freq[x] || 0) + 1;
  }
  return freq;
}

// Example usage
console.log(freqCount("geeksforgeeks"));

{ g: 2, e: 4, k: 2, s: 2, f: 1, o: 1, r: 1 }

Using Map (Simple and Efficient for Large Strings)

JS Map also store key value pairs and works for data types other than string as well.

function freqCount(s) {
  const m = new Map();
  for (let x of s) {
    m.set(x, (m.get(x) || 0) + 1);
  }
  return m;
}

// Example usage
console.log(freqCount("geeksforgeeks"));

Map(7) {
  'g' => 2,
  'e' => 4,
  'k' => 2,
  's' => 2,
  'f' => 1,
  'o' => 1,
  'r' => 1
}

Using Regular Expression (Works for Mixed Input)

If our input string contains mixed characters and we wish to find frequencies of only alphabetic characters, we can use regular expressions.

function freqCount(s) {
  return (s.match(/[a-z]/gi) || []).reduce((freq, x) => {
    freq[x] = (freq[x] || 0) + 1;
    return freq;
  }, {});
}

// Example usage
console.log(freqCount("geeksforgeeks"));

{ g: 2, e: 4, k: 2, s: 2, f: 1, o: 1, r: 1 }

Please refer Frequency of characters in Order of Appearance for different algorithms and their time complexities.