How to get the number of occurrences of each letter in specified string in JavaScript ?

23 Jun, 2022
Given a string, our task is finding the occurrence of a character in the string with the help of user-defined function. 
Example:
Input : "hello"
Output : h occur 1 times
         e occur 1 times
         l occur 2 times
         o occur 1 times
Explanation : here "hello" have 1 h, so it have 1 value.
               as same e have 1, l have 2 , o have 1.
Example 2:
Input : "did"
Output: d occur 2 times 
        i occur 1 times
Approach 1: In this approach we use a map data structure to store the number of times characters occur.
First we initialize map with key each character of string and value for each is 0.
We iterate over string and increment value of the character.
Finally, print key-values of the map.
Example: 
Javascript
<script> //function to print occurrence of characterfunction printans( ans ){  for( let [ key ,value] of ans)  {    // if()    console.log(`${key}  occurs  ${value} times` );         } } // function count occurrence of characterfunction count( str , outp_map ){  for( let i = 0 ;i < str.length ;i++)  {     let k = outp_map.get(str[i]);    outp_map.set(str[i], k+1) ;                }  //calling  print function  printans(outp_map);} //function create map to count characterfunction count_occurs( test , callback ){  //checking string is valid or not  if( test.length === 0 )  {    console.log(" empty string ");    return ;  }  else  {    // map for storing count values    let ans = new Map();    for( let i = 0 ;i < test.length;i++)    {      ans.set(test[i], 0);    }         callback( test ,ans);       } } // test stringlet test =  "helloworld";count_occurs( test ,count); </script>
 Output:
h  occurs  1 times
e  occurs  1 times
l  occurs  3 times
o  occurs  2 times
w  occurs  1 times
r  occurs  1 times
d  occurs  1 times
Approach 2: In this approach, we use nested for loop to iterate over string and count for each character in the string. 
First initialize count with value 0 for ith value of string.
Now we iterate over string if ith value matches with the character, increase the count value by 1.
Finally, print the value of count.
Example: 
Javascript
<script> // function that count character occurrences in stringfunction count_occur( str ){  // checking string is valid or not  if( str.length == 0 )  {    console.log("Invalid string")    return;  }  //cor loop to iterate over string  for( let i = 0 ;i < str.length ;i++)  {    //variable counting occurrence    let count = 0;    for( let j = 0 ;j < str.length ;j++)    {      if( str[i] == str[j] && i > j  )      {       break;      }      if( str[i] == str[j]  )      {        count++;      }    }    if( count > 0)    console.log(`${str[i]} occurs ${count} times`);       } } // test stringlet test_str = "gfghello";count_occur( test_str);</script>
 Output:
g occurs 2 times
f occurs 1 times
h occurs 1 times
e occurs 1 times
l occurs 2 times
o occurs 1 times
Approach-3: In this approach we will use the for loop in order to iterate over the complete string itself. This is the simplest approach we may implement in order to find out the resulted task.
We will initialize a count variable variable which will store the count of each and every character in a string.
Then using a for loop we will check how many times a character in string has been occurred or repeated or present.
Javascript
// JavaScript code for the above approach... let countCharacters = (string) => {  let count = 1;  for (let i = 0; i < string.length; i++) {    if (string[i] === string[i + 1]) {      count++;    } else {      console.log(`${string[i]} occur ${count} times`);      count = 1;    }  }}; countCharacters("hello"); // This code is contributed by Aman Singla...
Output:
h occur 1 times
e occur 1 times
l occur 2 times
o occur 1 times
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