Count the number of occurrences of a character in a string in Javascript

Question

I need to count the number of occurrences of a character in a string.

For example, suppose my string contains:

var mainStr = \"str1,str2,str3,str4\";         


        

Answer 1

s = 'dir/dir/dir/dir/'
for(i=l=0;i<s.length;i++)
if(s[i] == '/')
l++

Answer 2

I was working on a small project that required a sub-string counter. Searching for the wrong phrases provided me with no results, however after writing my own implementation I have stumbled upon this question. Anyway, here is my way, it is probably slower than most here but might be helpful to someone:

function count_letters() {
var counter = 0;

for (var i = 0; i < input.length; i++) {
    var index_of_sub = input.indexOf(input_letter, i);

    if (index_of_sub > -1) {
        counter++;
        i = index_of_sub;
    }
}

http://jsfiddle.net/5ZzHt/1/

Please let me know if you find this implementation to fail or do not follow some standards! :)

UPDATE You may want to substitute:

    for (var i = 0; i < input.length; i++) {

With:

for (var i = 0, input_length = input.length; i < input_length; i++) {

Interesting read discussing the above: http://www.erichynds.com/blog/javascript-length-property-is-a-stored-value

Answer 3

I have found that the best approach to search for a character in a very large string (that is 1 000 000 characters long, for example) is to use the replace() method.

window.count_replace = function (str, schar) {
    return str.length - str.replace(RegExp(schar), '').length;
};

You can see yet another JSPerf suite to test this method along with other methods of finding a character in a string.

Answer 4

The fifth method in Leo Sauers answer fails, if the character is on the beginning of the string. e.g.

var needle ='A',
  haystack = 'AbcAbcAbc';

haystack.split('').map( function(e,i){ if(e === needle) return i;} )
  .filter(Boolean).length;

will give 2 instead of 3, because the filter funtion Boolean gives false for 0.

Other possible filter function:

haystack.split('').map(function (e, i) {
  if (e === needle) return i;
}).filter(function (item) {
  return !isNaN(item);
}).length;

Answer 5

var a = "acvbasbb";
var b= {};
for (let i=0;i<a.length;i++){
    if((a.match(new RegExp(a[i], "g"))).length > 1){
        b[a[i]]=(a.match(new RegExp(a[i], "g"))).length;
    }
}
console.log(b);

In javascript you can use above code to get occurrence of a character in a string.

Answer 6

If you are using lodash, the _.countBy method will do this:

_.countBy("abcda")['a'] //2

This method also work with array:

_.countBy(['ab', 'cd', 'ab'])['ab'] //2