The function will count the amount of existences of a character specific into the string

index.js

// You'll need to include the prototypes.js file before use it.
// To NodeJs: let proto = require('prototypes');
// To Html: <script src="path/prototypes.js"></script>

// Note: ChanceJs plugin could be omited since it's only used to verify the correct working for any case.

let Chance = require('chance'); // Plugin to generate content random 
let chance = new Chance();

let text = chance.paragraph(); // Generating paragraph random
let char = chance.character(); // Generating character random

let result = text.count(char);

console.log('char', char);
console.log('result', result);

prototypes.js

if (!Array.prototype.count) {
    String.prototype.count = function (toSearch) {
        if (toSearch == null) throw new TypeError("Array.prototype.count() - can't be null the search parameter");
        
        var str = this,
            r = 0,
            idx = 0;
        
        do {
            idx = str.indexOf(toSearch) + 1;
            if(idx != 0){
                r++;
                str = str.slice(idx, str.length);
            }
        }while(idx != 0);

        return r;
    };
}