JSFiddle - React, Tailwind, and code Playground

JavaScript

var arr = ['a', 'b', 'c', 'a'];

function count(books, pattern) {   
    var results = [];
    for (var index in arr) {
        if (arr[index] === pattern) {
          results.push(index);
        } 
    }
    return results;
}

console.log(count(arr, 'a'));