JSFiddle - React, Tailwind, and code Playground

HTML

<pre id="result"></pre>

JavaScript

let list = ['a', 'b', 'c', 'b', 'a', 'b', 'c', 'a', 'a', 'a'];

function counter(array) {
	return array.reduce((acc, value, index) => {
  		acc[value] = value in acc ? acc[value]  + 1: 1
    	return acc;
    }, {});
}

// output
let result = document.getElementById('result');
result.innerHTML = JSON.stringify(counter(list), null, 2);