JSFiddle - React, Tailwind, and code Playground

JavaScript

var items = ['1', '2', 'callback', '4'];

var testCallback = function (item, items) {
    var outputArray = [];
	    if (item == 'callback') {
            for (i = 0; i < items.length; i++) {
		    outputArray.push(testCallback('5', items));
            console.log(outputArray);
            };
		} else {
			outputArray.push(item);
		};

	return outputArray.join(',');
};

var testFunction = function (items) {
	var output = '';
	for (i = 0; i < items.length; i++) {
        console.log(i);
          output += testCallback(items[i], items);           
    };

    console.log(output)   
};

testFunction(items);