test2.js

function foo(arg1, arg2) {
    return {
        input: arg1,
        delimiter: arg2,
        push: function (newString) {
            this.input = this.input ? this.input + this.delimiter + newString : newString;
        },
        pop: function () {
            const LAST_INDEX = this.input.lastIndexOf(this.delimiter);
            this.input = (LAST_INDEX !== -1) ? this.input.substring(0, LAST_INDEX) : '';
        },
        concat: function (newString) {
            return this.input ? this.input + this.delimiter + newString : newString;
        },
        indexesOf: function (n) {
            let indexesString = '';
            for (let i = 0; i < this.input.length; i++) {
                if (n === this.input[i]) {
                    indexesString += i + ', ';
                }
            }

            let lastIndexofComma = indexesString.lastIndexOf(',');
            return indexesString.substring(0, lastIndexofComma);
        }
    }
}