test.js

function foo() {
  return {
    input: this.input,
    delimiter: this.delimier,
    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);
    }
  }
}