String Stack

/*  Create a Stack constructor that
      - creates an object that stores strings within a single string
      - that has a way of delimiting strings
      - has a push, pop, concat method that works exactly like their array counterparts
      - has an indexesOf method that behaves just like Array.prototype.indexOf but will also find all indexes if more than one exist
      
      - does your constructor initialise with a single string or an array of strings?
      - does it have to be called as a constructor / class - could it just be an object / function?
      
*/

function Stack(values) {
    if(!(this instanceof Stack)) {
        return new Stack(values);
    }
    this.storage = '';
    this.concat.apply(this, values);
}

Stack.prototype.push = function(item) {
    const delimiter = '***';
    const index = this.storage.length;
    this.storage = this.storage.concat(`${delimiter}${item}`);    
    return index;
};

Stack.prototype.concat = function(...items) {
    const delimiter = '***';
    items.forEach(item => this.storage = this.storage.concat(`${delimiter}${item}`));    
    return this;
};

Stack.prototype.pop = function(item) {
    const delimiter = '***';
    const lastDelimterPosition = this.storage.lastIndexOf(delimiter);
    const lastItem = this.storage.substr(lastDelimterPosition, this.storage.length);
    this.storage = this.storage.substr(0, lastDelimterPosition);
    return lastItem;
};

Stack.prototype.indexesOf = function(value) {
    let storage = this.storage;
    let position = 0;
    const indexes = [];
    while(storage.length) {
        let index = storage.indexOf(value);
        if(index > -1) {
            position += index;
            indexes.push(position);
            storage = storage.substr(index + value.length, storage.length);
            position = this.storage.length - storage.length;
        }
        else {
            storage = '';
        }
    }
    return indexes.length > 1 ? indexes : indexes.length ? indexes[0] : -1;
}

var stack = Stack(['another!', 'newitem!', 'another!']);