Emulate Array Objects

Question

Question from Object-Oriented JavaScript book: Imagine Array() doesn't exist and the array literal notation doesn't exist either. Create a constructor called MyArray() that behaves as close to Array() as possible.

I thought it would be a good challenge to test my skills. This is what I came up with, but it doesn't work and is very incomplete.. I am a bit stumped:

function MyArray(){

// PRIVATE FIELDS -----------------------

var initialData = arguments;
var storage;

// PRIVATE METHODS ----------------------

function refresh(){ //this doesn't work :(
    for(var i = 0; i < storage.length; i++){
        this[i] = storage[i]
    }
};

function initialize(){
    storage = initialData;
    refresh();
}

function count(){
    var result = 0;
    for(var item in this){
        //console.log(item, parseInt(item), typeof item);
        if(typeof item == 'number'){
            result++;
        }
    }
    return result;
};

initialize();

// PUBLIC FIELDS -------------------------

this.length = count();

// PUBLIC METHODS ------------------------

//todo:
this.push = function(item){
    refresh();
}

this.pop = function(){}

this.join = function(){}

this.toString = function(){}

}

var c = new MyArray(32,132,11);

console.log(c, c.length);

This isn't for any production code or any project.. just to try to learn JavaScript a lot more. Can anyone try to help me with this code?

The key thing about Array objects is that the "length" property is magic. Once Harmony object proxies become real, things like this won't be that hard at all. — Pointy
– Pointy, Commented Jul 2, 2012 at 21:04
So you're using arrays to replicate arrays? Even as an exercise it doesn't make much sense to me. If you want to learn inheritance and advanced topics I suggest you start with Essential JavaScript Design Patterns — elclanrs
– elclanrs, Commented Jul 2, 2012 at 21:06
@elclanrs: this was an exercise from the Object-Oriented JavaScript book.. — JC JC
– JC JC, Commented Jul 2, 2012 at 21:08
I like the idea. Have done something similar in C many years ago, but I don't know how I'd approach it now in JS (at least not in a meaningful and efficient way) with the lack of pointers. Interesting question, thanks :) — Bob Davies
– Bob Davies, Commented Jul 2, 2012 at 21:20

Alex · Accepted Answer · 2012-07-03 10:00:03Z

2

The thing is that you can use arguments object. It's not an array that was created with Array() so you won't break rules of the exercise. Here's what you need to do:

this.length = 0;
for (var i in arguments) {
   this[this.length] = arguments[i];
   this.length++;
}

I forgot to mention that ANY object is an associative array, so it's not a wrong thing to apply associative arrays for the exercise as we don't use the Array() object itself.

To author of the question: in your example you use: this["i"] = storage[i] that equals to this.i = storage[i]. Try to remove quotes and use it like this[i] = storage[i]

edited Jul 3, 2012 at 10:00

answered Jul 3, 2012 at 9:32

Alex

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2 Comments

Florent Over a year ago

Beware with for in loops, they may iterate over built in properties such as toString.

Alex Over a year ago

That's TRUE for general cases where you use objects, but in this case I use arguments-object: when iterating through it we get only function parameters(arguments)

Bergi · Accepted Answer · 2012-07-02 21:12:26Z

0

for(var item in this){
    if(typeof item == 'number')

A property name is always a string. You'll need to check if it is the string representation of a number from 0 to MaxArrayLength. You could for example do

for (var i=0; i<4294967296; i++)
    if (i in this)
         result = i;

You might also be interested in these articles or the official specification for Array behaviour.

edited Jul 2, 2012 at 21:12

answered Jul 2, 2012 at 21:04

Bergi

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2 Comments

JC JC Over a year ago

i'm trying to just filter through this[1], this[2], this[3] which would be like "indexes" but in property format.. created through my refresh() function, how would I be able to just filter through the number-named properties?

Bergi Over a year ago

I've exended my answer, was this what you wanted?

pwilcox83 · Accepted Answer · 2015-09-19 18:10:36Z

I'm currently looking through this book at the moment, I probably should try something more recent but its not that out of date yet and the principals are still sound...well they are in my opinion.

Anyway I went for a slightly different solution although I did take inspiration from the op in how he is handling strings. I think the challenge of this exercise is not to create any more arrays otherwise...its a bit of a crazy challenge, especially for people new to the language.

var MyArray = function () {
    var args = arguments;
    var length = 0;
    for each(var item in args) {
        this[length++] = item;
     }
    this.toString = function () {
    var result = args[0];
    for (var i = 1; i < args.length; i++) {
        result += ',' + args[i];
    }
    return result;
    }
  this.length = length;
  this.push = function (push) {
      var newLength = args.length++;
      args[newLength] = push;
      this[newLength] = push;
      return ++length;
  }
  this.pop = function () {
      delete args[--args.length];
      delete this[args.length];
      length--;
      return args;
  }
  this.join = function(joiner){
      if(typeof arguments[0] === "undefined"){
          joiner = ',';
      }
      var result = args[0];
      for (var i = 1; i < args.length; i++) {
          result += joiner + args[i];
      }
      return result;
  }
}
var a = new MyArray(1, 2, 3, 'test');
console.log(a.toString());
console.log(a[a.length - 1]);
console.log(a.push('boo'));
console.log(a.toString());
console.log(a.pop());
console.log(a.toString());
console.log(a.join(','));
a.join(' isn\'t ');

Deepika Chouhan · Accepted Answer · 2019-12-19 06:44:03Z

0

My solution is:

 function MyArray() {
    this.length = 0;

    for(i = 0; i < arguments.length; i++) {
        this[this.length] = arguments[i];
        this.length++;
    }
    this.toString = function(joiner = ',') {
        let str = this[0] ? this[0] : '';
        for(i=1;i<this.length; i++) {
            str+= joiner + this[i];
        }
        return str;
    };
    this.push = function(value) {
        this[this.length++] = value;
        return this.length;
    };
    this.pop = function() {
        let value = this[this.length -1];
        delete this[--this.length]
        return value;
    };
    this.join = function(joiner) {
        return this.toString(joiner);
    }
}

answered Dec 19, 2019 at 6:44

Deepika Chouhan

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