Implement Array-like behavior in JavaScript without using Array

Question

Is there any way to create an array-like object in JavaScript, without using the built-in array? I\'m specifically concerned with behavior like this:

var sup         


        

Answer 1

Yes, you can subclass an array into an arraylike object easily in JavaScript:

var ArrayLike = function() {};
ArrayLike.prototype = [];
ArrayLike.prototype.shuffle = // ... and so on ...

You can then instantiate new array like objects:

var cards = new Arraylike;
cards.push('ace of spades', 'two of spades', 'three of spades', ... 
cards.shuffle();

Unfortunately, this does not work in MSIE. It doesn't keep track of the length property. Which rather deflates the whole thing.

The problem in more detail on Dean Edwards' How To Subclass The JavaScript Array Object. It later turned out that his workaround wasn't safe as some popup blockers will prevent it.

Update: It's worth mentioning Juriy "kangax" Zaytsev's absolutely epic post on the subject. It pretty much covers every aspect of this problem.

Answer 2

You could also create your own length method like:

Array.prototype.mylength = function() {
    var result = 0;
    for (var i = 0; i < this.length; i++) {
        if (this[i] !== undefined) {
            result++;
        }
    }
    return result;
}

Answer 3

Interface and implementation

The case is a simple implementation of the original array packaging, you can replace the data structure and refer to the common interface can be implemented.

export type IComparer<T> = (a: T, b: T) => number;

export interface IListBase<T> {
  readonly Count: number;
  [index: number]: T;
  [Symbol.iterator](): IterableIterator<T>;
  Add(item: T): void;
  Insert(index: number, item: T): void;
  Remove(item: T): boolean;
  RemoveAt(index: number): void;
  Clear(): void;
  IndexOf(item: T): number;
  Sort(): void;
  Sort(compareFn: IComparer<T>): void;
  Reverse(): void;
}


export class ListBase<T> implements IListBase<T> {
  protected list: T[] = new Array();
  [index: number]: T;
  get Count(): number {
    return this.list.length;
  }
  [Symbol.iterator](): IterableIterator<T> {
    let index = 0;
    const next = (): IteratorResult<T> => {
      if (index < this.Count) {
        return {
          value: this[index++],
          done: false,
        };
      } else {
        return {
          value: undefined,
          done: true,
        };
      }
    };

    const iterator: IterableIterator<T> = {
      next,
      [Symbol.iterator]() {
        return iterator;
      },
    };

    return iterator;
  }
  constructor() {
    return new Proxy(this, {
      get: (target, propKey, receiver) => {
        if (typeof propKey === "string" && this.isSafeArrayIndex(propKey)) {
          return Reflect.get(this.list, propKey);
        }
        return Reflect.get(target, propKey, receiver);
      },
      set: (target, propKey, value, receiver) => {
        if (typeof propKey === "string" && this.isSafeArrayIndex(propKey)) {
          return Reflect.set(this.list, propKey, value);
        }
        return Reflect.set(target, propKey, value, receiver);
      },
    });
  }
  Reverse(): void {
    throw new Error("Method not implemented.");
  }
  Insert(index: number, item: T): void {
    this.list.splice(index, 0, item);
  }
  Add(item: T): void {
    this.list.push(item);
  }
  Remove(item: T): boolean {
    const index = this.IndexOf(item);
    if (index >= 0) {
      this.RemoveAt(index);
      return true;
    }
    return false;
  }
  RemoveAt(index: number): void {
    if (index >= this.Count) {
      throw new RangeError();
    }
    this.list.splice(index, 1);
  }
  Clear(): void {
    this.list = [];
  }
  IndexOf(item: T): number {
    return this.list.indexOf(item);
  }
  Sort(): void;
  Sort(compareFn: IComparer<T>): void;
  Sort(compareFn?: IComparer<T>) {
    if (typeof compareFn !== "undefined") {
      this.list.sort(compareFn);
    }
  }
  private isSafeArrayIndex(propKey: string): boolean {
    const uint = Number.parseInt(propKey, 10);
    const s = uint + "";
    return propKey === s && uint !== 0xffffffff && uint < this.Count;
  }
}

Case

const list = new List<string>(["b", "c", "d"]);
const item = list[0];

Reference

• proxy
• [Symbol.iterator]()

Answer 4

Is this what you're looking for?

Thing = function() {};
Thing.prototype.__defineGetter__('length', function() {
    var count = 0;
    for(property in this) count++;
    return count - 1; // don't count 'length' itself!
});

instance = new Thing;
console.log(instance.length); // => 0
instance[0] = {};
console.log(instance.length); // => 1
instance[1] = {};
instance[2] = {};
console.log(instance.length); // => 3
instance[5] = {};
instance.property = {};
instance.property.property = {}; // this shouldn't count
console.log(instance.length); // => 5

The only drawback is that 'length' will get iterated over in for..in loops as if it were a property. Too bad there isn't a way to set property attributes (this is one thing I really wish I could do).

Answer 5

Now we have ECMAScript 2015 (ECMA-262 6th Edition; ES6), we have proxy objects, and they allow us to implement the Array behaviour in the language itself, something along the lines of:

function FakeArray() {
  const target = {};

  Object.defineProperties(target, {
    "length": {
      value: 0,
      writable: true
    },
    [Symbol.iterator]: {
      // http://www.ecma-international.org/ecma-262/6.0/#sec-array.prototype-@@iterator
      value: () => {
        let index = 0;

        return {
          next: () => ({
            done: index >= target.length,
            value: target[index++]
          })
        };
      }
    }
  });

  const isArrayIndex = function(p) {
    /* an array index is a property such that
       ToString(ToUint32(p)) === p and ToUint(p) !== 2^32 - 1 */
    const uint = p >>> 0;
    const s = uint + "";
    return p === s && uint !== 0xffffffff;
  };

  const p = new Proxy(target, {
    set: function(target, property, value, receiver) {
      // http://www.ecma-international.org/ecma-262/6.0/index.html#sec-array-exotic-objects-defineownproperty-p-desc
      if (property === "length") {
        // http://www.ecma-international.org/ecma-262/6.0/index.html#sec-arraysetlength
        const newLen = value >>> 0;
        const numberLen = +value;
        if (newLen !== numberLen) {
          throw RangeError();
        }
        const oldLen = target.length;
        if (newLen >= oldLen) {
          target.length = newLen;
          return true;
        } else {
          // this case gets more complex, so it's left as an exercise to the reader
          return false; // should be changed when implemented!
        }
      } else if (isArrayIndex(property)) {
        const oldLenDesc = Object.getOwnPropertyDescriptor(target, "length");
        const oldLen = oldLenDesc.value;
        const index = property >>> 0;
        if (index > oldLen && oldLenDesc.writable === false) {
          return false;
        }
        target[property] = value;
        if (index > oldLen) {
          target.length = index + 1;
        }
        return true;
      } else {
        target[property] = value;
        return true;
      }
    }
  });

  return p;
}

I can't guarantee this is actually totally correct, and it doesn't handle the case where you alter length to be smaller than its previous value (the behaviour there is a bit complex to get right; roughly it deletes properties so that the length property invariant holds), but it gives a rough outline of how you can implement it. It also doesn't mimic behaviour of [[Call]] and [[Construct]] on Array, which is another thing you couldn't do prior to ES6—it wasn't possible to have divergent behaviour between the two within ES code, though none of that is hard.

This implements the length property in the same way the spec defines it as working: it intercepts assignments to properties on the object, and alters the length property if it is an "array index".

Unlike what one can do with ES5 and getters, this allows one to get length in constant time (obviously, this still depends on the underlying property access in the VM being constant time), and the only case in which it provides non-constant time performance is the not implemented case when newLen - oldLen properties are deleted (and deletion is slow in most VMs!).

Answer 6

The answer is: there's no way as of now. The array behavior is defined in ECMA-262 as behaving this way, and has explicit algorithms for how to deal with getting and setting of array properties (and not generic object properties). This somewhat dismays me =(.