ES6 Proxy set property trap not firing for array length

Question

When working with JavaScript ES6 Proxies, the set property trap for array.length does not fire when assigning array indexes directly.

For example:

const proxy = new Proxy([], {
    set: function(obj, name, value) {
        console.log(`set: ${name}`);
        obj[name] = value;
        return true;
    }
});
proxy.push(0);
proxy[1] = 1;

Chrome 51 and Firefox 47 outputs:

set: 0  
set: length  
set: 1

While I would expect:

set: 0  
set: length  
set: 1  
set: length

Is this per spec? I couldn't find any information on this.

score 2 · Accepted Answer · answered Jun 23 '16 at 11:14

2

There is no need to explicitly set the length property when a value is assigned to an index. The reason why it's set with push is indeed defined in the specification:

Repeat, while items is not empty

a. Remove the first element from items and let E be the value of the element.

b. Let setStatus be Set(O, ToString(len), E, true).

c. ReturnIfAbrupt(setStatus).

d. Let len be len+1.

Let setStatus be Set(O, "length", len, true).

Basically: If an error happens then set the correct length in case the array already has been expanded.

answered Jun 23 '16 at 11:14

a better oliver

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1

This is a very good question but i don't see how exactly you answer the question by **There is no need to explicitly set the** `length` **property when a value is assigned to an index.** When I make `proxy[99] = 100` the `length` property never gets trapped by `set` but magically gets trapped by `get` returning 100 when accessed by `console.log(proxy.length)`. So I suppose 100 is not a calculated but an assigned value to the `length` property. When I do a `.pop()` afterwards `length` gets trapped by `set` and updates to 99. How did the `length` property become 100 in the first place. – Redu Apr 15 '18 at 01:09
2

@Redu The `.length` does get updated *behind* the scenes [when assigning to an index](http://www.ecma-international.org/ecma-262/6.0/#sec-array-exotic-objects-defineownproperty-p-desc). You could also consider every instance to have a magical getter/setter `.length` that always knows the number of items (and holes). Regardless, this update cannot be trapped by a proxy, because it happens on the array instance (proxy target) itself - unlike the `push` or `pop` methods, which do an explicit assignment on the (proxy) object that they were invoked on. – Bergi Apr 15 '18 at 10:17
@Bergi Thanks for your attention. I have even tried the `defineProperty` trap hoping direct value assignment at index might somehow redefine the `length` property alas no way. Then i suppose another way is to check like `(prop === "length" || target.length <= prop)` in the `set` trap to catch by index insertions to the end of the array. Is this reasonable..? – Redu Apr 15 '18 at 13:54
1

@Redu Yes, that sounds reasonable (but cast `prop` to an integer, and use the same algorithm as in the spec section I linked). – Bergi Apr 15 '18 at 14:08

ES6 Proxy set property trap not firing for array length

1 Answers1