Assign and immediately delete property

Question

I am no javascript programmer and am totally puzzled by what this code does and what it is used for:

function map(x) {
    x = Object.create(null);
    x.x = 0;
    delete x.x;
    return x;
}

It's part of what you get when using the dart2js compiler.

I'm not trying to understand the whole context, but what does assigning a property and deleting it directly again help achieve?

This looks like outsmarting some internal JS engine behaviour.

EDIT: As requested, here's the complete out.js as generated by dart2js (input is the "Hello world!" example from Wikipedia): https://gist.github.com/Blutkoete/59be155b2642832e9acd383df0857d02

EDIT 2: gurvinder372's link indicates is has to do with "delegating to anonymous JS objects for performance", but I'll probably need a lot of experience with JS to understand that.

The code doesn't make much sense at all. It expects a parameter which is directly overridden - that's the first "error". And assigning a value to a property and deleting it thereafter doesn't change anything really. Maybe it's a leftover of code like `function map(x) { x = x || {}; }` which should make sure that a certain property is not set - but also then it wouldn't make sense to do that like that — baao, Apr 17 '18 at 11:01
It's a performance micro-optimisation. The function works the same as `function map() { return Object.create(null); }` would - it creates an empty object to be used as a lookup map. — Bergi, Apr 18 '18 at 11:51
Deleting a property will cause V8 to put the object into dictionary mode, but I’m not sure what the parameter is for. Maybe a leftover from minification? — Ry-, Apr 18 '18 at 11:52

score 5 · Accepted Answer · answered Apr 18 '18 at 11:48

Well... This is an interesting topic and understanding this trick takes to read a little bit on the object representation of V8 compiler. I am not an expert on this but the topic was interesting enough to intrigue me to search for some answer. So here is what i have found.

First of all deleting a property seems to be a trick to change the internal structure of how object properties are kept and accessed. In other words deleting a property switches the object to dictionary mode where the properties are kept in a hash map. So when a dummy property is deleted immediately after it's created gives you an object in dictionary mode.

V8 can handle minor divergences like this just fine, but if your code assigns all sorts of random properties to objects from the same constructor in no particular order, or if you delete properties, V8 will drop the object into dictionary mode, where properties are stored in a hash table. This prevents an absurd number of maps from being allocated.

Taken from this nice article A tour of V8: object representation

Also, this question is interesting about pros and cons of dictionary mode: https://stackoverflow.com/questions/23455678/pros-and-cons-of-dictionary-mode — Arashsoft, May 14 '18 at 14:38

Assign and immediately delete property

1 Answers1