(Map|Set|WeakMap)#set() returns `this` ?

Domenic Denicola domenic at domenicdenicola.com
Mon Dec 3 16:32:50 PST 2012

+1 (to all sentiments in the message :)
From: Nathan Wall<mailto:nathan.wall at live.com>
Sent: ‎12/‎3/‎2012 17:53
To: Andrea Giammarchi<mailto:andrea.giammarchi at gmail.com>; Rick Waldron<mailto:waldron.rick at gmail.com>
Cc: es-discuss at mozilla.org<mailto:es-discuss at mozilla.org>
Subject: RE: (Map|Set|WeakMap)#set() returns `this` ?

I'm not a person of influence, but as a JS developer, I agree with Andrea on this. I think Map#set() should return the value. I would expect the same behavior as obj[key] = value. I find Andrea's use case (m.get(k) || m.set(k, v)) more compelling than the method chaining possibilities.


Date: Mon, 3 Dec 2012 14:21:24 -0800
Subject: Re: (Map|Set|WeakMap)#set() returns `this` ?
From: andrea.giammarchi at gmail.com
To: waldron.rick at gmail.com
CC: es-discuss at mozilla.org

IMHO, a set(key, value) should return the value as it is when you address a value

var o = m.get(k) || m.set(k, v); // o === v

// equivalent of

var o = m[k] || (m[k] = v); // o === v

a set with a key that returns `this` is a non case so almost as useless as the void return is.

Usefulness comes with use cases ... except this jQuery chainability thingy that works fine for jQuery structure ( an ArrayLike Collection ) who asked for map.set(k0, v0).set(k1, v1).set(k2, v2) ? Or even map.set(k0,v0).get(k1) ? what are use cases for this?

I am honestly curious about them because I cannot think a single one ... specially with the Set

s.add(k0).add(k1).add(k2) ... this code looks weird inlined like this ...

Thanks for your patience

On Mon, Dec 3, 2012 at 2:04 PM, Rick Waldron <waldron.rick at gmail.com<mailto:waldron.rick at gmail.com>> wrote:

On Mon, Dec 3, 2012 at 4:28 PM, Andrea Giammarchi <andrea.giammarchi at gmail.com<mailto:andrea.giammarchi at gmail.com>> wrote:
I wonder what was the use case that convinced TC39 to return `this` with these methods.

Assuming you read the notes, I proposed the agenda item based on the best practice of ensuring meaningful returns, and in the case of mutation methods, |this| is a meaningful return.

Accordingly, this will never work:
var query = map.has('queried') ? map.get('queried') : map.set('queried', $('myquery'));

Previously, map.set() had a useless void return...

And it will be something like:
var query = map.has('queried') ? map.get('queried') : map.set('queried', $('myquery')).get('queried');

which is ugly and I don't really understand where map.set(k0, v0).set(k1, v1).set(k2, v2) could be useful.

Accessing the object post-mutation allows for more expressive use of the API.


Thanks for clarifications ( a use case would be already good )


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