问题
I have researched on net about the benefits of immutablejs over Object.freeze() but didn't find anything satisfying!
My question is why I should use this library and work with non native data structures when I can freeze a plain old javascript object?
回答1:
I don't think you understood what immutablejs offers. It's not a library which just turns your objects immutable, it's a library around working with immutable values.
Without simply repeating their docs and mission statement, I'll state two things it provides:
Types. They implemented (immutable) infinite ranges, stacks, ordered sets, lists, ...
All of their types are implemented as Persistent Data Structures.
I lied, here's a quote of their mission statement:
Immutable data cannot be changed once created, leading to much simpler application development, no defensive copying, and enabling advanced memoization and change detection techniques with simple logic. Persistent data presents a mutative API which does not update the data in-place, but instead always yields new updated data.
I urge you to read the articles and videos they link to and more about Persistent Data Structures (since they're the thing immutablejs is about), but I'll summarise in a sentence or so:
Let's imagine you're writing a game and you have a player which sits on a 2d plane. Here, for instance, is Bob:
var player = {
name: 'Bob',
favouriteColor: 'moldy mustard',
x: 4,
y: 10
};
Since you drank the FP koolaid you want to freeze the player (brrr! hope Bob got a sweater):
var player = Object.freeze({
name: 'Bob',
...
});
And now enter your game loop. On every tick the player's position is changed. We can't just update the player object since it's frozen, so we copy it over:
function movePlayer(player, newX, newY) {
return Object.freeze(Object.assign({}, player, { x: newX, y: newY }));
}
That's fine and dandy, but notice how much useless copying we're making: On every tick, we create a new object, iterate over one of our objects and then assign some new values on top of them. On every tick, on every one of your objects. That's quite a mouthful.
Immutable wraps this up for you:
var player = Immutable.Map({
name: 'Bob',
...
});
function movePlayer(player, newX, newY) {
return player.set('x', newX).set('y', newY);
}
And through the ﾉ*✧ﾟ magic ✧ﾟ*ヽ of persistent data structures they promise to do the least amount of operations possible.
There is also the difference of mindsets. When working with "a plain old [frozen] javascript object" the default actions on the part of everything is to assume mutability, and you have to work the extra mile to achieve meaningful immutability (that's to say immutability which acknowledges that state exists). That's part of the reason freeze exists: When you try to do otherwise, things panic. With Immutablejs immutability is, of course, the default assumption and it has a nice API on top of it.
That's not to say all's pink and rosy with cherry on top. Of course, everything has its downsides, and you shouldn't cram Immutable everywhere just because you can. Sometimes, just freezeing an object is Good Enough. Heck, most of the time that's more than enough. It's a useful library which has its niche, just don't get carried away with the hype.
回答2:
According to my benchmarks, immutable.js is optimized for write operations, faster than Object.assign(), however, it is slower for read operations. So the descision depends the type of your application and its read/write ratio. Following are the summary of the benchmarks results:
-- Mutable
Total elapsed = 103 ms = 50 ms (read) + 53 ms (write).
-- Immutable (Object.assign)
Total elapsed = 2199 ms = 50 ms (read) + 2149 ms (write).
-- Immutable (immutable.js)
Total elapsed = 1690 ms = 638 ms (read) + 1052 ms (write).
-- Immutable (seamless-immutable)
Total elapsed = 91333 ms = 31 ms (read) + 91302 ms (write).
-- Immutable (immutable-assign (created by me))
Total elapsed = 2223 ms = 50 ms (read) + 2173 ms (write).
Ideally, you should profile your application before introducing any performance optimization, however, immutability is one of those design decision must be decided early. When you start using immutable.js, you need to use it throughout your entire application to get the performance benefits, because interop with plain JS objects using fromJS() and toJS() is very costly.
PS: Just found out that deep freeze'ed array (1000 elements) become very slow to update, about 50 times slower, therefore you should only use deep freeze in development mode only. Benchmarks results:
-- Immutable (Object.assign) + deep freeze
Total elapsed = 45903 ms = 96 ms (read) + 45807 ms (write).
回答3:
Both of them don't make the object deeply immutable.
However, using Object.freeze you'll have to create the new instances of the object / array by yourself, and they won't have structural sharing. So every change which will require deeply copying everything, and the old collection will be garbage collected.
immutablejs on the other hand will manage the collections, and when something changes, the new instance will use the parts of the old instance that haven't changed, so less copying and garbage collecting.
回答4:
Object.freeze does not do any deep freezing natively, I believe that immutable.js does.
The same with any library -- why use underscore, jquery, etc etc.
People like re-using the wheels that other people built :-)
回答5:
The biggest reason that comes to mind - outside of having a functional api that helps with immutable updates, is the structural sharing utilized by Immutable.js. If you have an application that needs enforced immutability (ie, you're using Redux) then if you're only using Object.freeze then you're going to be making a copy for every 'mutation'. This isn't really efficient over time, since this will lead to GC thrasing. With Immutable.js, you get structural sharing baked in (as opposed to having to implement an object pool/a structural sharing model of your own) since the data structures returned from immutable are Tries. This means that all mutations are still referenced within the data structure, so GC thrashing is kept to a minimum. More about this is on Immutable.js's docsite (and a great video going into more depth by the creator, Lee Byron):
https://facebook.github.io/immutable-js/
回答6:
There are a couple of major differences between Object.freeze() and immutable.js.
Let's address the performance cost first. Object.freeze() is shallow. It will make the object immutable, but the nested properties and methods inside said object can still be mutated. The Object.freeze() documentation addresses this and even goes on to provide a "deepFreeze" function, which is even more costly in terms of performance. Immutable.js on the other hand will make the object as a whole (nested properties, method, etc) immutable at a lower cost.
Additionally should you ever need to clone an immutable variable Object.freeze() will force you to create an entirely new variable, while Immutable.js can reuse the existing immutable variable to create the clone more efficiently. Here's an interesting quote about this from this article:
"Immutable methods like .set() can be more efficient than cloning because they let the new object reference data in the old object: only the changed properties differ. This way you can save memory and performance versus constantly deep-cloning everything."
In a nutshell, Immutable.js makes logical connections between the old and new immutable variables, thus improving the performance of cloning and the space frozen variables take in memory. Object.freeze() sadly does not - every time you clone a new variable from a frozen object you basically write all the data anew, and there is no logical connection between the two immutable variables even if (for some odd reason) they hold identical data.
So in terms of performance, especially if you constantly make use of immutable variables in your program, Immutable.js is a great choice. However, performance is not everything and there are some big caveats to using Immutable.js. Immutable.js uses it's own data structure, which makes debugging, or even just logging data to the console, a royal pain. It also might lead to a loss of basic JavaScript functionality (for example, you cannot use ES6 de-structuring with it) The Immutable.js documentation is infamously impossible to understand (because it was originally written for use only within Facebook itself), requiring a lot of web-searching even when simple issues arise.
I hope this covers the most important aspects of both approaches and helps you decide which will work best for you.
来源：https://stackoverflow.com/questions/36726138/why-should-i-use-immutablejs-over-object-freeze