When is javascript deoptimizer invoked? - javascript

I understand that js' JIT compiler will deoptimize the warm or hot code in case datatype changes in a loop (like one element in array is string whereas rest were int).
But i have few scenarios where i'm not able to understand will the code be deoptimized or not
Same loop is used for two arrays where one array contains strings and other ints. Will compiler deoptimize the code here or create two copies? (I understand it should be two copies).
In case of array of object. Considering all scenarios like
Manipulated sub-property is of different type.
Same sub-properties for each object but one object has one property missing.
Missing property is not manipulated inside the loop.
Missing property is manipulated inside loop (null case is handled).
All objects have different properties (New property is added, or manipulation is done using property location).

Related

Post representation in JavaScript?

Question 1: If there a post such as this one. How would I represent it in JavaScript? I thought was JSO and parse. Other programming language like C++ and JAVA a main function and have map references but where does JavaScript store it's objects?
Question 2: Oh and do I add Event Handlers to each of the HTML representatives elements?
The basic data structure in JavaScript is the object, you would probably use one (or, more likely, a structured collection of them along with more specific data structures such as arrays) to describe the data structure that makes up a webpage.
The main function in Java and C++ is the entry point to the program. In JavaScript, the entry point is the top of the program.
JavaScript stores its objects in memory and references to them in variables and as properties of objects.
You would only add an event handler to an HTML Element object if you wanted to listen for an event on it.
People are saying store objects in variables but it's I was also told its better to hash them and create references to them in an array.
You use the data type most appropriate for the data you are working with.
If you have a:
simple piece of data, use a variable.
collection of ordered data, use an array (or maybe a Set if you are using ES6)
collection of unordered data, use an object (or maybe a Map if you are using ES6)
… and nest to whatever level makes sense.
You might also create a custom data type with a constructor function or (if you are using ES6) a Class.

Most efficient way to use an object as a hash key

I have a situation in which I would greatly benefit from the use of function caching (memoization).
However, my function takes a single argument, whose value is a very large and multi-dimensional Array.
The standard way to do caching ( and the only way that I can think of) is to create a cache property on the function itself (cache is a hash). Each run of the function, you can check for the existence of myFunc.cache[arg] and simply return that value if it exists, otherwise you perform the calculations as normal and add the argument as a property of cache afterwards.
However, it appears that javascript does not try to evaluate the strings used as hash keys when you are creating them and always just treats them as strings. example
I could apply JSON.stringify to the argument, but because the array will large and nested, I am wondering if there is a more efficient way to identify unique arguments.
This question is basically what I was asking without knowing it:
JavaScript Hashmap Equivalent
In my case, the simplest solution is just to manually make the 0th index of my arrays an ID that I can use as the hash key.
Without a means of doing this, or something similar, you have to either create your own hashmap (outlined in the linked question) or wait for the official Map object to be implemented.

JavaScript in Chrome: Read performance differs between two objects created with different strategies

[UPDATE The discussion of "slow mode" below also pointed me to Why the convertToFastObject function make it fast?, which has a nice discussion of the V8 internal discussion that #benjamin-gruenbaum referenced.]
I was wondering about the performance of JavaScript one for optimizing reads, so I wrote a JSPerf test suite, and the results are non-intuitive.
Firstly, the question "Performance of key lookup in JavaScript object" discusses the internals of V8s object creation, but I don't see why the method of creation would change the read performance after it has been created.
Overview
I want to test read access on various data structures. I chose 3 structures:
An object literal, created and assigned in one statement
An object literal, created in one statement, then values assigned in subsequent statements
An array literal, created in one statement, but populated as an associative array, which presumably gets translated to an object in the background.
Test
http://jsperf.com/object-read-varies-by-creation-method
Methodology
In the setup method. I create 3 data structures and populate them with about random words from /usr/share/dict/words, in randomized order (but the same order on each test run). Each test case retrieves the same 10 random words from /usr/share/dict/words, but not necessarily words that are in the list, to simulate misses as well as hits.
Results
What I see on most browsers is pretty intuitive: the two objects perform better than the pseudo-associative array. My assumption is that the array literal is being proxied by an object in those cases, increasing the overall time it takes to access.
What I don't get is the Chrome 33 case, where creating and assigning an object as a single statement performs dramatically better than populating the structure after creation.
If the test included setup time, I could understand that difference, but as it stands, I don't fully get why the two object literals don't perform about the same.
The best guess I've come up with so far is that Chrome's object creation algorithm is able to optimize its hash tree better during an act of creation that includes all keys at once then it is when it's inserting the keys one at a time after the fact.
Can anyone more familiar with these structures account for the difference?

How to query javascript's object space

JavaScript creates a bunch of objects in memory. Even "classes" are really just prototype objects in JavaScript.
I want to create a list of all of the objects currently instantiated on a page. I realize this might include JavaScript basic objects like String.prototype and Number.prototype. I think that's great!
Bonus points for also telling me how to query JavaScript's object space more specifically. Perhaps, say, find all objects currently in memory with a property key matching "name".

Under the hood, are Javascript objects hash tables? [duplicate]

This question already has answers here:
How does JavaScript VM implements Object property access? Is it Hashtable?
(4 answers)
Closed 2 years ago.
I was wondering about how Objects are implemented under the hood in Javascript engines (V8, Spidermonkey, etc). Are they really just Hash Tables? If so, how do they handle collisions?
First of all, the answer is probably somewhat different for different JS engines. Also, I assume you're specifically asking about the property storage; obviously objects have a bunch of other state too (prototype chain link being an obvious one).
In the case of Spidermonkey, objects basically have a linked list of (propname, information about property) pairs, until they have too many properties, when I believe they still keep the linked list (because order matters for properties in JS in practice) but add an out-of-band hashtable that maps property names to entries in the linked list.
There may also be other reasons for the switch to the hashtable; the details haven't exactly been fixed over time and are likely subject to change in the future.
The linked lists and hashtables are actually shared across objects; as long as two objects have the same property names and corresponding property information (which does NOT include the value, for properties with a stored value) and the properties were set in the same order, they're able to share the property linked list.
The actual property values, when those need to be stored, are stored in an array in the object (or more precisely, two arrays; one allocated inline with the object, whose size is fixed at object-creation time, one dynamically allocated and resized as needed for properties that are added later).
Take a look at a similar question How does JavaScript VM implements Object property access? and my answer. Here I described the optimization technique used by JS engines and how it affects the key lookup performance. I hope that knowing of these details let you write more efficient JS code.
One can think of an object as an associative array (a.k.a. map, dictionary, hash, lookup table). The keys in this array are the names of the object's properties.
I found this on MDN
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Property_accessors
In JS arrays are associative arrays and objects are also the same.
In JS arrays are basically objects with properties as sequential numbers.

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