How is a JavaScript hash map implemented

Question

I currently work with OpenLayers and have a huge set of data to draw into a vector layer  greater than 100000 vectors    I m now trying to put all these vectors into a JavaScript hash map to analyze the performance   I want to know how is the hash map in JavaScript implemented  is it a real hash function or just a wrapped function that uses a simple data structure and a search algorithm

User · Answer

JavaScript objects cannot be implemented purely on top of hash maps.

Try this in your browser console:

var foo = {
    a: true,
    b: true,
    z: true,
    c: true
}

for (var i in foo) {
    console.log(i);
}

...and you'll recieve them back in insertion order, which is de facto standard behaviour.

Hash maps inherently do not maintain ordering, so JavaScript implementations may use hash maps somehow, but if they do, it'll require at least a separate index and some extra book-keeping for insertions.

Here's a video of Lars Bak explaining why v8 doesn't use hash maps to implement objects.

User · Answer

Should you try this class Map    x000D   x000D  var myMap   new Map    x000D   x000D     setting the values x000D  myMap set  1    value1    x000D  myMap set  2    value2    x000D  myMap set  3    value3    x000D   x000D  myMap size     3 x000D   x000D     getting the values x000D  myMap get  1           value associated with  value1  x000D  myMap get  2              value associated with  value1  x000D  myMap get  3             value associated with  value3  x000D   x000D   x000D    Notice  key and value can be any type   https   developer mozilla org en-US docs Web JavaScript Reference Global Objects Map

User · Answer

While plain old JavaScript objects can be used as maps  they are usually implemented in a way to preserve insertion-order for compatibility with most browsers  see Craig Barnes s answer  and are thus not simple hash maps   ES6 introduces proper Maps  see MDN JavaScript Map  of which the standard says      Map object must be implemented using either hash tables or other mechanisms that  on average  provide access times that are sublinear on the number of elements in the collection

User · Answer

every javascript object is a simple hashmap which accepts a string or a Symbol as its key  so you could write your code as  var map          add a item map key1    value1     or remove it delete map key1      or determine whether a key exists key1 in map   javascript object is a real hashmap on its implementation  so the complexity on search is O 1   but there is no dedicated hashcode   function for javascript strings  it is implemented internally by javascript engine  V8  SpiderMonkey  JScript dll  etc     2020 Update  javascript today supports other datatypes as well  Map and WeakMap  They behave more closely as hash maps than traditional objects

User · Answer

I was running into the problem where i had the json with some common keys  I wanted to group all the values having the same key  After some surfing I found hashmap package  Which is really helpful   To group the element with the same key  I used multi key    value    key2    value2           This package is somewhat similar to Java Hashmap collection  but not as powerful as Java Hashmap

User · Answer

Here is an easy and convenient way of using something similar to the Java map   var map         map name 1   map value 1       map name 2   map value 2       map name 3   map value 3       map name 4   map value 4         And to get the value   alert  map  map name 1            fives the value of map value 1          etc

User · Answer

lt html gt   lt head gt   lt script type  text javascript  gt  function test    var map    m1   12  m2   13  m3   14  m4   15       alert map  m3        lt  script gt   lt  head gt   lt body gt   lt input type  button  value  click  onclick  test     gt   lt  body gt   lt  html gt

[javascript] How is a JavaScript hash map implemented?

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