JavaScript object vs. array lookup performance

Question

What is the performance difference between retrieving the value by key in a JavaScript object vs iterating over an array of individual JavaScript objects?

In my case, I have a JavaScript object containing user information where the keys are the user's IDs and the values are each user's information.

The reason I ask this is because I would like to use the angular-ui-select module to select users, but I can't use that module with a Javascript object - it requires an array.

How much, if anything, am I sacrificing by switching from a lookup by key, to a lookup by iteration?

By key:

var user = users[id];

By iteration

var user;

for (var i = 0; i < users.length; i ++) {
  if (users[i].id == id) { 
    user = users[i]; break;
  }
}

Answer 1

The answer to this is browser dependent, however, there are a few performance tests on jsperf.com on this matter. It also comes down to the size of your data. Generally it is faster to use object key value pairs when you have large amounts of data. For small datasets, arrays can be faster.

Array search will have different performance dependent on where in the array your target item exist. Object search will have a more consistent search performance as keys doesn't have a specific order.

Also looping through arrays are faster than looping through keys, so if you plan on doing operations on all items, it can be wise to put them in an array. In some of my project I do both, since I need to do bulk operations and fast lookup from identifiers.

A test:

http://jsben.ch/#/Y9jDP

Answer 2

This problem touches all programming languages. It depends on many factors:

• size of your collection -arrays will get slower when you are searching for the last key, and array is quite long
• can elements repeat them selves-if yes, than you need a array. If no: you need either a dictionary (map) or you need to write a add method that for each add will iterate your array and find possible duplicates-that can be troublesome, when dealing with large lists
• average key usage - you will lose performance, if the most requested userId is at the end of the list.

In your example map would be a better solution. Secondly, you need to add a break to yor code:)

var user;

for (var i = 0; i < users.length; i ++) {
  if (users[i].id == id) {
     user = users[i]; break;
  }
}

Or you will lose performance:)

Answer 3

关联数组比带有编号索引的数组要慢得多，因为关联数组通过字符串比较来工作，这比数字比较慢很多！