Using an array as a hash table

Question

This is basically a programming style question in javascript.

Sometimes when I'm coding I find myself looping through a collection of data, like for example, a collection of users:

var users = [
    {
        id: 'USER-435',
        name: 'James',
        email: '[email protected]'
    },{
        id: 'USER-7897',
        name: 'Mark',
        email: '[email protected]'
    },{
        id: 'USER-2345',
        name: 'Harry',
        email: '[email protected]'
    }
]

Important: This data comes from the server and it needs to be in order.

If I want to get the properties of a specific user by its ID, I will have to loop through the array to find it.

So what I have done is to loop through the array once and create properties in the array with the user's ID as its key. This way I can access each user using its ID without looping through the array. Because the = operator creates a reference to the object and not a copy, each property added to the array will be a reference.

The only problem I have found is that if the ID of the user is a number it will be part of the array. So, if we have a collection with 3 users, with one of their ID been 120, that will set the length of the array to 121. A quick could be add the property as 'id-120', but it doesn't feel very clean.

I can also create a separate object for the collection instead of creating new properties in the array, but that will create a new object I need to take care of.

I just wanted to ask what people think about this type of pattern and if you have a better way of doing it.

JayKuri · Accepted Answer · 2014-10-09 14:59:27Z

2

You only have a few options to accomplish this. It's a pretty common pattern to re-map an array to an object to do lookups by key.

I would not worry too much about creating a 'users_by_id' object, it is unlikely to cause you much trouble (unless your code is already very complex)

But, I have a suggestion for your particular problem. Why not create a property on the array called 'by_id' and make that the object that you index by id.

users.by_id['USER-2345'];

It solves the 'extra variable' problem, the key-clobbering problem and the code becomes self-explanatory to boot.

Good luck.

PS - also check out underscore.js or lodash - they have routines that make this kind of thing very easy. They'll save you a ton of time.

answered Oct 9, 2014 at 14:59

JayKuri

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KooiInc · Accepted Answer · 2014-10-09 15:48:12Z

0

I would use Array.filter here. Something like:

users.filter( function (v) { return v.id === 'USER-435'; } );

In a function:

function findUser(id) {
  var user = users.filter( function (v) { return v.id === id; } );
  return user.length ? user[0] : {id: 'not found'};
}

This will need looping, but until your array contains, say, more than a million elements, I wouldn't worry about that. To quote Tony Hoare:

We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil.

See also MDN
And (to be fair) also THE FALLACY OF PREMATURE OPTIMIZATION

edited Oct 9, 2014 at 15:48

answered Oct 9, 2014 at 14:58

KooiInc

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2 Comments

ajorquera Over a year ago

The main question is how to avoid looping through the array

ajorquera Over a year ago

Sorry but @JayKuri 's answer it's much cleaner

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