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I am new to constructing a JavaScript Array prototype. Please only paste previous links that are directly appropriate as I have been sourcing on SO and on w3school. https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Array/prototype

I would like to create a method for an array that checks if the 'id' field within the object does not already exist in the array. If not, then 'push' object into the array, else do nothing.

Each object has a unique 'id' field.

eg

var array = [{ id: 001, title: 'some title'},
               { id: 002, title: 'other title'}]

var object = {id: 001, title: 'some title'}


// if object.id does not exist in array...
array.push(object)
//else do nothing

I would like the function to take the 'id' field as an argument so this function has wider use.

Are there any drawbacks to extending the array.Prototype? Otherwise I can do a for loop instead to do the check without the prototype constructor.

meteorBuzz
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    Why not just use an object with the id as the key? – bhspencer Mar 14 '15 at 13:55
  • Javascript arrays are "sparse" (Google fodder...) - if your id's are always numeric you could take advantage of this behaviour instead of creating an object/map. – Emissary Mar 14 '15 at 14:00

3 Answers3

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Since your ids are integers you could just set that index in the array directly.

var array = [];
var object = {id: 001, title: 'some title'};
array[object.id] = object;

That will overwrite the element in that location such that you won't get duplicates with the same id. Your js engine will automatically switch to using a sparse array rather than a contiguous array if your ids are far apart.

Emissary
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bhspencer
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  • bhspencer - this is perfect and easy to implement. Thanks for pointing out how the engine will work. I sourced what this means and is important to my use case. To prevent unnecessary writes to the client database, is there an underscore method to check if value exists, _.find(list, predicate, [context]) – meteorBuzz Mar 14 '15 at 14:22
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    you don't need to use underscore to see if an array contains an element. But if you want to the answer is here http://stackoverflow.com/questions/237104/array-containsobj-in-javascript – bhspencer Mar 14 '15 at 14:25
  • bhspencer, the method using 'array[object.id] = object;' writes the id as the index position. it is not the desired behaviour I prefer. – meteorBuzz Mar 14 '15 at 15:03
  • What is it that you would prefer? Keep in mind that sparse arrays in JS are not a problem. – bhspencer Mar 14 '15 at 15:07
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Instead of trying to change the behavior of Array consider using an object instead of array with the id as the key.

var objects = {};
var object = {id: 001, title: 'some title'}
objects[object.id] = object;

That way you can also retrieve your objects from the parent object by their id. e.g.

var result = objects[001];
bhspencer
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I found brads's answer using the function containsObject How do I check if an array includes an object in JavaScript?

This does a check so there is no extra writes to the database such as overwrites. Thank you everyone for inputting towards the question. It has helped.

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meteorBuzz
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