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Need help understanding: Filtering an array of objects in Javascript

leanminmachine on October 04, 2018

I am still trying to wrap my head around filtering an array of objects in Javascript; spent a couple of hours figuring this out. I have this array...

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Dian Fay • Oct 4 '18

includes returns a boolean value but forEach does not (or rather, it returns undefined). So the result of the includes call is simply thrown away if you use forEach instead of some.

Two obvious improvements to the working method:

iterate restaurant.menuEntities with some as well, if it's possible that the result you're looking for is not in the very first one
use reduce instead of filter; if you search, there have been numerous guides posted here

Additionally, if you're retrieving the restaurant & menu records from a relational database you could short-circuit the whole thing by filtering in your query instead of in application logic.

leanminmachine • Oct 4 '18

Hahhaa for the menuEntities yeah I'll definitely have to use .some if there's more than one.. Thanks for the reminder & explanation, makes sense that the results of includes is thrown away with forEach

Guillaume Martigny • Oct 4 '18

Just to summarize:

forEach if you need to act on every item (display all prices)
- map same, but return a new array with the new values (add 10% tax to all prices)

filter if you need to remove items (get prices under 10$)

includes if you need to know if the array contains at least one (does one prices is exactly equal to 10$)
- some same, but you can use a function as assertion (does one price is under 10$)

reduce if you need to transform an array into one value (sum all prices)

ps: Try to never modify a variable (except iterator).

this.filteredRestaurants = this.sortedRestaurants.filter( // ...

Dian Fay • Oct 4 '18

The reduce accumulator doesn't have to be one value -- it's quite useful in places you'd use both filter and map to choose some elements of an array and generate derived values, and you only have to traverse the array once.

Guillaume Martigny • Oct 4 '18

For large data set I would agree (even tho I never thought of it). But for sub 100 items, the lose in clarity isn't much worth it.

Jason Matthews • Oct 4 '18

Looks like the answer to why your initial solution didn't work has been answered. I just wanted to leave a few suggestions.

Avoid using == in favor of ===. == Will attempt to type convert and unless that's what you need you can run into some false positives. If it is what you explicitly need you're better off converting and then comparing with === yourself
Instead of checking _.isEmpty(thing) == false, you can write !isEmpty(thing). It'll be easier to read when/if you or anyone else has to come back and do additional work on this code.
You're mixing two flavors of syntax with an ES5 function definition and an ES6 fat arrow function definition. It's not wrong but unless you have a good reason for it, it's probably better to stick to one again for code legibility down the line.

Adam Crockett 🌀 • Oct 6 '18

Don't forget filtter and find. Find gives just one result and filter gives many reults.
If I'm looping through a crowd of people to find on person.
If I'm looking for a particular group to filter through.

diek • Oct 4 '18

Hi, a filter must return boolean for every element in the array, you are not doing that. The filter will fill your result with only the elements that returned true. You should check the mdn docs for the Array.prototipe.filter, it comes with examples and very well explained.