How to filter arrays depending on the values of arrays inside the array?

Question

Why doesn't this test pass?

it('lodash_test', function() {
    var arr = [{a: "x", b: [{x: "1", y: "2"}]}, {a: "x", b: [{x: "1", y: "2"}]}, {a: "x", b: [{x: "33", y: "2"}]}]
    var result = _.filter(arr, function(obj) {
        obj.b.forEach(function(item_b) {
            if(item_b.x=="1") {
                return true;
            }
        });
    });
    expect(result).to.be.eql([{a: "x", b: [{x: "1", y: "2"}]}, {a: "x", b: [{x: "1", y: "2"}]}]);
});

T.J. Crowder · Accepted Answer · 2016-05-20 17:25:38Z

1

Because forEach's return value is completely ignored, and your _.filter predicate function never returns anything.

You probably wanted to use some:

it('lodash_test', function() {
    var arr = [{a: "x", b: [{x: "1", y: "2"}]}, {a: "x", b: [{x: "1", y: "2"}]}, {a: "x", b: [{x: "33", y: "2"}]}]
    var result = _.filter(arr, function(obj) {
        return obj.b.some(function(item_b) {
    //  ^^^^^^^      ^^^^
            if(item_b.x=="1") {
                return true;
            }
        });
    });
    expect(result).to.be.eql([{a: "x", b: [{x: "1", y: "2"}]}, {a: "x", b: [{x: "1", y: "2"}]}]);
});

Array#some calls the predicate for each entry in the array, stopping the first time the predicate returns a truthy value; its return value is true if the predicate ever returned a truthy value, or false if not.

So the return I've underscored in the above is the return from the _.filter predicate; your original return true is the return from some (formerly forEach).

edited May 20, 2016 at 17:25

answered May 20, 2016 at 17:22

T.J. Crowder

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3 Comments

scaryguy Over a year ago

You're a genius :) Thank you! By the way, do you think that it is a performant way to do the job? Are there any better alternatives?

T.J. Crowder Over a year ago

@scaryguy: I think for a small data set, it doesn't matter. It's simple and gets the job done. Worry about performance if and when performance is an issue. :-) Having said that, based on the structure of the data, I'm not immediately seeing how you could do it other than what you're doing; you have to loop through the inner arrays to know if they have x == "1". The JavaScript engine's going to optimize those predicate calls (and even if it didn't, they don't cost anything worth worrying about).

scaryguy Over a year ago

I'm going to use this for an ad server. Instead of doing the same thing in MongoDB, I'm fetching a base number of records and I'm iterating over them to filter odd ones. Maybe it's a very broad question for a "comment" but do you think so this should be more performant than a "db query"? If you'd like to answer, I can add another question for this :)

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