/* Code Comments */

Using Lodash Reduce And Some To Accomplish Complex Filtering

July 23, 2019

I came across a situation where I had a collection of collections. Some of the internal collections, however, I didn’t want to keep around because of the values it held. If all of the values resolved to false, I would throw it out - but if even a single one was truthy, I’d keep it around.

I needed a way to economically filter them out. If I was dealing with arrays, I’d use .reduce and .some. Faced with objects, however, I looked to Lodash’s methods for both — which conveniently handle not only arrays, but all collections (which includes objects).

NB: The following example is contrived, but illustrative of the kind of situation I was facing.

const init = {
    first: {
        title: "first",
        names: {
            a: true,
            b: false,
            c: true,
        }
    },
    second: {
        title: "second",
        names: {
            d: false,
            e: false,
            f: false,
        }
    }
}

So, passing the init through a function, we’ll call it clean, I want to see the following object:

const output = {
    first: {
        title: "first",
        names: {
            a: true,
            b: false,
            c: true,
        }
    },
}

Lodash’s some method fit my use case well. I wanted an escape hatch as quickly as possible if I found a true value

My first step was understanding how to use the _.some. The documentation paints the picture:

Checks if predicate returns truthy foranyelement of collection. Iteration is stopped once predicate returns truthy. The predicate is invoked with three arguments: (value, index|key, collection).

Unfortunately, while it’s clear that this is possible, the examples don’t show how it’s used, focusing more on the magic that Lodash enables.

Here’s a simple example of _.some in practice given our init object:

function checkSome (collection) {
  return _.some(collection.names, (val, key, collection) => {
    return val;
  })
}
console.log(checkSome(init.first)); // true
console.log(checkSome(init.school)); // false

Okay, so that’s the second step — but, how do I actually get to it? With the _.reduce.

A reminder on how the _.reduce works:

Reduces collection to a value which is the accumulated result of running each element in collection thru iteratee, where each successive invocation is supplied the return value of the previous. If accumulator is not given, the first element of collection is used as the initial value. The iteratee is invoked with four arguments: (accumulator, value, index|key, collection).

function reducer(accumulator, value, key, collection) {
  // if some of the values return true when passed into checkSome
  if (checkSome(value)){
	  // add them to the accumulator
    accumulator[key]=value;
  }
  return accumulator;
}

function clean(collection) {
  return _.reduce(collection, reducer, {})
}
const reducedInit = clean(init)

Voila! My reducedInit returns only the desired values.


Stephen Weiss

Thanks for reading! My name's Stephen Weiss. I live in Chicago with my wife, Kate, and dog, Finn.
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