Lodash FP flow filter key is undefined

Question

I am trying to adopt Lodash functional programming functions and replacing _.chain with the flow. However, filter key passed into the flow is undefined:

import flow from 'lodash/fp/flow';
import filter from 'lodash/fp/filter';

flow(
  filter((value, key) => console.log(value, key)) // key is undefined
)({ name: "Tobi", year: 2017, });

So I wrote a long answer below describing some of the problems here, but what are you *really* trying to do? Given your input, what would you like your output to be? — Mulan, Feb 16 '17 at 21:58
Thank you for the detailed answer. I will analyze your answer tomorrow. What I am trying to do is to move from _.chain to flow. Generally, I use _.chain to get some specific part of data from the response JSON or reformat it to be usable for some components or sort it or do all these things together. — Shota, Feb 16 '17 at 22:16
you might be interested in a couple of my recent answers regarding Lodash's `flow`: [*Function composition with Lodash flow*](http://stackoverflow.com/a/42139851/633183) and [*Composing functions of varying arity using Lodash flow*](http://stackoverflow.com/a/42166494/633183) – let me know if I can help you in any other way ^_^ — Mulan, Feb 16 '17 at 22:19

Mulan · Accepted Answer · 2020-09-27T19:33:37.353

You have several problems about assumptions you've made here

1. Objects aren't considered a collection

Your code is basically doing this

// f = your filtering function
filter(f, {name: "Tobi", year: 2017})
// => ["Tobi", 2017]

That's because filter will implicitly convert the Object to an Array of the object's values if an Object is given

To convert an Object to an Array of key/value pairs, use Lodash's _.toPairs function

_.toPairs({name: "Tobi", year: 2017})
// => [ [ "name", "Tobi" ], [ "year", 2017 ] ]

2. Your filtering function parameters are backwards/wrong

Even if we were to utilize _.toPairs, your code would be doing this

_.filter((value, key) => ..., _.toPairs(data),)

Not only is the order backwards, but looking at the data, notice we'll be passing a single Array (a [key, value] pair) to the filtering function. That means we'll have to update the function to destructure the element like so

_.filter(([key, value]) => console.log(key, value), _.toPairs(data))
// name Tobi
// year 2017
// => []

Putting it all together

const app = _.flow (
  _.toPairs,
  _.filter(([key, value]) => console.log(key, value)))

app ({name: "Tobi", year: 2017})
// name Tobi
// year 2017
// => []

Why this is still weird and pointless

Well for starters, the filtering function just performs a console.log so there's really no point to anything we've done here.

I assume the point of the console.log was to just see if filter was even working, but you'll have to ask yourself what it even means to "filter an object".

What sort of output are you even expecting ?
Are we expecting an Object to be returned ?
Are we looking to return an Array of key/value pairs? Probably not, but then what? – maybe look at _.fromPairs that would convert the pairs back to an Object

Anyway, once you answer these questions, maybe you'll have a better idea of how to structure your program

Sensible Guess?

let isDigit = x => !Number.isNaN(Number(x))

// only keep object assignments where the value is a number
let app = _.flow(
  _.toPairs,
  _.filter(([key, value]) => isDigit(value)),
  _.fromPairs)

app ({name: "Tobi", year: 2017})
// => { year: 2017 }

originally I didn't notice you were using `lodash/fp` – i've updated my answer accordingly — Mulan, Feb 16 '17 at 22:12

Lodash FP flow filter key is undefined

1 Answers1