Higher Order Functions in ES6: Easy as a => b => c;

Originally published at StrongBlog

ES6 is nigh! As more and more libraries & Thought Leaders start incorporating ES6 into their code, what used to be nice-to-know ES6 features are becoming required knowledge. And it’s not just new syntax – in many cases, new language features can make an expression that was cumbersome to write in ES5 easy in ES6, enabling and encouraging the use of this type of expression. We’re going to look at one such case here: how arrow functions make it easier to write higher-order functions in ES6.

A higher order function is a function that does one or both of the following:

  1. takes one or more functions as arguments
  2. returns a function as its result.

The purpose of this post is not to convince you to adopt this new style right away, although I certainly encourage you to give it a try! The purpose is to familiarize you with this style, so that when you run into it in someone’s ES6-based library, you won’t sit scratching your head wondering what you’re looking at as I did the first time I saw it. If you need a refresher in arrow syntax, check out this post first.

Hopefully you’re familiar with arrow functions that return a value:

const square = x => x * x;

square(9) === 81; // true

But what’s going on in the code below?

const has = p => o => o.hasOwnProperty(p);
const sortBy = p => (a, b) => a[p] > b[p];

What’s this “p returns o returns o.hasOwnProperty…”? How can we use has?

Understanding the syntax

To illustrate writing higher order functions with arrows, let’s look at a classic example: add. In ES5 that would look like this:

function add(x){
  return function(y){
    return y + x;
  };
}

var addTwo = add(2);
addTwo(3);          // => 5
add(10)(11);        // => 21

Our add function takes x and returns a function that takes y which returns y + x. How would we write this with arrow functions? We know that…

  1. an arrow function definition is an expression, and
  2. an arrow function implicitly returns the results of a single expression

…so all we must do is make the body of our arrow function another arrow function, thus:

const add = x => y => y + x;
// outer function: x => [inner function, uses x]
// inner function: y => y + x;

Now we can create inner functions with a value bound to x:

const add2 = add(2);// returns [inner function] where x = 2
add2(4);            // returns 6: exec inner with y = 4, x = 2
add(8)(7);          // 15

Our add function isn’t terribly useful, but it should illustrate how an outer function can take an argument (x) and reference it in a function it returns.

Sorting our users

So you’re looking at an ES6 library on github and encounter code that looks like this:

const has = p => o => o.hasOwnProperty(p);
const sortBy = p => (a, b) => a[p] > b[p];

let result;
let users = [
  { name: 'Qian', age: 27, pets : ['Bao'], title : 'Consultant' },
  { name: 'Zeynep', age: 19, pets : ['Civelek', 'Muazzam'] },
  { name: 'Yael', age: 52, title : 'VP of Engineering'}
];

result = users
  .filter(has('pets'))
  .sort(sortBy('age'));

What’s going on here? We’re calling the Array prototype’s sort and filter methods, each of which take a single function argument, but instead of writing function expressions and passing them to filter and sort, we’re calling functions that return functions, and passing those to filter and sort.

Let’s take a look, with the expression that returns a function underlined in each case.

Without higher order functions

result = users
  .filter(x => x.hasOwnProperty('pets')) //pass Function to filter
  .sort((a, b) => a.age > b.age);        //pass Function to sort

With higher order functions

result = users
  .filter(has('pets'))  //pass Function to filter
  .sort(sortBy('age')); //pass Function to sort

In each case, filter is passed a function that checks if an object has a property called “pets.”

Why is this useful?

This is useful for a few reasons:

Imagine we want only users with pets and with titles. We could add another function in:

result = users
  .filter(x => x.hasOwnProperty('pets'))
  .filter(x => x.hasOwnProperty('title'))
  ...

The repetition here is just clutter: it doesn’t add clarity, it’s just more to read and write. Compare with the same code using our has function:

result = users
  .filter(has('pets'))
  .filter(has('title'))
  ...

This is shorter and easier to write, and that makes for fewer typos. I consider this code to have greater clarity as well, as it’s easy to understand its purpose at a glance.

As for reuse, if you have to filter to pet users or people with job titles in many places, you can create function to do this and reuse them as needed:

const hasPets = has('pets');
const isEmployed = has('title');
const byAge = sortBy('age');

let workers = users.filter(isEmployed);
let petOwningWorkers = workers.filter(hasPets);
let workersByAge = workers.sort(byAge);

We can use some of our functions for single values as well, not just for filtering arrays:

let user = {name: 'Assata', age: 68, title: 'VP of Operations'};
if(isEmployed(user)){   // true
  //do employee action
}
hasPets(user);          // false
has('age')(user);       //true

A Step Further

Let’s make a function that will produce a filter function that checks that an object has a key with a certain value. Our has function checked for a key, but to check value as well our filter function will need to know two things (key and value), not just one. Let’s take a look at one approach:

//[p]roperty, [v]alue, [o]bject:
const is = p => v => o => o.hasOwnProperty(p) && o[p] == v;

// broken down:
// outer:  p => [inner1 function, uses p]
// inner1: v => [inner2 function, uses p and v]
// inner2: o => o.hasOwnProperty(p) && o[p] = v;

So our new function called “is” does three things:

  1. Takes a property name and returns a function that…
  2. Takes a value and returns a function that…
  3. Takes an object and tests whether the object has the property specified with the value specified, finally returning a boolean.

Here is an example of using this is to filter our users:

const titleIs = is('title');
// titleIs == v => o => o.hasOwnProperty('title') && o['title'] == v;

const isContractor = titleIs('Contractor');
// isContractor == o => o.hasOwnProperty('title') && o['title'] == 'Contractor';

let contractors = users.filter(isContractor);
let developers  = users.filter(titleIs('Developer'));

let user = {name: 'Viola', age: 50, title: 'Actress', pets: ['Zak']};
isEmployed(user);   // true
isContractor(user); // false

A note on style

Scan this function, and note the time it takes you to figure out what’s going on:

const i = x => y => z => h(x)(y) && y[x] == z;

Now take a look at this same function, written slightly differently:

const is = prop => val => obj => has(prop)(obj) && obj[prop] == val;

There is a tendency when writing one line functions to be as terse as possible, at the expense of readability. Fight this urge! Short, meaningless names make for cute-looking, hard to understand functions. Do yourself and your fellow coders a favor and spend the extra few characters for meaningful variable and function names.

One more thing. . .

What if you want to sort by age in descending order rather than ascending? Or find out who’s not an employee? Do we have to write new utility functions sortByDesc and notHas? No we do not! We can wrap our functions, which return Booleans, with a function that inverts that boolean, true to false and vice versa:

//take args, pass them thru to function x, invert the result of x
const invert = x => (...args) => !x(...args);
const noPets = invert(hasPets);

let petlessUsersOldestFirst = users
  .filter(noPets)
  .sort(invert(sortBy('age')));

Conclusion

Functional programming has been gaining momentum throughout the programming world and ES6 arrow functions make it easier to use this style in JavaScript. If you haven’t encountered FP style code in JavaScript yet, it’s likely you will in the coming months. This means that even if you don’t love the style, it’s important to understand the basics of this style, some of which we’ve gone over here. Hopefully the concepts outlined in this post have helped prepare you for when you see this code in the wild, and maybe even inspired you to give this style a try!

Comments

I think you did a very good (difficult to do a great) job, particularly with your examples. Your post motivates me to dive deeper into FP.

- Brent Enright,

Thanks Brent!! Feedback like this makes my day!

Thanks. This gave me a bit to think about and mull over. It's not something that I have spent a lot of time on, but it's pretty important as I try to better grok the functional programming thing.

- Brian,

Nice!! FP is fun, I hope you keep it up!

type IPropertyKey = string | number | symbol;
const h = Object.prototype.hasOwnProperty;
export const hasOwnProperty = (obj, property: IPropertyKey): boolean => h.call(obj, property);

Another reason to use Object.prototype.hasOwnProperty.call() https://github.com/jquery/jquery/issues/4665

More and more people are using const a = Object.create(null); a.something = 'abc'. There is no prototype... so hasOwnProperty is undefined there. But you can use Object.prototype. hasOwnProperty.call(a, 'something');

- Darcy,

That is a good point and one I had not considered. Your comment should serve as ample warning to those considering copy/pasting from this post pell-mell, methinks. Thank you for pointing this out! Here's the JavaScript version for readers not versed in Typescript:

const hasOwnProperty = (obj, propname) =>
  Object.prototype.hasOwnProperty.call(object, propname);

Darcy: why are "more and more people" using a = Object.create(null)? Is there some percieved performance benefit? Strikes me as a bit... well it would certainly be better if it weren't necessary.

Follow-up from Darcy:

To answer your question about why more and more people are using Object.create(null):

Object.create() is useful for some types of prototype composition. (ES6 class provides nicer syntax for many types of composition... but there is still use for Object.create() for special cases. mixins being one example.) One use case for Object.create(null) is for when you don't want OOTB methods from Object.prototype on your object. But I wouldn't do so without thinking about the consequences first. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/create has some discussion on it.

A popular use case is when the object is being used as a hash map where keys are strings (and ES6 Map is not available). This is what JQuery was doing (it was used as a cache). By having no prototype, the cache is safe for keys like 'constructor', and other values on prototype chain. In some use cases the object may store user generated keys and someone could create a key like 'hasOwnProperty' that blocks the 'hasOwnProperty' method on prototype chain. In this case, if cache.hasOwnProperty() is called, it would throw an error. Of course it really depends on the use cases of the hash map. If you know there won't be any key collisions, it does not matter.

Performance is arguably better too because with a null prototype, there is one less prototype in the chain to resolve a key's value. And it could help prevent the need for even using hasOwnProperty in some cases. But I don't think performance is really the motivating reason for Object.create(null). JS engines are pretty fast (relative to other work) at resolving a value by looking up a key in prototype chain.

Thank you for the follow-up, Darcy! As I suspected, Object.create(null) is being used for "something weird" (to wit: trying to create an object that behaves like Map where Map is not available). Regarding performance, I will quibble slightly here and insist that unless one has actually tested and measured the performance impact, this is not work doing "for performance" (I know you're not strictly suggesting it but still).

  1. Some things we think improve performance actually make no difference
  2. The JS JITC & engine are good at optimizing code (so we don't need to do so much manually and may even get in the way of it's optimizations) and
  3. The performance benefit must be weighed against the cost, such as, in this case, breaking normal object behavior (to wit: .hasOwnProperty is not a function).

Thank you for the suggestion and the follow-up!!

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