Smelly Code

JS one-liners 😻

March 01, 2019🍵 4 min

I am a huge fan of Arrow functions. They have changed the way we write code. They have cured the cloudy behaviour of this. They let us define functions without function keyword 😅. This article collates one-liners — written using arrow functions — with their explanations.

Remove Duplicates from Array

Interviewer often asks candidates to write a function which removes duplicates from an array. This can be done in many ways. We’ll discuss only those which are relevant to this post.

const removeDuplicates = arr => arr.filter((item, index) => index === arr.indexOf(item));

As you can see we are using filter and indexOf methods of the Array class. We get the index of the item using indexOf method and compare it with the current index. If both are equal then the item is unique else its repeated.

To remove duplicates we can also use Set data structures. We create a set from the array and then convert set back to the array using spread operator or Array.from method.

// Using spread operator. const removeDuplicates1 = array => [ Set(array)]; // Using Array.from method const removeDuplicates2 = array => Array.from(new Set(array));

Flatten Array

Flattening an array means converting a multi-dimension array to one-dimensional array. eg.

// 2-dimensional array.
[[1, 2, 3], [4, 5, 6]]
// After flattening
[1, 2, 3, 4, 5, 6]
// Multi-dimentional array
[1, [2, [3, 4, [5], 6], 7], 8], 9]
// After flattening
[1, 2, 3, 4, 5, 6, 7, 8, 9]

We can use Array.concat method with spread operator to flatten an array.

// Flattens an array(doesn't flatten deeply). const flatten = array => [].concat(...array);

This approach flattens an array at only one level. Below function fully flattens an array.

const flattenDeep = arr => arr.reduce( (fArr, item) => fArr.concat(Array.isArray(item) ? flatten(item) : item), [] );

Merge Arrays

To merge arrays we can use Array.concat method with spread operator.

// Merge arrays const merge = (...arrays) => [].concat(...arrays);

Pipe (Function Composition)

We often need to “pipe” the output of one function as an input to another function. Here’s a simple implementation of the pipe function using Function Composition.

// Pipe fn const pipe = (...fns) => arg => fns.reduce((v, fn) => fn(v), arg); // Example const addTwo = x => x + 2; const double = x => x * 2; const square = x => x * x; const fn = pipe(addTwo, double, square); console.log(fn(1)); // square(double(addTwo(1))) -> 36

Compose(Pipe reverse)

Just like the pipe function, compose also supplies the output of one function as an input to another but it does it in reverse order.

const compose = (...fns) => fns.reduce((a, b) => (...args) => a(b(...args))); // Example const addTwo = x => x + 2; const double = x => x * 2; const square = x => x * x; const fn = compose(addTwo, double, square); console.log(fn(1)); // -> 4 addTwo(double(square(1)))

Range Function

You might have used range function of Lodash. Here’s a one-liner implementation of range using Array.from method. It generate range from start to end (exclusive). It also takes an optional step parameter.

// Generates range [start, end) with step const range = (start, end, step = 1) => Array.from( { length: Math.ceil((end - start) / step) }, (_, i) => start + i * step );

Color Function

Generates a random hex color code.

// Generates random hex color code. const color = () => '#' + Math.floor(Math.random() * 0xffffff) .toString(16) .padEnd(6, '0');


Before we conclude there’s one thing that should be kept it mind while writing one-liners. One-liners do more things with less code, which tempts people to write more one-liners. But sometimes, writing less code makes it less readable and hard to debug. So if you are writing code which you feel should or can be broken into multiple lines. Just go ahead and do it. You’ll thank yourself later 🙂.

One liners are great but one should always make sure that they don’t take a toll on the legibility of code.

Here’s the gist of all the one-liners used in this post. If there’s any one-liner which should be added to the list, please feel free to share. I’ll add it. Thanks for Reading 🙏🏻.

Hi, I am Hitesh.