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Gal Elmalah
Gal Elmalah

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9 Great Tips To Use Javascript Array Methods Like a Pro!

As a developer, I always reflect upon the code I write and read. Through this process, I have collected a bunch of useful tips.
In this post, I'm going to share those tips that relate to array methods.

Although I'm talking about array methods, these tips apply in other situations. Keep them in mind while you code.

The tips

Some are better naming conventions and rules on when to apply them.
Some are little tricks to make your code cleaner.
Most of them are very opinionated 😄.

I tried categorizing them into general and specific tips to make it easier to digest and use as a reference.


📚Table of contents


General tips

Name your functions

Future readers of the code shouldn't have to think about what's that function is doing. Be a nice human and use meaningful names.

const numbers = [1,2,3,4];

 // BAD - I need to think about what this function is doing
numbers.filter(num => num % 2 === 0);

// GOOD - I can read this line and immediately tell what's going on.
const isEven = num => num % 2 === 0);
numbers.filter(isEven); 

Don't pass arguments from one function to another

Array methods call functions that were sent to them with specific arguments. There is no need to explicitly pass those arguments through another function.

const numbers = [1,2,3,4];
const multiplyByTwo = num => num * 2;

// BAD - There is no need to explicitly pass num.
numbers.map(num => multiplyByTwo(num));

// GOOD 
numbers.map(multiplyByTwo); 

Use partial application

Do you need more than whats passed to you by the array method? Use partial application.

don't know what's a partial application? check this out.

const numbers = [1,2,3,4];

// BAD
const multiplyBy = (num, multiplier) => num * multiplier;
numbers.map(num => multiplyBy(num, 2));

const multiplyBy = multiplier => num => num * multiplier;
// GOOD
numbers.map(multiplyBy(2)); 

// GOOD - more verbose
const multiplyByTwo = multiplyBy(2);
numbers.map(multiplyByTwo); 

Break long chains or Assign them to a variable/function

When I see 3 or 4 levels of array methods chained together without anything indicating the result of that chain, I ask myself, Why? Why do I have to go over each line and figure out what the result is going to be?
There are two ways we can solve this.

  1. Break the chain - assign the result of each line to a variable and operate on that variable.
  2. Assign the result to a function or a variable with a meaningful name.

let's say we want to find all employees that are above 18, give them a random bonus and then get the sum of their salaries.

const employees = [{name:"bruce banner", age:21, salary: 1500}, ...];
const isAboveEighteen = (employ) => employ.age > 18;
const addRandomBonus = (employ) => ({...employ, salary: employ.salary*(Math.random() + 1)});
const toSumOfSalaries = (sum, employ) => sum + employ.salary;

// BAD - I need to think how each line effect the next one and what will be the outcome
employees
  .filter(isAboveEighteen)
  .map(addRandomBonus)
  .reduce(toSumOfSalaries);

// Breaking the chain
const aboveEighteenEmployees = employees.filter(isAboveEighteen);
const salariesWithBonus = aboveEighteenEmployees.map(addRandomBonus);
const sumOfSalaries = salariesWithBonus.reduce(toSumOfSalaries);

// Assign the result
// *If we need to reuse this then we would use a function
const aboveEighteenAfterBonusSumOfSalaries = employees
      .filter(isAboveEighteen)
      .map(addRandomBonus)
      .reduce(toSumOfSalaries);


Map tips

When transforming from type A to B use "toB" as the function name and "A" or "fromA" as the function argument

For example, let's say we want to transform error codes into human-readable error messages.

In this example A is errorCode and B is errorMessage.

const errorCodes = [1, 2, 3]; 
const errorCodesMessages = {1: "your code is great!", 2: "your code is awesome!".... };

const toErrorMessage = (fromErrorCode) => errorCodesMessages[fromErrorCode];
errorCodes.map(toErrorMessage);

In this example, it's clear from our code what we intend to do.
toErrorMessage function conveys that we are transforming to B.
Our array should tell us that we are operating on error codes. But, if we screw up the naming of the array, then it's clear from the function argument we are operating on A.

Performing actions

We can use the same convention we used in the above example, but it feels a bit awkward and overly verbose.
Instead, for actions, we will just state the action we are performing.

Let's say we want to add a unique id to an array of users

const users = [{name: "john doe", email: "johnDoe@evilcorp.com"}, ....];
const addId = obj => ({...obj, id: uuid()});
users.map(addId);

Filter tips

Use it when it's the right tool for the job

When is filter the right tool for the job? When you want to get a subset of an array based on some condition.
In other cases, where you want to get a specific member, assert that a condition holds for at least one member or for all of them, use find, some, or every - know your tools and when to use them.

Don't really know these methods? Read about them find, some and every

const peoples = [{ name: "Bruce Wayne", country: "USA", city: "Gotham",... }, ...];

// find
const isBruceWayne = person => person.name === "Bruce Wayne";
peoples.filter(isBruceWayne)[0]; // BAD
peoples.find(isBruceWayne); // GOOD 

// some
const isFromTheUSA = person => person.country === "USA";
// has peoples from USA?
!!peoples.filter(isFromTheUSA)[0];  // BAD
peoples.some(isFromTheUSA);  // GOOD

// every
const isNotFromTheUSA = person => person.country !== "USA";
// everyone from the USA?
!peoples.filter(isNotFromTheUSA)[0]  // BAD
peoples.every(isFromTheUSA)  // GOOD

Make it sound like a question

This one applies to all conditional statements.
If the return value of our function is a boolean i.e true or false, then we should write our function in a way that will read like a question.

const numbers = [1,2,3,4]

// BAD - Reads like a statment
const even = num => num % 2 === 0);
numbers.filter(even);

// GOOD - Reads like a question
const isEven = num => num % 2 === 0);
numbers.filter(isEven); 

Some common prefixes are is, has, should...

Check for multiple conditions in one pass

If you want to check for multiple conditions in one pass, use ramda anyPass and allPass functions when you want to combine multiple conditions while adhering to SRP.

For example, let's say we want to get all the even numbers that are bigger than 10 OR odd numbers that are smaller than 10.

import R from 'ramda';
const numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16];
// This is a simple example but you can imagine how it can get out of control.
const isAValidNumber = num => {
  if (num % 2 === 0 && num > 10) {
    return true;
  }
  if (num % 2 && num < 10) {
    return true;
  }
};

// Good - We've split our logic into small, reusable functions, that do one thing.
const isEven = num => num % 2 === 0;
const isOdd = num => !isEven(num);
const isBiggerThanTen = num => num > 10;
const isSmallerThanTen = num => num < 10;
const isValidNumber = R.anyPass([
  R.allPass([isBiggerThanTen, isEven]),
  R.allPass([isSmallerThanTen, isOdd]),
]);
numbers.filter(isValidNumber);

We added some code, but look at it, it's so clear what we want to achieve!

If you don't like the functional approach or your team is not familiar with ramda, there are other ways to get the same result, like plugging the functions we wrote into the if statements in the first implementation of isValidNumber.

Want to apply multiple functions in one pass using map? check out ramda pipe or compose.


Got any tips to share?

Those were my greatest hits for using array methods.
Got any tips of your own? Found something new or interesting in this post?
Leave a comment and share this post with your fellow developers😄

Discussion (8)

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dglsparsons profile image
Douglas Parsons

Great post. Some really good examples and clear improvements made.

One to be careful with though, is your point on Don't pass arguments from one function to another. It's more verbose, but sometimes you can get nasty surprises with functions taking more arguments than you might think!

For example:

['1', '7', '11'].map(parseInt) 
> [1, NaN, 3]
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Where the more explicit passing stops this weird behaviour.

['1', '7', '11'].map(n => parseInt(n))
> [1, 7, 11]
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This happens because map passes the index of the array you're operating on to parseInt as well as the number, and this totally changes the behaviour.

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galelmalah profile image
Gal Elmalah Author

Thanks, I’m glad you liked it :)
That’s a great point, thanks for pointing it out!

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nikla profile image
Nikla

Good examples and write up. Definitely gave me some takeaway things to think about when using these methods. 👍

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galelmalah profile image
Gal Elmalah Author

Thanks Nikla 😄

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seanitzel profile image
Sean Dvir

Awesome write up, learned some new tricks...

Time for some refactoring :D

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galelmalah profile image
Gal Elmalah Author

Thanks Sean! ya gever!

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zdev1official profile image
ZDev1Official

Amazing!

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galelmalah profile image
Gal Elmalah Author

Thanks!