Pure Functions in JavaScript. Why should we know about them as early as possible?

Problem

I'm 26 years old this year, and I've been maintaining a few projects. Sometimes I come across cases where some of my teammates do things like this:

function convertBirthdayToAges(person) {
  const year = new Date().getFullYear(); // 2021
  return person.map(p => p.age = year - p.year);
}
...  
const persons = [{name: 'Nguyễn Văn A', year: 2000}];
convertBirthdayToAges(persons);
console.log(persons); // [{name: 'Nguyễn Văn A', year: 2000, age: 21}]

At first glance, the above function seems normal, but if you notice, after persons goes through the convertBirthdayToAges function, it is attached with an additional attribute age.

Or another example like this:

let year = 2020;
function afterManyYear(num) {
  return year + num;
}
afterManyYear(5) // 2025;

....  

year = 2025;
afterManyYear(5) // 2030;

In the example above, initially when calling afterManyYear(5), the result is 2025, but later, when year is changed to 2025, afterManyYear(5) now returns 2030.

This may seem normal, but imagine in a maintenance phase where you don't know where year was changed, it can be a disaster. You may argue why not declare year with const: const year = 2020;? Well, I think when they chose to declare with let, they already had it in mind that year could be changed at any time.

If you are someone who regularly does these things and see the inconvenience they bring, then it's time for you to know about the concept of Pure Functions.

What is a Pure Function?

Pure function is exactly what its name suggests: "Pure function".

It is a JS function that satisfies two conditions:

• The same inputs always return the same outputs.
• No side-effects.

The same inputs always return the same outputs

Pretty straightforward. For example:

function add(x, y) {
  return x + y;
}
add(1, 2); // 3
add(1, 2); // 3

For each pair of x,y passed in, the return value never changes.

This function won't satisfy:

let x = 1;
function add(y) {
  return x + y;
}
add(1); // 2
x = 2;
add(1); // 3

When a function guarantees this condition, understanding and debugging become much easier.

No side-effects

Side-effects are the "effects" that come along with a function, such as:

• Changing the value of an input.
• console.log
• HTTP calls (fetch/AJAX).
• Changing a file (fs).
• Querying the DOM.
• …

In general, besides the things listed above, side-effects also include any tasks within a function that are unrelated to the final computation result.

In the example in the opening part, we saw that convertBirthdayToAges modified the value of the input persons. If by any chance, persons loses one of its attributes, then it becomes a hassle.

To solve this issue, instead of directly modifying persons, let's return a new object:

function convertBirthdayToAges(person) {
  const year = new Date().getFullYear();
  return [...person.map(p => p.age = year - p.year)];
}

const persons = [{name: 'Nguyễn Văn A', year: 2000}];
const newPersons = convertBirthdayToAges(persons);
console.log(persons); // [{name: 'Nguyễn Văn A', year: 2000}];

In the above example, I used the spread syntax (…) to create a new array object. Note that it can only create a "shallow" copy of an object. To create a "deep" copy, I recommend using the clone package available on npm.

An application cannot be without side effects

That's right, your application cannot function without including the "effects" listed above, except for extremely simple ones. They cannot function without reading/writing to a database or selecting an element in the DOM. But the important thing is to minimize as much as possible or structure independent side-effect code segments.

For example, an update data function:

// In this example, Person is a sequelize model

// This update function is the only one that directly has side-effects
function update(payload) {
  return Person.update(payload);
}

function updatePerson(body) {
  const name = body.name.trim();
  const year = +body.year;
  return update({ name, year });
}

Summary

Pure functions are not a new concept, but the benefits they bring during the development and maintenance of a product are extremely valuable based on my working experience.

Through the examples above, I hope you realize the benefits of applying pure functions to current and future projects. A slight change in expression can bring many benefits in maintaining code later on.