Register to get access to free programming courses with interactive exercises

Dispatch functions by key JS: Polymorphism

Dispatching by associative array key of is more powerful than it might seem at first sight. It is not always enough to select different data; sometimes you have to execute other code.

Suppose we need to generate an array with the last names of our users. The main condition is that for girls, we take their maiden name.

Let's implement this task using good old if statements:

const lastNames = users.map((user) => {
  if (user.gender === 'female') {
    return user.maidenName;
  }
  return user.lastName;
});

Now let's solve this problem by dispatching by object keys, but unlike in the previous lesson, the values will be functions:

// Define a key-value, where the key is a gender and the value is a function,
// which returns the desired value for the gender
const mapping = {
  female: (user) => user.maidenName,
  male: (user) => user.lastName,
};

// Select the desired function by the name of the gender and call it
// The user is passed to the function
const lastNames = users.map((user) => mapping[user.gender](user));

In some situations, this solution is better. It clearly shows dependencies, especially if there are a lot of them. In addition, data configuration allows you to take it elsewhere and extend the behavior without rewriting the code that uses that data.

Another example is parsing data depending on its type:

import yaml from 'js-yaml';

const mapping = {
  yml: yaml.load,
  json: JSON.parse
};

const parse = (type, data) => mapping[type](data);

Recommended materials

  1. An example of using dispatch by key in Hexlet projects

Hexlet Experts

Are there any more questions? Ask them in the Discussion section.

The Hexlet support team or other students will answer you.

About Hexlet learning process

For full access to the course you need a professional subscription.

A professional subscription will give you full access to all Hexlet courses, projects and lifetime access to the theory of lessons learned. You can cancel your subscription at any time.

Get access
130
courses
1000
exercises
2000+
hours of theory
3200
tests

Sign up

Programming courses for beginners and experienced developers. Start training for free

  • 130 courses, 2000+ hours of theory
  • 1000 practical tasks in a browser
  • 360 000 students
By sending this form, you agree to our Personal Policy and Service Conditions

Our graduates work in companies:

<span class="translation_missing" title="translation missing: en.web.courses.lessons.registration.bookmate">Bookmate</span>
<span class="translation_missing" title="translation missing: en.web.courses.lessons.registration.healthsamurai">Healthsamurai</span>
<span class="translation_missing" title="translation missing: en.web.courses.lessons.registration.dualboot">Dualboot</span>
<span class="translation_missing" title="translation missing: en.web.courses.lessons.registration.abbyy">Abbyy</span>
Suggested learning programs

From a novice to a developer. Get a job or your money back!

Frontend Developer icon
Profession
beginner
Development of front-end components for web applications
start anytime 10 months

Use Hexlet to the fullest extent!

  • Ask questions about the lesson
  • Test your knowledge in quizzes
  • Practice in your browser
  • Track your progress

Sign up or sign in

By sending this form, you agree to our Personal Policy and Service Conditions
Toto Image

Ask questions if you want to discuss a theory or an exercise. Hexlet Support Team and experienced community members can help find answers and solve a problem.