introduction.md

Introduction

JavaScript is a dynamic language, supporting object-oriented, imperative, and declarative (e.g. functional programming) styles.

(Re-)Assignment

There are a few primary ways to assign values to names in JavaScript - using variables or constants. On Exercism, variables are always written in camelCase; constants are written in SCREAMING_SNAKE_CASE. There is no official guide to follow, and various companies and organizations have various style guides. Feel free to write variables any way you like. The upside from writing them the way the exercises are prepared is that they'll be highlighted differently in the web interface and most IDEs.

Variables in JavaScript can be defined using the const, let or var keyword.

A variable can reference different values over its lifetime when using let or var. For example, myFirstVariable can be defined and redefined many times using the assignment operator =:

let myFirstVariable = 1;
myFirstVariable = 'Some string';
myFirstVariable = new SomeComplexClass();

In contrast to let and var, variables that are defined with const can only be assigned once. This is used to define constants in JavaScript.

const MY_FIRST_CONSTANT = 10;

// Cannot be re-assigned.
MY_FIRST_CONSTANT = 20;
// => TypeError: Assignment to constant variable.

💡 In a later Learning Exercise, the difference between constant assignment / binding and constant value is explored and explained.

Function Declarations

In JavaScript, units of functionality are encapsulated in functions, usually grouping functions together in the same file if they belong together. These functions can take parameters (arguments), and can return a value using the return keyword. Functions are invoked using () syntax.

function add(num1, num2) {
  return num1 + num2;
}

add(1, 3);
// => 4

💡 In JavaScript there are many different ways to declare a function. These other ways look different than using the function keyword. The track tries to gradually introduce them, but if you already know about them, feel free to use any of them. In most cases, using one or the other isn't better or worse.

Exposing to Other Files

To make a function, a constant, or a variable available in other files, they need to be exported using the export keyword. Another file may then import these using the import keyword. This is also known as the module system. A great example is how all the tests work. Each exercise has at least one file, for example lasagna.js, which contains the implementation. Additionally, there is at least one other file, for example lasagna.spec.js, that contains the tests. This file imports the public (i.e. exported) entities to test the implementation:

// file.js
export const MY_VALUE = 10;

export function add(num1, num2) {
  return num1 + num2;
}

// file.spec.js
import { MY_VALUE, add } from './file';

add(MY_VALUE, 5);
// => 15