introduction.md

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

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.

Variables in JavaScript can be defined using the const, let or var keyword. A variable can reference different objects over its lifetime when using let or var, but can not be reassigned when using const. For example, myFirstVariable can be defined and redefined many times using the = operator:

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

Constants in JavaScript can only be defined using const. These are meant to be assigned only once. For clarity, on Excercism they must start with capital letters and are normally written in block capitals with words separated by underscores. For example:

const MY_FIRST_CONSTANT = 10

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

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 arguments, and can return a value using the return keyword. Methods are invoked using () syntax.

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

add(1, 3)
// => 4

To make a function, constant, or variable available in other files, they need to be exported using the export keyword. Another file may then import these using the import keyword.

// 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