Practical Swift Introduction for JS Developers

This is a quick-start guide to developing apps using Swift, from the perspective of a front-end developer who knows JavaScript as the main language. I will be comparing both languages, pointing out the similarities and main differences.

This is not meant to teach you everything there is about Swift — that's what the official swift documentation is for. This guide is meant to get you running as fast and easily as possible.

Swift is a programming language built for developing apps for the Apple ecosystem. It’s somewhat similar to C and Objective-C, which doesn’t mean much to me — if you’re in the same position, this guide is for you.

For this quick-start guide, I will assume you already know modern JavaScript fairly well. I will constantly compare JavaScript and Swift, just to make it easier for us JS-folk.

Basic syntax

  • Comments work like in JavaScript. // starts an inline comment, /* ... */ defines a multi-line one.

    • Multiline comments can be nested.
  • Semicolons aren’t necessary or common.

Variables and Constants


Variables and constants have to me declared before they can be used. The keywords used for declaring them are sort of opposite to JavaScript's.

  • Variables are defined with var.
  • Constants are defined with let.

If a value isn’t going to change, always declare it as a constant.

var myVariable = 10
myVariable = 11
myVariable = "String" // Error!

let myConstant = 10
myConstant = 11       // Error!

Multiple constants or variables can be declared at once.

var a = 1, b = 2, c = 3
let a = 1, b = 2, c = 3


Variable and constant names are very free. They can contain most characters, including emojis and non-latin characters. They cannot start with a number.


Like JavaScript, swift has type inference, which figures out which type to give your variable when you declare it with a value.

let two  = 2        // inferred to be an Int
let pi   = 3.14     // inferred to be a Double
let name = "Snoopy" // inferred to be a String

Swift has two main types of numbers: Integers and Doubles. Integers are whole numbers, while Doubles have decimal points. Math can only happen when all the values are of the same type: If we need to calculate an Integer and a Double together, one of them will have to be converted so they match.

There are other types of numbers in Swift, but no need to discuss them now!

Swift is type-safe. It’s important to know that trying to change a variable's type in this manner will cause an error.

welcome = "Welcome!"
welcome = "Hi!"
welcome = 1 // Error! At compile time.

Empty variables

If you need to declare an empty variable, you have to declare its type. This is called a type annotation.

var welcome: String

Type conversion

Types are never implicitly converted.

let text = "Slices on a medium pizza: "
let slices = 8
let pizzaSize = text + slices          // Error!
let pizzaSize = text + String(slices)  // Works

Variables in strings

However, there's an easier way to do this: The \() syntax. It allows us to add a variable, or a math operation, inside a string declaration. Like JavaScript's backtick syntax with ${}.

let mediumPizza = 8
let largePizza = 12
let mediumStatement = "A medium pizza has \(mediumPizza) slices."
// A medium pizza has 8 slices
let myPizza = "I have \(mediumPizza + largePizza) slices of pizza."
// I have 20 slices of pizza.

That's a lot of pizza!

Multi-line strings

Multiline strings are declared with """String""".

let quote = """
“You have power over your mind - not outside events.
Realize this, and you will find strength.”
— Marcus Aurelius, Meditations

Arrays and Dictionaries

Dictionaries are objects


var shoppingList = ["tea", "crackers", "tulips"]

shoppingList[0] = "coffee"

// shoppingList is now
// ["coffee", "crackers", "tulips"]


// shoppingList is now
// ["coffee", "crackers", "tulips", "asparagus"]


var priceList = [
   "tea": 4.99,
   "crackers": 2.50,
   "tulips": 4.00,
   "coffee": 5.99,

priceList["asparagus"] = 2.00

// priceList is now
// [
//    "tea: 4.99,
//    "crackers": 2.50,
//    "tulips": 4.00,
//    "coffee": 5.99,
//    "asparagus": 2.00,
// ]

Empty arrays and dictionaries

var emptyArray = [String]()
var emptyDictionary = [String: Float]()


Think of tuples like groups of information, somewhere in between arrays and objects in JavaScript. They contain related values, of any type.

let http404Error = (404, "Not Found)
// Tuple type (Int, String)

Tuples can be decomposed:

Let (statusCode, statusMessage) = http404Error
print("The status code is \(statusCode)") // 404

print() is the equivalent of JavaScript's console.log().

An element inside a tuple can be accessed using index numbers (like JavaScript arrays):

print("The status code is \(http404error.0)") // 404

Elements in a tuple can be named when declaring it:

let http200Status = (statusCode: 200, description: "OK")

The elements can then be accessed using their names (like JavaScript objects):

print("The status code is \(https200Status.statusCode)") // 200

Tuples are not recommended for complex data structure. Use a class or structure instead.

Basic functionality


  • if
  • switch


  • for-in
  • while
  • repeat-while

The condition or loop doesn't need parentheses. The body must be surrounded by braces.

let individualScores = [75, 43, 103, 87, 12]
var teamScore = 0
for score in individualScores {
   if score > 50 {
      teamScore += 3
   } else {
      teamScore += 1
// Prints "11"