Go's language notes for JavaScript Devs

These are my personal notes with code samples to explain key concepts of Go's language. I think these notes can be useful for anyone starting to learn Go or even using it as a cheat sheet.

• Variables
• Logs
• Variadic parameters
• Statically typed
• Strings
• Arrays and Slices
• Maps
• Structs
• Embedded structs
• Methods
• Interfaces and Polymorfism
• Anonymous functions
• Pointers
• JSONs

Variables

// Global scope declaration
var x int

func main() {
    // Shorthand declaration (recommended)
    x := 10
    fmt.Println(x) // 10
}

Logs

func main() {
    fmt.Println("Hello from Go!") // Hello from Go
    x := "Hello from Go!"

    // Print with format

    // %q a single-quoted character literal safely escaped with Go syntax.
    fmt.Printf("%q\n", x) // Hello from Go!

    y := 10
    fmt.Printf("%b\n", y) // 1010 (prints number in binary)
    fmt.Printf("%d\n", y) // 10   (prints number in decimal)
    fmt.Printf("%x\n", y) // a    (prints number in hexadecimal)

    // Print the type of the value
    fmt.Printf("%T\n", y) // int
}

Variadic parameters (x...)

Variadic parameter is to what we call in JavaScript spread operator:

func main() {
    numbers(1, 2, 3, 4)
}

func numbers(x ...int) {
    fmt.Println(x) // [1 2 3 4]
    fmt.Printf("%T", x) // []int (it prints the data type)
}

Statically typed

Go is statically typed:

// Statically typed means:
func main() {
    a := 10
    fmt.Printf("%T\n", a) // int

    a = "now is string value" // this will show an error as is not dynamically typed
}

Strings

Strings are immutable

A string is also an array of bytes, this means that you can access to the indexes of the string doing something like this: str[0] and you will get the value of the letter specified in ASCII

If you're more familiarized with JavaScript or Python you'd be expecting to see the letter instead of the ASCII value, there's also a way to get this value:

func main() {
    text := "Hello world"
    fmt.Println(text) // Hello world
    fmt.Println(text[0]) // 72
    fmt.Println(text[1]) // 101
    fmt.Printf("%T\n", text[0]) // uint8 (byte = uint8)

    // Get the letter instead of the ASCII value (by doing a conversion)
    letter := string(text[0])
    fmt.Println(letter) // H

    // You can also make a conversion from string to array of bytes:
    bs := []byte(text)
    fmt.Println(bs) // [72 101 108 108 111 32 119 111 114 108 100]
}

Arrays and Slices

Arrays are usually declared like this: var a [4]int whereas slices are normally declared like this: x := []int{1, 2, 3, 4}

Slices are a bit more flexible than arrays since arrays are limited by a length from the beginning. Slices are built on arrays so slices have all the capabilities of an array and they are more flexible

For more explanation between arrays and slices: https://blog.golang.org/slices-intro

Operations over arrays/slices:

func main() {
    var x [5]int
    // Array's elements are initialized by default depending on the data type
    fmt.Println(x) // [0, 0, 0, 0, 0]

    // Get the length
    len(x) // 5 
}

Slices using composite literals

func main() {
    // this is a composite literal => dataType{}
    x := []int{3, 50 ,1 , 56, 5}

    // you can iterate over arrays/slices using the for-range loop
    for i, v := range x {
        fmt.Println(i, v) // index, value
    }

    // Get a slice from the slice
    fmt.Println(x[1:3]) // [50, 1]

    // Append to a slice
    fmt.Println(append(x, 4, 6)) // [3, 50 ,1 , 56, 5, 4, 6]
    y := []int{1, 2, 3, 4}

    // Append two slices
    fmt.Println(append(x, y...)) // [3, 50 ,1 , 56, 5, 1, 2, 3, 4]

    // Delete an element in the slice (remove the third element)
    append(x[:2], x[3:]...) // [3, 50, 56, 5]
}

Maps

func main() {
    x := map[string]int{
        "Ana": 30,
        "Joe": 40,
    }
    fmt.Println(x) // map[Ana:30 Joe:40]
    fmt.Println(x["Joe"]) // 40
    fmt.Println(x["None"]) // 0 (zero indexed for non-assigned indexes in the Map)

    // To check if the value is actually indexed or not
    v, ok := x["Nah"]
    fmt.Println(v) // 0
    fmt.Println(ok) // false

    // A common idiomatic statement in Go
    if v, ok := x["Joe"]; ok {
        // Do something when it DOES exists
    }

    // Append element to Map
    x["John"] = 40

    // Delete element from Map
    delete(x, "Joe")
    fmt.Println(x) // (Will print without the removed element)
}

Structs

The syntax is very similar to JS literal objects but internally they are a very different thing, in fact, in Go they don't call it objects but struct

Structs is a composite data type

type person struct {
    name string
    age  int
}

func main() {
    x := person{
        name: "John Doe",
        age:  40,
    }
    fmt.Println(x) // {John Doe 40}
    fmt.Println(x.name, x.age) // John Doe 40
}

Embedded structs

type person struct {
    name string
    age  int
}

type student struct {
    alumni person
    school string
}

func main() {
    x := student{
        alumni: person{
            name: "John Doe",
            age:  40,
        },
        school: "MIT",
    }
    fmt.Println(x)
    fmt.Println(x.alumni.name, x.alumni.age)
}

Methods

Go does not have classes. However, you can define methods on types. A method is a function with a special receiver argument. The receiver appears in its own argument list between the func keyword and the method name:

type person struct {
    name string
    age  int
}
// This function will be attached to any variable whose type is person
func (p person) sayHello() string {
    return "Hello, my name is " + p.name
}

func main() {
    x := person{
        name: "John Doe",
        age:  40,
    }
    y := person{
        name: "Ana Doe",
        age:  40,
    }
    fmt.Println(x.sayHello()) // Hello, my name is John Doe
    fmt.Println(y.sayHello()) // Hello, my name is Ana Doe
}

Interfaces and Polymorfism

// TL;DR:
// The type person is also type human
// The type student is also type human

type person struct {
    name string
    age  int
}

type student struct {
    alumni person
    school string
}

func (p person) sayHello() string {
    return "Hello, my name is " + p.name
}

func (s student) sayHello() string {
    return "Hello, my name is " + s.alumni.name
}

// Any type that has a sayHello() function will also be type human
// This means that person and student are also type human
type human interface {
    sayHello() string
}

func humanCanPass(h human) {
    fmt.Println("Hey! I'm a human", h)
}

func main() {
    p := person{
        name: "John Doe",
        age:  40,
    }
    s := student{
        alumni: person{
            name: "Ana Doe",
            age:  30,
        },
        school: "MIT",
    }
    fmt.Println(p.sayHello())
    fmt.Println(s.sayHello())

    humanCanPass(p)
    humanCanPass(s)
}

Anonymous function

// (func expression)
func main() {
    x := func (s string){
        fmt.Println("Hey!", s)
    }

    x("John")
}

Returning a function is also valid:

func main() {
    x := thisIsValid()
    fmt.Println(x("John")) // Hello John

}

func thisIsValid() func(s string) string {
    return func(s string) string {
        return "Hello " + s
    }
}

Pointers

& gives you the address (reference) in memory

* de-references the address to its value stored

func main() {
    x := 21
    fmt.Println(x) // 21
    fmt.Println(&x) // 0xc0000b6020 (of course this will vary on every machine)

    y := &x
    fmt.Println(y) // 0xc0000b6020
    fmt.Println(*y) // 21

    // This is valid too
    fmt.Println(*&x) // 21
}

JSONs

Example of parsing a slice of person struct to a JSON string

import (
    "encoding/json"
    "fmt"
)
// Keys must be capitalized
type person struct {
    Name     string
    LastName string
    Age      int
}

func main() {
    p1 := person{
        Name:     "John",
        LastName: "Doe",
        Age:      40,
    }
    p2 := person{
        Name:     "Ana",
        LastName: "Doe",
        Age:      32,
    }
    people := []person{p1, p2}

    bs, err := json.Marshal(people)

    if err != nil {
        fmt.Println("The error => ", err)
    }

    fmt.Println(string(bs)) // [{"Name":"John","LastName":"D....
}

Example of parsing a JSON string to a person struct slice (The opposite as the prev example)

import (
    "encoding/json"
    "fmt"
)

type person struct {
    Name     string
    LastName string
    Age      int
}

func main() {
    jsonString := `[{"Name":"John","LastName":"Doe","Age":40},{"Name":"Ana","LastName":"Doe","Age":32}]`
    jsonBytes := []byte(jsonString)
    people := []person{}

    err := json.Unmarshal(jsonBytes, &people)

    if err != nil {
        fmt.Println("The error => ", err)
    }

    fmt.Println(people[0].Name) // John
    fmt.Println(people[0].LastName) // Doe
    fmt.Println(people[1].Name) // Ana
}

I will keep updating this post with more topics...