This cheat sheet is based on Go 1.26.
Table of Contents
- Program structure
- Variables, constants, and types
- Control flow
- Functions
- Arrays, slices, and maps
- Structs, methods, and tags
- Interfaces
- Errors
- Goroutines, channels, and synchronization
- Context
- Generics
- Iterators
- Testing
- Standard-library quick reference
- Tips and common mistakes
- Modules and packages
- Further reading
Program structure
package main // Every executable must be in package main.
import (
"fmt"
"os"
)
func main() {
fmt.Println("Hello, Go 1.26")
os.Exit(0)
}
- Every Go source file starts with a
packagedeclaration. - An executable program uses
package mainand amainfunction. - Group standard-library and third-party imports separately.
Variables, constants, and types
Variable declarations and inference
// var can infer the type from an initializer.
var name = "Gopher" // string
var age int // No initializer: specify a type. Zero value: 0.
var id int64 = 42 // Use an explicit type when int is not what you want.
// Short declaration: inside functions only.
city := "Tokyo"
// Declare multiple variables in a block.
var (
x, y int = 1, 2
msg string
)
Use var name = "Gopher" when an initializer makes the type clear. Without an initializer, write the type explicitly. A := declaration is only valid inside a function, and at least one name on its left side must be new.
Types and zero values
| Category | Types | Zero value |
|---|---|---|
| Numbers | integers, floats, complex numbers |
0 (0 + 0i for complex) |
| Strings | string |
"" |
| Booleans | bool |
false |
| Arrays | [N]T |
every element is T's zero value |
| Structs | struct |
every field has its zero value |
| Reference-containing types | pointers, slices, maps, channels, functions, interfaces | nil |
| Defined types | e.g. type UserID int
|
the underlying type's zero value |
| Category | Types | Notes |
|---|---|---|
| Boolean | bool |
true or false
|
| String | string |
immutable sequence of bytes (usually UTF-8, but invalid UTF-8 is allowed) |
| Signed integers |
int, int8, int16, int32, int64
|
int is platform-sized (32 or 64 bit) |
| Unsigned integers |
uint, uint8, uint16, uint32, uint64, uintptr
|
uintptr holds a pointer's raw bits |
| Aliases |
byte, rune
|
aliases for uint8 and int32
|
| Floating point |
float32, float64
|
IEEE 754 |
| Complex |
complex64, complex128
|
float32 or float64 real and imaginary parts |
var i8 int8 = 127 // -128 to 127
var f64 float64 = 3.141592653589793
var c complex128 = 1 + 2i
s := "こんにちは"
runes := []rune(s) // Unicode code points
bytes := []byte(s) // raw bytes
// rune = int32 alias for a Unicode code point
var ch rune = '🐹'
Conversion and constants
i := 42
f := float64(i) // Conversions are always explicit; no implicit coercion.
text := fmt.Sprintf("%d", i) // Number to string.
n, err := strconv.Atoi("123") // String to number.
const Pi = 3.14159
// iota: auto-incremented integer constant.
type Weekday int
const (
Sunday Weekday = iota // 0
Monday // 1
Tuesday // 2
)
// Bit-flag pattern.
type Permission uint
const (
Read Permission = 1 << iota // 1
Write // 2
Execute // 4
)
Pointers
x := 10
p := &x // Take the address of x.
*p = 20 // Dereference the pointer.
fmt.Println(x) // 20
p1 := new(int) // *int; the pointed-to value is 0.
// Go 1.26+: new accepts an expression as well as a type.
// This does not compile with Go 1.25 or earlier.
p2 := new(42) // *int; the pointed-to value is 42.
p3 := new(Person{Name: "Alice"}) // *Person initialized with a literal.
Control flow
if
// An initialization statement is scoped to this if/else chain.
if err := doSomething(); err != nil {
return fmt.Errorf("do something: %w", err)
}
if x > 0 {
fmt.Println("positive")
} else if x < 0 {
fmt.Println("negative")
} else {
fmt.Println("zero")
}
for
// C-style loop.
for i := 0; i < 5; i++ {
}
// Go has no while keyword; use for instead.
for n > 0 {
n--
}
// Infinite loop.
for {
if done {
break
}
}
// range returns an index and a value for slices and arrays.
for i, v := range []int{10, 20, 30} {
fmt.Println(i, v) // 0 10 / 1 20 / 2 30
}
// Discard the index.
for _, v := range slice {
}
// Strings range over Unicode code points (runes), not bytes.
for i, r := range "日本語" {
fmt.Printf("%d: %c\n", i, r)
}
// Maps.
for k, v := range m {
fmt.Println(k, v) // Iteration order is unspecified.
}
switch
// Expression switch (fallthrough requires an explicit keyword).
switch os := runtime.GOOS; os {
case "darwin":
fmt.Println("macOS")
case "linux":
fmt.Println("Linux")
default:
fmt.Println("Other")
}
// Condition-less switch is a cleaner alternative to a long if-else chain.
switch {
case x > 100:
fmt.Println("big")
case x > 10:
fmt.Println("medium")
default:
fmt.Println("small")
}
// Type switch: identify the dynamic type of an interface value.
func describe(i any) {
switch v := i.(type) {
case int:
fmt.Printf("int: %d\n", v)
case string:
fmt.Printf("string: %q\n", v)
default:
fmt.Printf("unknown: %T\n", v)
}
}
defer
defer schedules a call to run just before the surrounding function returns. Use it for cleanup: closing files, response bodies, mutexes, and contexts. Deferred calls run in LIFO order.
func writeFile(path string) error {
f, err := os.Create(path)
if err != nil {
return err
}
defer f.Close() // Runs when writeFile returns, regardless of how.
defer fmt.Println("third")
defer fmt.Println("second")
defer fmt.Println("first")
// Prints: first, second, third.
return nil
}
Note
Deferred call arguments are evaluated whendeferis encountered. In a long-running loop, a deferred cleanup does not run until the function returns, so resources accumulate. Close them promptly or move the loop body into a helper function.
Functions
Basic
// Multiple return values.
func divide(a, b float64) (float64, error) {
if b == 0 {
return 0, errors.New("division by zero")
}
return a / b, nil
}
result, err := divide(10, 3)
// Named return values (useful as documentation for short functions).
func minMax(nums []int) (min, max int) {
min, max = nums[0], nums[0]
for _, n := range nums[1:] {
if n < min {
min = n
}
if n > max {
max = n
}
}
return // Naked return.
}
Variadic functions
func sum(nums ...int) int {
total := 0
for _, n := range nums {
total += n
}
return total
}
sum(1, 2, 3)
nums := []int{1, 2, 3}
sum(nums...) // Spread a slice into variadic arguments.
First-class functions and closures
Functions are values in Go: you can assign them to variables, pass them as arguments, and return them. A closure captures variables from its surrounding scope and keeps them alive after the outer function returns.
// Assign a function to a variable.
add := func(a, b int) int { return a + b }
// Pass a function as an argument.
apply := func(f func(int, int) int, x, y int) int {
return f(x, y)
}
fmt.Println(apply(add, 3, 4)) // 7
// This closure captures n.
func counter() func() int {
n := 0
return func() int {
n++
return n
}
}
c := counter()
c() // 1
c() // 2
init
An init function runs automatically while a package is initialized, before main. A package may have more than one init function. Prefer explicit initialization in main for ordinary application setup; reserve init for package-level registration.
// Runs automatically at package initialization.
func init() {
// Register a driver or handler.
}
Arrays, slices, and maps
Memory layout
Array (fixed length, value type)
┌───┬───┬───┬───┬───┐
│ 0 │ 1 │ 2 │ 3 │ 4 │ ← contiguous memory (placement decided by the compiler)
└───┴───┴───┴───┴───┘
Slice (variable-length view)
┌────────┬──────┬──────┐
│ ptr │ len │ cap │ ← 3-word slice header
└────┬───┴──────┴──────┘
↓
┌───┬───┬───┬───┬───┬───┐
│ 0 │ 1 │ 2 │ 3 │ │ │ ← underlying array
└───┴───┴───┴───┴───┴───┘
↑ ↑
len=4 cap=6
Note: actual placement (stack/heap) is decided by escape analysis.
Arrays
Arrays have a fixed length, and that length is part of their type: [3]int and [4]int are different types. Arrays are values, so assigning or passing one copies all its elements.
var a [3]int // [0 0 0]
b := [3]int{1, 2, 3}
c := [...]int{4, 5, 6} // Length inferred from the literal.
// Arrays are copied by value.
d := b
d[0] = 99 // b is unchanged.
Slices
A slice is a variable-length view over part or all of an underlying array. Unlike an array, its length is not part of its type. Assigning a slice copies only its header, so two slices may share elements.
// Create a slice.
s := []int{1, 2, 3}
s2 := make([]int, 5) // len=5, cap=5
s3 := make([]int, 3, 10) // len=3, cap=10
// append may allocate a new backing array when cap is exceeded.
s = append(s, 4, 5)
s = append(s, []int{6, 7}...)
// Slicing shares the backing array.
sub := s[1:3] // s[1] and s[2]; shares s's array.
limited := s[1:3:3] // Third index limits the resulting cap.
// copy produces an independent slice.
clone := make([]int, len(s))
copy(clone, s)
// Delete element at index i while preserving order.
i := 2
s = append(s[:i], s[i+1:]...)
fmt.Println(len(s), cap(s))
Caution
A subslice shares its backing array with the original. Mutating one can mutate the other. Usecopywhen an independent slice is required.
Maps
Maps associate keys with values. Iteration order is unspecified. Keys must be comparable; slices, maps, and functions cannot be map keys.
// Create a map.
m := map[string]int{"a": 1, "b": 2}
m2 := make(map[string]int)
m["c"] = 3
delete(m, "a") // delete is a built-in function.
// Check for key existence.
v, ok := m["missing"] // ok is false when the key is absent.
if ok {
fmt.Println(v)
}
// Iteration order is unspecified.
for k, v := range m {
fmt.Println(k, v)
}
Caution
The zero value of a map isnil. Reading from a nil map is safe, but writing to one panics. Initialize it withmakeor a map literal first.
Structs, methods, and tags
Structs and embedding
type Person struct {
Name string // Exported: accessible from other packages.
Age int
addr string // Unexported: package-private.
}
// Named-field initialization (preferred).
p1 := Person{Name: "Alice", Age: 30}
p2 := new(Person) // *Person; all fields are zero values.
// Embedding promotes fields and methods.
type Employee struct {
Person // Fields and methods of Person are promoted.
Department string
}
e := Employee{Person: Person{Name: "Carol", Age: 28}, Department: "Eng"}
fmt.Println(e.Name) // Transparent access to Person.Name.
Methods
// Value receiver: p is a copy of the caller.
func (p Person) Greet() string {
return "Hi, I'm " + p.Name
}
// Pointer receiver: can modify the original Person.
func (p *Person) Birthday() {
p.Age++
}
p := Person{Name: "Dave", Age: 20}
p.Birthday() // Go automatically passes &p.
fmt.Println(p.Age) // 21
When to use pointer receivers
- The method needs to modify the receiver → pointer receiver.
- The struct is large (avoid copy overhead) → pointer receiver.
- Otherwise, either works.
Keep receiver kinds consistent across all methods of one type.
Struct tags
Struct tags are metadata attached to fields. The language does not interpret them; packages such as encoding/json and ORMs read them through reflection.
type User struct {
ID int `json:"id" db:"user_id"`
Email string `json:"email,omitempty"` // Omitted from JSON when empty.
}
// encoding/json reads tags through reflection.
data, _ := json.Marshal(User{ID: 1, Email: ""})
// → {"id":1} (Email omitted by omitempty)
Interfaces
An interface defines a set of methods. A type satisfies an interface implicitly — no implements keyword. This lets code depend on behavior rather than a concrete type.
interface value layout (the internal representation is not part of the spec)
var s Shape = Circle{Radius: 5}
s (interface value)
┌──────────────┬──────────────┐
│ type info │ value/ptr │
│ (Circle) │ {Radius:5} │
└──────────────┴──────────────┘
If either part is nil while the other is not, the interface value itself is not nil.
(See nil interface trap in Tips.)
type Shape interface {
Area() float64
Perimeter() float64
}
type Circle struct{ Radius float64 }
func (c Circle) Area() float64 { return math.Pi * c.Radius * c.Radius }
func (c Circle) Perimeter() float64 { return 2 * math.Pi * c.Radius }
// Circle implicitly satisfies Shape.
var s Shape = Circle{Radius: 5}
fmt.Println(s.Area())
Type assertions and type switches
// Safe type assertion; the single-result form panics on failure.
c, ok := s.(Circle)
if ok {
fmt.Println("radius:", c.Radius)
}
// any is an alias for interface{} (Go 1.18+).
func printAny(v any) {
fmt.Printf("%T: %v\n", v, v)
}
Small interfaces used everywhere
// io.Reader and io.Writer are the most widely used interfaces in the stdlib.
type Reader interface { Read(p []byte) (n int, err error) }
type Writer interface { Write(p []byte) (n int, err error) }
// Implementing fmt.Stringer controls how fmt.Println prints a value.
type Stringer interface { String() string }
func (p Person) String() string {
return fmt.Sprintf("%s (%d)", p.Name, p.Age)
}
fmt.Println(Person{Name: "Eve", Age: 22}) // Eve (22)
Errors
// error is a built-in interface: type error interface { Error() string }
f, err := os.Open("file.txt")
if err != nil {
return fmt.Errorf("open file: %w", err) // %w wraps err for errors.Is/As.
}
defer f.Close()
Custom errors
type NotFoundError struct {
Resource string
ID int
}
func (e *NotFoundError) Error() string {
return fmt.Sprintf("%s (id=%d) not found", e.Resource, e.ID)
}
func findUser(id int) (*User, error) {
if id <= 0 {
return nil, &NotFoundError{Resource: "user", ID: id}
}
// ...
return nil, nil
}
errors package
import "errors"
// Sentinel error for errors.Is comparisons.
var ErrNotFound = errors.New("not found")
func fetchUser(id int) (*User, error) {
if id <= 0 {
return nil, fmt.Errorf("fetchUser: %w", ErrNotFound) // Wrap with %w.
}
return nil, nil
}
_, err := fetchUser(-1) // fetchUser returns (*User, error); discard the value.
// errors.Is traverses the error chain to find a matching sentinel.
if errors.Is(err, ErrNotFound) {
fmt.Println("not found")
}
// errors.As traverses the chain and extracts a value of the target type.
func findItem(id int) error {
return fmt.Errorf("findItem: %w", &NotFoundError{Resource: "item", ID: id})
}
var nfe *NotFoundError
if errors.As(findItem(42), &nfe) {
fmt.Println("resource:", nfe.Resource) // "item"
}
// Go 1.26+: a type-safe generic alternative to errors.As.
if nfe, ok := errors.AsType[*NotFoundError](findItem(42)); ok {
fmt.Println("resource:", nfe.Resource) // "item"
}
// errors.Join combines multiple errors into one (Go 1.20+).
err1 := errors.New("err1")
err2 := errors.New("err2")
combined := errors.Join(err1, err2)
panic and recover
Use panic for broken program invariants or truly unrecoverable situations; return error for expected failures. recover only works when called directly from a deferred function.
func mustPositive(n int) int {
if n <= 0 {
panic(fmt.Sprintf("must be positive, got %d", n))
}
return n
}
func safeDiv(a, b int) (result int, err error) {
defer func() {
if r := recover(); r != nil {
err = fmt.Errorf("recovered: %v", r)
}
}()
return a / b, nil
}
Goroutines, channels, and synchronization
GMP model (Go runtime scheduler)
G = Goroutine (lightweight thread; initial stack ~2 KB, grows as needed)
M = OS thread
P = Processor (controlled by GOMAXPROCS)
P1 P2
┌────────────┐ ┌────────────┐
│ G1 running │ │ G3 running │
│ ┌────────┐ │ │ ┌────────┐ │
│ │ G2 │ │ │ │ G4 │ │ ← local run queue
│ │ G5 │ │ └────────────┘
└────────────┘
M1 M2 ← OS threads
Goroutines
// go keyword: start a function concurrently.
go func() {
fmt.Println("goroutine")
}()
// Wait for goroutines to finish.
var wg sync.WaitGroup
for i := 0; i < 5; i++ {
wg.Add(1)
go func(n int) {
defer wg.Done()
fmt.Println(n)
}(i) // Pass i as an argument to avoid closure capture issues.
}
wg.Wait()
Channels
ch := make(chan int) // Unbuffered: send blocks until a receiver is ready.
bch := make(chan int, 10) // Buffered: send blocks only when the buffer is full.
// Unbuffered: send and receive must be in separate goroutines.
go func() { ch <- 42 }()
v := <-ch
// Buffered: send and receive can be in the same goroutine within capacity.
bch <- 1
bch <- 2
fmt.Println(<-bch, <-bch) // 1 2
// Receiving from a closed, drained channel returns (zero, false).
v, ok := <-ch // ok is false when the channel is closed and empty.
// range receives values until the channel is closed and drained.
ch2 := make(chan int, 3)
ch2 <- 1; ch2 <- 2; ch2 <- 3
close(ch2) // The sender closes the channel.
for v := range ch2 {
fmt.Println(v) // 1, 2, 3
}
Channel direction
// Directional types make intent explicit and catch mistakes at compile time.
func producer(ch chan<- int) { // Send-only.
for i := 0; i < 5; i++ {
ch <- i
}
close(ch)
}
func consumer(ch <-chan int) { // Receive-only.
for v := range ch {
fmt.Println(v)
}
}
ch := make(chan int)
go producer(ch)
consumer(ch)
select
// select waits on multiple channel operations simultaneously.
// It picks one ready case at random when several are ready.
select {
case v := <-ch1:
fmt.Println("ch1:", v)
case v := <-ch2:
fmt.Println("ch2:", v)
case ch3 <- data:
fmt.Println("sent to ch3")
case <-time.After(1 * time.Second):
fmt.Println("timeout")
default:
fmt.Println("no channel ready") // Non-blocking.
}
sync and atomic
// Mutex: protect shared state from concurrent access.
var mu sync.Mutex
var count int
mu.Lock()
count++
mu.Unlock()
// RWMutex: more efficient when reads are much more frequent than writes.
var rw sync.RWMutex
rw.RLock() // Multiple goroutines can read simultaneously.
// ... read ...
rw.RUnlock()
// Once: run an initialization function exactly once.
var once sync.Once
once.Do(func() { /* called only the first time */ })
// atomic: low-overhead operations for simple counters and flags.
var n atomic.Int64
n.Add(1)
fmt.Println(n.Load())
Avoid goroutine leaks
When a goroutine sends on a channel, always give it a way to stop if the receiver exits early.
select {
case ch <- result:
case <-ctx.Done(): // Exit when the caller cancels.
return
}
Context
Context tree
context.Background()
└── WithCancel → ctx, cancel
└── WithTimeout(ctx, 5s) → ctx2, cancel2
└── WithValue(ctx2, key, val) → ctx3
↓
Cancellation and deadlines propagate to child contexts.
Use context.Context to propagate deadlines, cancellation, and request-scoped metadata. Call the returned cancel function even when a timeout will eventually fire.
import "context"
// Timeout context.
ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
defer cancel() // Always call cancel to release resources.
req, err := http.NewRequestWithContext(ctx, http.MethodGet, "https://example.com", nil)
if err != nil {
return err
}
resp, err := http.DefaultClient.Do(req)
if err != nil {
if errors.Is(err, context.DeadlineExceeded) {
fmt.Println("timeout")
}
return err
}
defer resp.Body.Close()
// Cancellable context.
ctx2, cancel2 := context.WithCancel(context.Background())
go func() {
<-ctx2.Done() // Block until cancelled.
fmt.Println("cancelled:", ctx2.Err())
}()
cancel2()
// context.WithValue carries request-scoped metadata.
// Use it only for small amounts of data; never as a substitute for function parameters.
type ctxKey string
const userKey ctxKey = "user"
ctx3 := context.WithValue(context.Background(), userKey, "alice")
user, ok := ctx3.Value(userKey).(string) // Type-assert with an ok check.
Generics
Go 1.18 introduced type parameters for reusable, type-safe code.
Type parameters
// T and U can be any type.
func Map[T, U any](s []T, f func(T) U) []U {
result := make([]U, len(s))
for i, v := range s {
result[i] = f(v)
}
return result
}
doubled := Map([]int{1, 2, 3}, func(n int) int { return n * 2 })
// [2 4 6]
Type constraints
import "cmp"
// cmp.Ordered allows <, >, == (int, float64, string, etc.).
func Min[T cmp.Ordered](a, b T) T {
if a < b {
return a
}
return b
}
Min(3, 5) // 3 (int)
Min(3.14, 2.71) // 2.71 (float64)
Min("b", "a") // "a" (string)
// Custom constraint with ~ for underlying types.
type Number interface {
~int | ~int64 | ~float64
}
func Sum[T Number](nums []T) T {
var total T
for _, n := range nums {
total += n
}
return total
}
Generic types
type Stack[T any] struct {
items []T
}
func (s *Stack[T]) Push(v T) {
s.items = append(s.items, v)
}
func (s *Stack[T]) Pop() (T, bool) {
if len(s.items) == 0 {
var zero T
return zero, false // Return the zero value of T.
}
v := s.items[len(s.items)-1]
s.items = s.items[:len(s.items)-1]
return v, true
}
s := Stack[string]{}
s.Push("Go")
s.Push("Generics")
v, _ := s.Pop() // "Generics"
Iterators
Since Go 1.23, a function returning iter.Seq can be ranged over directly. When the caller breaks early, yield returns false so the producer can stop.
import "iter"
// Backward yields n-1 through 0.
func Backward(n int) iter.Seq[int] {
return func(yield func(int) bool) {
for i := n - 1; i >= 0; i-- {
if !yield(i) { // Caller broke out of the loop.
return
}
}
}
}
for i := range Backward(3) {
fmt.Println(i) // 2, 1, 0
}
// maps.Keys returns an iter.Seq; slices.Collect converts it to a slice.
keys := slices.Collect(maps.Keys(m))
Testing
// Place tests in xxx_test.go files.
package mypackage_test
import (
"testing"
"github.com/yourname/yourproject/mypackage"
)
func TestAdd(t *testing.T) {
got := mypackage.Add(2, 3)
if got != 5 {
t.Errorf("Add(2, 3) = %d; want 5", got)
}
}
// Table-driven tests are the idiomatic Go pattern.
func TestDivide(t *testing.T) {
tests := []struct {
name string
a, b float64
want float64
wantErr bool
}{
{"normal", 10, 2, 5, false},
{"division by zero", 10, 0, 0, true},
}
for _, tt := range tests {
t.Run(tt.name, func(t *testing.T) {
got, err := mypackage.Divide(tt.a, tt.b)
if (err != nil) != tt.wantErr {
t.Errorf("unexpected error: %v", err)
}
if got != tt.want {
t.Errorf("got %v, want %v", got, tt.want)
}
})
}
}
// Benchmark: run with go test -bench=.
func BenchmarkAdd(b *testing.B) {
b.ReportAllocs() // Report memory allocations per operation.
for i := 0; i < b.N; i++ {
mypackage.Add(2, 3)
}
}
Test helpers
// t.Helper makes failures report the caller's line, not the helper's.
func requireNoError(t *testing.T, err error) {
t.Helper()
if err != nil {
t.Fatal(err)
}
}
func TestWriteFile(t *testing.T) {
t.Setenv("APP_ENV", "test") // Restored automatically after the test.
dir := t.TempDir() // Deleted automatically after the test.
t.Cleanup(func() {
// Register arbitrary cleanup: reset mocks, close clients, etc.
})
path := filepath.Join(dir, "output.txt")
requireNoError(t, os.WriteFile(path, []byte("hello"), 0o644))
}
go test ./... # Run all tests.
go test -race ./... # Detect data races.
go test -cover ./... # Show coverage.
Standard-library quick reference
fmt
// Output.
fmt.Print("no newline")
fmt.Println("with newline")
fmt.Printf("formatted %s %d %v\n", "str", 42, true)
fmt.Fprintf(os.Stderr, "error: %v\n", err)
// Build strings.
s := fmt.Sprintf("Hello, %s!", name)
msg := fmt.Errorf("wrap: %w", err)
// Common format verbs.
// %v default %+v struct with field names %#v Go syntax
// %T type name %d decimal integer %f floating-point
// %s string %q quoted string %x hexadecimal
// %p pointer address %b binary %e exponent
strings
import "strings"
strings.Contains("Hello", "ell") // true
strings.HasPrefix("Hello", "He") // true
strings.HasSuffix("Hello", "lo") // true
strings.Count("cheese", "e") // 3
strings.Index("Hello", "ll") // 2
strings.Replace("oink oink", "oink", "moo", 1) // "moo oink"
strings.ReplaceAll("oink oink", "oink", "moo") // "moo moo"
strings.ToUpper("hello") // "HELLO"
strings.ToLower("HELLO") // "hello"
strings.TrimSpace(" hello ") // "hello"
strings.Trim("##hello##", "#") // "hello"
strings.Split("a,b,c", ",") // ["a" "b" "c"]
strings.Join([]string{"a", "b"}, "-") // "a-b"
strings.Fields("a b\tc") // ["a" "b" "c"]
// strings.Builder is efficient for repeated concatenation in a loop.
// A plain + is fine for a small number of concatenations.
var sb strings.Builder
for i := 0; i < 5; i++ {
fmt.Fprintf(&sb, "%d", i)
}
sb.String() // "01234"
strconv
import "strconv"
strconv.Itoa(42) // "42"
strconv.Atoi("42") // 42, nil
strconv.ParseFloat("3.14", 64) // 3.14, nil
strconv.FormatFloat(3.14, 'f', 2, 64) // "3.14"
strconv.ParseBool("true") // true, nil
strconv.FormatBool(true) // "true"
sort and slices
import "sort"
nums := []int{5, 2, 4, 1, 3}
sort.Ints(nums) // [1 2 3 4 5]
words := []string{"b", "a", "c"}
sort.Strings(words)
// Custom sort.
sort.Slice(nums, func(i, j int) bool {
return nums[i] > nums[j] // Descending.
})
import (
"cmp"
"slices"
)
slices.Sort(nums) // Ascending.
idx, found := slices.BinarySearch(nums, 3) // Requires sorted input.
// cmp.Compare avoids integer overflow in comparison functions.
slices.SortFunc(nums, func(a, b int) int { return cmp.Compare(b, a) }) // Descending.
maps (Go 1.21+)
import (
"maps"
"slices"
)
m := map[string]int{"a": 1, "b": 2}
m2 := maps.Clone(m) // Shallow copy.
maps.Copy(m2, m) // Copy all entries from m into m2.
delete(m, "a") // delete is a built-in function.
keys := slices.Collect(maps.Keys(m)) // Collect keys into a slice.
time
import "time"
now := time.Now()
t := time.Date(2026, 6, 1, 12, 0, 0, 0, time.Local)
// Go uses a reference time (Mon Jan 2 15:04:05 MST 2006) as the layout.
now.Format("2006-01-02 15:04:05")
now.Format(time.RFC3339) // "2026-06-22T12:00:00+09:00"
parsed, _ := time.Parse("2006-01-02", "2026-06-22")
later := now.Add(24 * time.Hour)
diff := later.Sub(now) // time.Duration
fmt.Println(diff.Hours()) // 24.0
time.Sleep(100 * time.Millisecond)
ticker := time.NewTicker(500 * time.Millisecond)
defer ticker.Stop()
os and filepath
import (
"os"
"path/filepath"
)
// File I/O.
data, err := os.ReadFile("file.txt")
err = os.WriteFile("out.txt", data, 0644)
// Directory operations.
entries, _ := os.ReadDir(".")
for _, e := range entries {
fmt.Println(e.Name(), e.IsDir())
}
os.MkdirAll("a/b/c", 0755)
os.Remove("file.txt")
os.RemoveAll("dir/")
// Environment variables.
home := os.Getenv("HOME")
os.Setenv("KEY", "value")
// Path manipulation.
filepath.Join("a", "b", "c") // "a/b/c"
filepath.Base("/a/b/file.txt") // "file.txt"
filepath.Dir("/a/b/file.txt") // "/a/b"
filepath.Ext("file.txt") // ".txt"
abs, _ := filepath.Abs("relative") // Absolute path.
encoding/json
import "encoding/json"
type Point struct {
X int `json:"x"`
Y int `json:"y"`
}
// Encode.
data, err := json.Marshal(Point{X: 1, Y: 2}) // []byte
pretty, err := json.MarshalIndent(Point{X: 1}, "", " ")
// Decode.
var p Point
err = json.Unmarshal(data, &p)
// Streaming.
enc := json.NewEncoder(os.Stdout)
enc.Encode(p)
dec := json.NewDecoder(os.Stdin) // Accepts any io.Reader.
dec.Decode(&p)
// When the type is unknown.
var v any
json.Unmarshal(data, &v)
m := v.(map[string]any)
net/http
import (
"bytes"
"context"
"errors"
"io"
"log"
"net/http"
"os"
"os/signal"
"syscall"
"time"
)
// Always set a timeout; the default client has none.
client := &http.Client{Timeout: 10 * time.Second}
// GET request.
resp, err := client.Get("https://example.com")
if err != nil {
return err
}
defer resp.Body.Close()
body, _ := io.ReadAll(resp.Body)
// Custom request with context.
req, err := http.NewRequestWithContext(ctx, http.MethodPost, url, bytes.NewReader(jsonData))
if err != nil {
return err
}
req.Header.Set("Content-Type", "application/json")
resp2, err := client.Do(req)
if err != nil {
return err
}
defer resp2.Body.Close()
// HTTP server with timeouts (required in production).
mux := http.NewServeMux()
mux.HandleFunc("GET /hello/{name}", func(w http.ResponseWriter, r *http.Request) {
name := r.PathValue("name") // Path parameters (Go 1.22+).
fmt.Fprintf(w, "Hello, %s!", name)
})
server := &http.Server{
Addr: ":8080",
Handler: mux,
ReadHeaderTimeout: 5 * time.Second,
ReadTimeout: 30 * time.Second,
WriteTimeout: 30 * time.Second,
IdleTimeout: 60 * time.Second,
}
go func() {
if err := server.ListenAndServe(); err != nil && !errors.Is(err, http.ErrServerClosed) {
log.Printf("server error: %v", err)
}
}()
// Graceful shutdown on SIGINT / SIGTERM.
shutdownCtx, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
defer stop()
<-shutdownCtx.Done()
ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
defer cancel()
if err := server.Shutdown(ctx); err != nil {
log.Printf("shutdown error: %v", err)
}
log/slog (Go 1.21+)
import "log/slog"
// Default logger writes text to stderr.
slog.Info("server started", "port", 8080)
slog.Error("request failed", "err", err)
// JSON handler for structured logging (e.g., cloud environments).
logger := slog.New(slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{
Level: slog.LevelDebug,
}))
logger.Info("user logged in", "userID", 42, "ip", "127.0.0.1")
// {"time":"...","level":"INFO","msg":"user logged in","userID":42,"ip":"127.0.0.1"}
// Add fields shared across multiple log calls.
logger.With("requestID", "abc123").Info("processing")
Tips and common mistakes
Goroutine leak
// Bad: the goroutine blocks forever if no one receives from ch.
go func() {
ch <- result
}()
// Good: always provide a cancellation path.
go func() {
select {
case ch <- result:
case <-ctx.Done():
return
}
}()
nil interface trap
// Bad: even when *MyError is nil, the interface value is non-nil.
func getError() error {
var err *MyError = nil
return err // The returned error interface is non-nil!
}
if getError() != nil { // true — this branch is taken unexpectedly.
fmt.Println("this prints unexpectedly")
}
// Good: return nil directly.
func getError() error {
return nil
}
Pre-allocate slice capacity
// Bad: append may reallocate the backing array many times.
result := []int{}
for i := 0; i < 10000; i++ {
result = append(result, i)
}
// Good: allocate the full capacity upfront.
result := make([]int, 0, 10000)
for i := 0; i < 10000; i++ {
result = append(result, i)
}
for-loop variable capture (fixed in Go 1.22)
// Go 1.21 and earlier: all goroutines share the same i variable.
for i := 0; i < 5; i++ {
go func() { fmt.Println(i) }() // May print 5 five times.
}
// Go 1.22+: each iteration creates a new i variable.
// The above code now correctly prints 0, 1, 2, 3, 4 in some order.
// Workaround for Go 1.21 and earlier.
for i := 0; i < 5; i++ {
i := i // Shadow i with a new variable for this iteration.
go func() { fmt.Println(i) }()
}
Modules and packages
# Initialize a module.
go mod init github.com/yourname/yourproject
# Go 1.26
go get go@1.26
go get github.com/some/package@v1.2.3 # Add a dependency.
go mod tidy # Remove unused dependencies.
go mod vendor # Vendor dependencies.
go build ./... # Build.
go test ./... # Test.
go test -race ./... # Detect data races.
go vet ./... # Static analysis.
go fmt ./... # Format.
Top comments (1)
Love it!