Error Handling & Defer

Errors in Go

This lesson explains the main concept clearly with beginner-friendly examples and background so you can learn why the code works.

Go does not have exceptions. Functions return an error value which must be checked.

func ReadFile(name string) ([]byte, error) {
    return os.ReadFile(name)
}

data, err := ReadFile("config.json")
if err != nil {
    log.Fatalf("cannot read file: %v", err)
}

Creating custom errors

var ErrNotFound = errors.New("resource not found")

Wrapping errors (Go 1.13+)

if err != nil {
    return fmt.Errorf("open %s: %w", name, err)
}

The defer statement

Runs after the surrounding function returns.

func copyFile(src, dst string) error {
    in, err := os.Open(src)
    if err != nil { return err }
    defer in.Close()

    out, err := os.Create(dst)
    if err != nil { return err }
    defer out.Close()

    _, err = io.Copy(out, in)
    return err
}