What is the difference between fmt.Println and log.Println?

Question

Accepted Answer

**fmt.Println** — stdout output without metadata. **log.Println** — stderr output with timestamp.

**fmt.Println:**
- Writes to `os.Stdout`.
- No timestamp or source info.
- Adds spaces between args.
- Returns `(n int, err error)`.

```go
fmt.Println("Hello, World!")
// Output: Hello, World!

fmt.Printf("Name: %s
", name)
fmt.Fprintf(w, "Response: %v
", data)
```

**log.Println:**
- Writes to `os.Stderr`.
- Adds timestamp: `2024/01/15 10:30:45`.
- Can add file/line info.
- Calls `os.Exit(1)` on write error.

```go
log.Println("Server started")
// Output: 2024/01/15 10:30:45 Server started

log.SetFlags(log.Ldate | log.Ltime | log.Lshortfile)
log.Println("Debug")
// Output: 2024/01/15 10:30:45 main.go:42 Debug

log.Printf("User %d logged in
", id)
```

**Structured logging:**
```go
import "log/slog"

slog.Info("request handled", "method", "GET", "path", "/api")
slog.Error("database error", "error", err)
```

**When to use fmt:**
- CLI output.
- HTTP responses.
- Test output.

**When to use log:**
- Application logging.
- Error tracking.
- Audit trails.

**Common mistakes:**
- Using `fmt.Println` for logs (no timestamps).
- Using `log.Println` for user output (stderr).
- Not configuring log output.
- Using `log.Fatal` (exits immediately).
- Not using structured logging in production.

Go: What is the difference between fmt.Println and log.Println?