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

Question

Accepted Answer

**fmt.Println** — stdout, no metadata. **log.Println** — stderr, with timestamp.

**fmt.Println:**
- Writes to `os.Stdout`.
- No timestamp.

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

**log.Println:**
- Writes to `os.Stderr`.
- Adds timestamp.

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

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

**When to use log:**
- Application logging.
- Debugging.

**Common mistakes:**
- Using `fmt.Println` for logs.
- Using `log.Println` for user output.
- Not configuring log output.
- Using `log.Fatal` (exits immediately).
- Not using structured logging.

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