Go HTTP Responses: When to Use `ResponseWriter.Write`, `io.WriteString`, and `fmt.Fprintf`?

Difference Between ResponseWriter.Write, io.WriteString, and fmt.Fprintf

In Go, there are multiple ways to write content to an HTTP response:

1. io.Writer

io.Writer is an interface that represents an output stream that allows writing bytes to a target. http.ResponseWriter implements io.Writer and is the general method for assembling a response body. It provides the Write() method.

2. io.WriteString

io.WriteString is a utility function that writes a string to an io.Writer. It checks if the receiver implements the WriteString() method and uses it if available. Otherwise, it converts the string to bytes and writes it.

3. fmt.Fprintf

fmt.Fprintf is a function from the fmt package that formats and writes values to an io.Writer. It accepts a format string and variable arguments and writes the formatted string to the output.

Performance Considerations

io.WriteString is typically recommended for writing strings because it can be more efficient. If the io.Writer implements WriteString(), it may avoid the overhead of converting the string to bytes. In particular, http.ResponseWriter implements WriteString(), which can improve performance.

fmt.Fprintf is less performant because it requires parsing and formatting the string before writing it.

Usage Recommendations

For writing simple strings, io.WriteString is the preferred choice. For writing formatted strings, fmt.Fprintf provides a convenient way to do so but with a performance trade-off.

Passing ResponseWriter

In cases where the response content is generated on-the-fly, it's more efficient to pass the http.ResponseWriter (which is an io.Writer) to the generator and let it write directly to the response. Examples include using json.Encoder to generate JSON responses.

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