Golang Convert String to io.Writer?

You can’t write into a string, strings in Go are immutable.

The best alternatives are the bytes.Buffer and since Go 1.10 the faster strings.Builder types: they implement io.Writer so you can write into them, and you can obtain their content as a string with Buffer.String() and Builder.String(), or as a byte slice with Buffer.Bytes().

You can also have a string as the initial content of the buffer if you create the buffer with bytes.NewBufferString():

s := "Hello"
buf := bytes.NewBufferString(s)
fmt.Fprint(buf, ", World!")
fmt.Println(buf.String())

Output (try it on the Go Playground):

Hello, World!

If you want to append a variable of type string (or any value of string type), you can simply use Buffer.WriteString() (or Builder.WriteString()):

s2 := "to be appended"
buf.WriteString(s2)

Or:

fmt.Fprint(buf, s2)

Also note that if you just want to concatenate 2 strings, you don’t need to create a buffer and use fmt.Fprintf(), you can simply use the + operator to concatenate them:

s := "Hello"
s2 := ", World!"

s3 := s + s2  // "Hello, World!"

Also see: Golang: format a string without printing?

It may also be of interest: What’s the difference between ResponseWriter.Write and io.WriteString?