StringJoin in Thread not working

Question

Here is what I have tried first.
Thread[f["x", {"1", "2"}]]

{f["x", "1"], f["x", "2"]}
But the weirdest thing happened when I substituted StringJoin for f
Thread[StringJoin["x", {"1", "2"}]]

"x12"
I thought it would be like this below
{"x1","x2"}

Is there anyone who can explain this?

kglr · Accepted Answer

Thread >> Possible Issues:

"Thread evaluates the whole expression before threading"

Wrap StringJoin with Unevaluated to prevent evaluation of StringJoin before Thread gets to work:
Thread[Unevaluated @ StringJoin["x", {"1", "2"}]]

{"x1", "x2"}

Alternatively, you can use
Thread[foo["x", {"1", "2"}]] /. foo -> StringJoin

{"x1", "x2"}

One Answer