What’s a “thunk”?

I remember when everyone started talking about using “thunks” in Redux to execute code without blocking the UI. And I remember getting tripped up on the word “thunk”. What’s a “thunk”? Why are we calling it that?

The term “thunk” goes all the way back to ALGOL 60! It’s a play on “think”, because the ALGOL compiler needed to “think” about what sort of subroutines to generate. ALGOL allows passing expressions as arguments to subroutines, not just constants. One way of allowing this is to substitute the expression for another subroutine that evaluates the expression when the original subroutine call happens. This new subroutine is the “thunk” - the result of the “thinking”.