Thunks

A thunk is a small routine that encapsulates a computation to be performed later. In programming, a thunk is typically a parameterless function or closure that delays evaluation of an expression until it is explicitly invoked.

Thunks serve several roles: they implement lazy evaluation, control evaluation order, or adapt calling conventions. In call-by-name languages, a thunk suspends a computation for later evaluation; in call-by-need, the result is memoized after the first use. By delaying execution, thunks can help avoid unnecessary work and enable more flexible evaluation strategies.

In practical programming, thunks appear in many forms. In JavaScript, a thunk is a function that wraps a computation and returns its result when called. Middleware such as redux-thunk enables action creators to return a function (instead of a plain action) that receives dispatch and getState to perform asynchronous or conditional work.

Example: a thunk can be defined as function thunk() { return expensiveComputation(); }. Calling thunk() executes the computation at that moment. Thunks are also used by compilers to implement tail calls or to suspend evaluation of expressions for optimization, and they appear in various functional languages as a standard mechanism to defer work and manage side effects.

The term likely originated in early compiler literature, referring to a small piece of code that defers a computation. The precise origin is not undisputed. Across languages, thunks remain a fundamental concept for controlling when and how expressions are evaluated, and for enabling lazy or asynchronous programming patterns.