What is a Thunk?

Nick Scialli January 31, 2020🚀 3 minute read

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“Thunk” is one of those programming terms that sounds intimidating, but many of us are actually familiar with and have used them. Let’s first see how Wikipedia defines thunks[1]:

In computer programming, a thunk is a subroutine used to inject an additional calculation into another subroutine. Thunks are primarily used to delay a calculation until its result is needed, or to insert operations at the beginning or end of the other subroutine.

This offers a little help, but maybe is a little too abstract.

So, what is a thunk really? It’s simply a function returned from another function. Let’s look at a quick example in JavaScript:

function delayedLogger(message, delay) {
  return function(logger) {
    setTimeout(function() {
    }, delay);

In this example, when we call the delayedLogger function, it returns a thunk. We can then pass that thunk a logger parameter, which will be executed after the specified delay.

const thunk = delayedLogger('See you in a bit', 2000);

In this example, we’ll see "See you in a bit" logged to the console after two seconds.

Bonus: Use in Redux

If you’re familiar with Redux, you likely know the concept of action creators: functions that return action objects. The following is an example of an action creator that makes an action that adds a product to a shopping cart:

function addToCart(product) {
  return {
    type: 'ADD_TO_CART',
    payload: product,

It turns out that we need a little more flexibility with our action creators: we need to be able to dispatch an action asynchronously: often after we perform a fetch request to save or load data from an API. We can solve this issue by using redux-thunk, a Redux middleware that allows us to return thunks from action creators [2].

Let’s mock up what this would look like. We’ll load a list of products from an API and then dispatch an action with those loaded products.

function loadProducts() {
  // Return a thunk
  return function(dispatch) {
      .then(res => res.json())
      .then(data => {
          type: 'ADD_PRODUCTS',
          payload: data.products,

And there we have it: A more practical application of the thunk concept!


  1. Wikipedia: Thunk
  2. redux-thunk

Nick Scialli

Nick Scialli is a software engineer at the U.S. Digital Service.

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