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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() {
logger(message);
}, 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);
thunk(console.log);
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) {
fetch('some-product-api-url')
.then(res => res.json())
.then(data => {
dispatch({
type: 'ADD_PRODUCTS',
payload: data.products,
});
});
};
}
And there we have it: A more practical application of the thunk
concept!
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References:
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