Mastering the 'compose' function in Redux with the power of Typescript

Struggling with the compose function in Typescript has been a recurring issue for me. Errors keep popping up, and I find the type definition in .d.ts to be quite perplexing. Take this example:

type Props = { t: number }; 
const foo = (props: {}) => <div {...props} />;
const moo = (props: Props) => <div {...props} />;
const bar = (props: Props) => <div {...props} />;
const Dar = compose(foo, moo)(bar);

const Test = () => <Dar />;

This snippet presents several issues. The error message complains that "bar" lacks the "foo" parameter, even though it does have it.

Moreover, I'm unable to use since Dar is evaluated as JSX.Element rather than a stateless function. Any suggestions or examples on how to effectively utilize compose in Typescript?

Appreciate any insights. Thanks!

typescriptredux

Answer №1

TypeScript's inference mechanism operates in a left-to-right fashion, whereas a generic function named compose combines functions by operating from right to left, potentially resulting in incorrect type assignments.

To avoid this issue, consider using a composition function that composes from left to right, such as the flow function.

Answer №2

compose is a technique that merges two functions together, assuming the result type of the first function matches the input type of the second function. 

const compose = (f,g) => x => f(g(x))

So basically, when you use compose in your code, you are essentially applying the function bar (which takes Props and returns JSX.Element) as an argument to moo (also taking Props and returning JSX.Element), and then passing the outcome to foo.

The issue here lies in the fact that moo doesn't expect a function from Props to JSX.Element, it wants just Props

If you intend to create a "stateless function," you must utilize this structure to inform Typescript of your intentions

const bar: React.FunctionComponent<Props> = (x: Props) => <div {...props} />;

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