Explained: 'React' refers to a UMD global
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Let's look at a question I got asked the other day:
typescript
type TakesOneArgument = (a: string) => void// Why does this not error in TypeScript?const func: TakesOneArgument = () => {}
In the type TakesOneArgument
, we're saying that the function takes a string
and returns void
. But when we use TakesOneArgument
to annotate a function that doesn't take any arguments, TypeScript doesn't complain.
Why is this?
Let's look at an example that might look less weird:
typescript
const array = ['a', 'b', 'c']// No errors!array.forEach(() => {})
Here, we're using a .forEach
method on an array of strings. The function we pass to .forEach
could receive several arguments:
typescript
array.forEach((item, index, array) => {// ...})
But we're not using any of those arguments. So why doesn't TypeScript complain?
Let's imagine we've ditched TypeScript, and we're living in JavaScript-land again. We've got a function that takes a single argument:
javascript
function takesOneArg(a) {}
What's going to happen if we call it with two arguments?
javascript
takesOneArg('a', 'b')
Well - nothing! The second argument passed to it will be silently ignored, and it's unlikely ever to cause a runtime error (unless you're doing something unsafe with arguments
).
This is exactly what's happening in the .forEach
case:
The function passed to .forEach
is always passed item
, index
and array
- but it doesn't always need to specify them.
typescript
// Both are fine!array.forEach((item, index, array) => {})array.forEach(() => {})
So - when you specify a function type, TypeScript doesn't force you to handle all the parameters. It's perfectly fine to use a function that takes fewer arguments than the specified type.
Let's circle back to our first example:
typescript
type TakesOneArgument = (a: string) => void// Why does this not error in TypeScript?const func: TakesOneArgument = () => {}
The reason this doesn't error is because, technically, func
is assignable to TakesOneArgument
. You could use that function anywhere that TakesOneArgument
is expected, and it would work.
But as you can see, it's a little unexpected. The cleanest way to annotate this would be:
typescript
const func = (a: string): void => {}
I don't want to dissuade you from typing your functions using type
. But this behaviour is definitely worth considering.
Plus, I go into depth elsewhere on why function types can give you worse errors.
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readonly T[]