Time to talk about one of the strangest TypeScript tricks around.
It's the loose autocomplete trick. And it's ACTUALLY USEFUL for app development.
Let me explain 🧵
Imagine you're building an `Icon` component.
Your brand has some known colors, like `primary` and `secondary`. But you also want to make sure that users can specify any color they want.
You might start by defining a `Color` type:
Your brand has some known colors, like `primary` and `secondary`. But you also want to make sure that users can specify any color they want.
You might start by defining a `Color` type:
Then, using that type in your `Icon` component:
Then, you might use the `Icon` component like this:
But there's an issue. We aren't getting `color` suggestions when we use the `Icon` component. If we try to autocomplete the `color` prop, we get no suggestions.
Ideally, we want `primary` and `secondary` to be part of that list. How do we manage that?
Ideally, we want `primary` and `secondary` to be part of that list. How do we manage that?
The solution is very odd-looking. We can intersect the `string` type in `Color` with an empty object:
Now, we'll get suggestions for `primary` and `secondary` when we use the `Icon` component.
What on earth?
Now, we'll get suggestions for `primary` and `secondary` when we use the `Icon` component.
What on earth?
This works because of a quirk of the TypeScript compiler.
When you create a union between a string literal type and `string`, TypeScript will eagerly widen the type to `string`. We can see this by hovering over `Color` without the intersection:
When you create a union between a string literal type and `string`, TypeScript will eagerly widen the type to `string`. We can see this by hovering over `Color` without the intersection:
So, before it's ever used, TypeScript has already forgotten that `"primary"` and `"secondary"` were ever part of the type.
But by intersecting `string` with an empty object, we trick TypeScript into retaining the string literal types for a bit longer.
But by intersecting `string` with an empty object, we trick TypeScript into retaining the string literal types for a bit longer.
Now, when we use `Color`, TypeScript will remember that `"primary"` and `"secondary"` are part of the type - and it'll give us suggestions accordingly.
This might feel pretty fragile to you. This doesn't seem like intended behavior from TypeScript.
Well, the team actually know about this trick. They test against it.
And someday, they may make it so that a plain `string` type will just work. But until then, keep this in mind.
Well, the team actually know about this trick. They test against it.
And someday, they may make it so that a plain `string` type will just work. But until then, keep this in mind.
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