问题
type WeirdCustomType = {[int]: boolean, ...};
What kind of structure is WeirdCustomType
? Is it simply an array of {int:boolean}
types ? (ie, key is an int, value is a boolean)? If so, what is the meaning of ... here? And where can I read about this particular type aliasing usage?
回答1:
WeirdCustomType
is an “Explicit inexact object type”
Its properties consist of:
- A) properties with integer keys with boolean values
- the brackets around
int
indicate that we’re referring to the property keys, and that those keys are of typeint
(which should actually benumber
, see last section of my answer) - this follows destructuring syntax for objects and their keys, and you can read more about “Computed object property names and destructuring” on the MDN docs
- the brackets around
- B) additional properties with other keys and value types
Here’s how an object of WeirdCustomType
may look:
const inexactObject: WeirdObjectType = {
1: ‘foo’,
2: ‘bar’,
baz: ‘abc’
}
The ellipses on WeirdCustomType
indicate explicitly (to make it extra clear) that this type allows for an object with extra properties where a normal object type is expected.
To disable that behavior, you can use the exact object type. As described in the documentation,
Unlike regular object types, it is not valid to pass an object with “extra” properties to an exact object type.
// @flow
var foo: {| foo: string |} = { foo: "Hello", bar: "World!" }; // Error!
Here’s a post on Medium that explains the motivation.
Currently,
{foo: number}
is the type for any object which has a propertyfoo
with typenumber
.{| foo: number |}
is the type for an object which ONLY has a propertyfoo
with typenumber
. We say the former syntax is an inexact object and the latter is an exact object.In a few releases, Flow will begin to treat
{foo: number}
as an exact object. To indicate inexactness, you must add an ellipsis to the end of an object type:{foo: number, ...}
. This new syntax forces the developers to opt in to inexactness.
note on int
type
Flow doesn’t actually have a primitive type int
. Integers are represented by the number
type. I believe that the [int]
should be [number]
.
See https://flow.org/en/docs/types/primitives/
来源:https://stackoverflow.com/questions/62353038/type-alias-in-flow-with-brackets-and-ellipsis