I recently began delving into typescript and stumbled upon a unique way of defining the type of an argument.
function postThread({userId}:{userId:string}) {
// implementation code here
}
As opposed to the usual method shown below:
function postThread({userId}:string) {
// implementation code here
}
Why is direct assignment of the type not possible in this case?
Let's examine the distinctions:
function createThread({user}:{user:string}) {
// code to run
}
In this scenario, object destructuring is used in the function parameter. The createThread function requires a single argument, which must be an object with a property called user of type string.
function createThread({user}:string) {
// code to run
}
In this version, it explicitly specifies that the function expects a single argument of type string. This does not involve object destructuring; instead, it simply states the expected type of the parameter. However, this appears to be inaccurate as you are attempting to destructure an object while the provided type is only string.
function postThread({ userId }: { userId: string }) {
console.log(userId) // outputs a string
}
postThread({ userId: 'abc123' })
This code snippet defines an object type { userId: string }, and extracts the userId property from the parameter using destructuring.
When calling the function, an object with a userId property is passed as an argument.
The equivalent way to write this functionality would be:
function postThread(props: { userId: string }) {
const userId = props.userId
console.log(userId) // outputs a string
}
postThread({ userId: 'abc123' })
function postThread({ userId }: string) {
// will not compile
}
The syntax { userId }: string is invalid because it tries to access the userId property of a string, resulting in a type error.
A more common alternative approach, which was probably intended in the second example, is demonstrated here:
function postThread(userId: string) {
console.log(userId) // outputs a string
}
postThread('abc123')
In this case, the function accepts a single string parameter directly.
When invoking the function, you simply provide a string value, rather than an object.
The initial example demonstrates the explicit addition of a type to the object parameter that is immediately destructured.
function postThread({userId}:{userId:string}) { // destructuring
// code to be executed
}
In the second scenario, a compiler error occurs. Once again, there is an attempt to destructure a property userId from an object parameter with an explicit type of string. However, a primitive string does not contain a property named userId.
function postThread({userId}: string) {
// ~~~~~~
// Property 'userId' does not exist on type 'String'.
}
Unless you have actually defined userId within the String interface:
interface String {
userId: string;
}
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