Suppose I have a class like this:
class Foo<T>{}
How can I determine the type of the instance of the class within a method? In other words, something along the lines of:
public barbaz(){
// This approach does not function
if(typeof(<T>) === 'number'){}
}
Although there may be some confusion surrounding mixing type and values, it is important to remember that this feature can actually be quite helpful at times. In the JavaScript language, which is a superset of other languages, there are no specific "Type parameters" called out.
It is necessary in certain cases (such as constructors or setters) to use a parameter with the type "T".
class Example<T> {
constructor(public value: T) {}
public doSomething(){
// This will work
if(typeof this.value === 'string'){}
}
}
Here is a clever solution to the problem at hand. Check out this example below:
interface HandleNumbers {
add(arg: number): number
}
interface HandleStrings {
concat(a: string, b: string): string;
}
class Api {
add = (arg: number) => arg + 1
concat = (a: string, b: string) => a.concat(b)
}
interface HandleHttp {
<T extends void>(): {};
<T extends string>(): HandleStrings;
<T extends number>(): HandleNumbers;
}
const handleHttp: HandleHttp = <_ extends string | number>() => new Api();
handleHttp<number>() // you can only call [add] method
handleHttp<string>() // you can only call [concat] method
handleHttp() // unable to call any method
Remember that your resulting JS code will include both the add and concat methods.
Therefore, your ability to invoke these methods is influenced by the provided generic type.
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