TypeScript reflection for interfaces

Question

Is there a way to find out if interface's property is defined as read only? Say,

interface ITest {
    readonly foo: number;
}

Now, is there some sort of reflection or trickery for TypeScript to get this information? E.g. something like:

let info = Reflect.get(ITest, 'foo');

if (info.isReadOnly()) { ... }

Answer 1

Since TypeScript interfaces do not exist at runtime, you cannot use reflection on them. To use reflection I created a class that implements the interface and reflected on the class. However, I was unable to tell if a property is readonly. Not sure if this is a lack of understanding on my part, or a defect. Here is what I tried:

Code

interface ITest {
  readonly foo: number;
  bar: number;
}
class TestImplementation implements ITest {
  readonly foo: number = 1;
  bar: number = 2;
}
function reflectOnTest() {
  var testImplementation = new TestImplementation();
  var properties: string[] = Object.getOwnPropertyNames(testImplementation);
  var fooDescriptor = Object.getOwnPropertyDescriptor(testImplementation, properties[0]);
  var barDescriptor = Object.getOwnPropertyDescriptor(testImplementation, properties[1]);
  console.log("foo writable = " + fooDescriptor.writable);
  console.log("bar writable = " + barDescriptor.writable);
}

Output is:

foo writable = true

bar writable = true