Type safety of "this" is potentially misleading in class methods

[nfriend]

Given the following:

class TestClass {

    myTestObject = {
        message: 'It worked!',
    }

    myTestMethod() {
        console.log(this.myTestObject.message);
    }
}

var myTestClassInstance = new TestClass();
myTestClassInstance.myTestMethod.apply({});

The compiler assumes that the scope of this is the instance of the TestClass. However, if the method is called via .apply(), as in the example above, it is possible to hijack the this scope of the method.

This is a bit of an edge case, since most TypeScript code shouldn't be circumventing the type safety by using the .apply() method. However, some third-party JavaScript libraries rely on the .apply() method, which undermines confidence in the this keyword.

The fix is to redefine the method as a property using the fat arrow syntax, i.e.:

myTestMethod = () => {
    console.log(this.myTestObject.message);
}

Perhaps the compiler should only enforce type safety on this if the method is defined such that the this keyword is lexically scoped?

[RyanCavanaugh]

See #15.