"this" in a method call in TypeScript?

Question

How do I want to make this work:

class TestClass {

   doMethod1 (arg1, arg2, cb)
   {
      this.doMethod2(arg1, arg2, function (result){cb (result)});
   }

  doMethod2 (arg1, arg2, cb) {
      this.doMethod3(arg1, arg2, function(result){cb (result)});
   }

  doMethod3 (arg1, arg2, cb) {
      var result = arg1 + arg2;
      cb(result);
   }
}

test = new TestClass;

test.doMethod3(1,1, cb); test.doMethod2(1,1,cb);

Both work.

test.doMethod1(1,1,cb);

EDIT: Actually, it does work.

I got around related lexical scoping issues by using the "fat arrow" syntax:

doMethod1 (arg1, arg2, cb)
   {
      this.doMethod2(arg1, arg2, (result) => {cb (result)});
   }

Ensures that the "this" in doMethod1 is the same as the "this" in the anonymous callback function.

Answer 1

To preserve the lexical scoping of this in TypeScript, you use Arrow Function Expressions.

They are defined in section 4.9.2 of the TypeScript Language Specification.

ArrowFormalParameters => { return AssignmentExpression ; }

Which in code can look like:

() => { alert(this.arg1); }