Why strictNullChecks option does not warn a non-nullable class property which is not initialized?

[tetsuharuohzeki]

Is this a bug or by design?

Environment

• TypeScript
  • 2.1.0-dev.20160715
  • 2.0.0

Steps to reproduce

compile the following code with tsc --strictNullChecks test.ts:

// test.ts

class A {
  a: Array<number>;
  constructor() {}
}

var a = new A();
// this would be the error which is like "Cannot read property 'length' of undefined"
// because `a.a` is not uninitialized in A's constructor.
a.a.length; 

Expected behavior

• I'd like to this should be a compile error by following reasons:
  • A.a is not "nullable" (null or undefined) nor optional class property a?
  • But A.a is not initialized in A's constructor.

Actual behavior

• TypeScript compiler successes the compilation without any errors.
  • I think this might be a pitfall.

[sandersn]

This is currently by design, but it's a design limitation. We would like to allow a to be initialised in the constructor or somewhere else, not just directly on the property. But there's no simple way to know which code is used for initialisation (where a would be allowed to be undefined) and which should require a to be defined.

I don't know if there's an existing bug about this, but we've spent hours arguing about it in design meetings. I'll see if I can find something.

[sandersn]

Here it is: #8476