Object is possibily undefined, but it's not

TypeScript Version: 2.4.1

Code

type cb = () => void;
function foo(cb?: cb) {
  const defaultFn = () => {};
  cb = cb || defaultFn;
  function bar() {
    cb(); // here I get the compiler error
  }
  bar();
}

Expected behavior:

The function foo should call second().

Actual behavior:

I get Object is possibily 'undefined' in that line where I left a comment.

I know it is not a good idea to reasign function arguments. Following code would work as expected:

type cb = () => void;
function foo(cb?: cb) {
  const defaultFn = () => {};
  const innerCb = cb || defaultFn; // don't override argument
  function bar() {
    innerCb()
  }
  bar();
}

But should the TypeScript compiler not be smart enough to recognize the closure? It only happens in the callback function onSomethingHappened. If I write it in the scope of the function foo it works as well. I only get the compiler error in the closure:

type cb = () => void;
function foo(cb?: cb) {
  const defaultFn = () => {};
  cb = cb || defaultFn;
  cb(); // no closure
}

PS: I know I could work with default arguments in the example above but my real production code is far more complex as this one 😉

type cb = () => void;

function foo(cb?: cb) {
  const defaultFn = () => {};
  cb = cb || defaultFn;

  function bar() {
    cb(); // <-- should check if undefined first, error as expected
  }

  assignUndefinedToCb(); // <------ TS won't know what did you do to `cb`

  bar(); // 💥

  function assignUndefinedToCb() {
    cb = undefined;
  }
}

See #9998 for more details.

Big thank you! It seems that TypeScript is more intelligent than I 😂 👍👍

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Object is possibily undefined, but it's not #16889

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Object is possibily undefined, but it's not #16889

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ikatyang commented on Jul 2, 2017

screendriver commented on Jul 2, 2017

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