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Suggestion: typechecking of 'this' context #15

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Suggestion: typechecking of 'this' context#15
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DuplicateAn existing issue was already createdNeeds ProposalThis issue needs a plan that clarifies the finer details of how it could be implemented.SuggestionAn idea for TypeScript

Description

@RyanCavanaugh
Ryan Cavanaugh (RyanCavanaugh)
opened on Jul 15, 2014

A common error in TypeScript is to lose the this context, especially in class situations:

class MyClass {
    x = 4;
    printX() { console.log(this.x); }
}
var m = new MyClass();
window.setInterval(m.printX, 1000); // Prints 'undefined', not '4'

The compiler should detect when this happens and issue an error:

var m = new MyClass();
// Error, `this` context might be lost
window.setInterval(m.printX, 1000);

We need a proposal here that determines why this would be an error, in a way that is not burdensome. For example, we do want it to be valid to call e.g. function#bind on m.printX, and it should probably valid to write something like if(m.printX) { m.printX() }.

This gets even trickier when we look at the members of window -- some of them can be invoked with any this, but others cannot. It should be possible to describe these differences.

Activity

  1. Ryan Cavanaugh (RyanCavanaugh)
    added
    Suggestion
    on Jul 15, 2014

  2. metaweta commented on Dec 13, 2014

    @metaweta
    Mike Stay (metaweta)
    on Dec 13, 2014

    A solution to this problem would need to distinguish between methods that should only be used on instances of MyClass and methods that can be used on any object satisfying an interface that MyClass satisfies. For example, all the methods on Array.prototype are "generic" in the sense that they can be applied to any "array-like" object, i.e. one with a length property and numeric properties, not just to instances of Array.

  3. mhegazy commented on Feb 20, 2016

    @mhegazy
    Mohamed Hegazy (mhegazy)
    on Feb 20, 2016
    Contributor

    This is now covered by #3694. PR is in #6739

  4. Mohamed Hegazy (mhegazy)
    closed this as completedon Feb 20, 2016
  5. Mohamed Hegazy (mhegazy)
    added
    DuplicateAn existing issue was already created
    on Feb 20, 2016
  6. microsoft
    locked and limited conversation to collaborators on Jun 18, 2018
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