Scope of this is lost when passing a member function to setInterval

TypeScript Version: 1.8.10

When passing a member function to setInterval, the scope of ‘this’ is lost within the callback, though the structure of the code (given experience of any object orientated language) indicates it shouldn’t be.

Example code

export class SetIntervalTest {
  private someNumber: number = 1;

  trigger() {
      setInterval(this.setIntervalCallback, 400);
  }

  private setIntervalCallback() {
      console.log(this.someNumber);
  }
}  

Expected behavior When console.log(this.someNumber); is called, the scope of this is within the scope of the SetIntervalTest interval instance, printing ‘1’.

Actual behavior: The scope of this is pulled from global scope, resulting in this.someNumber being ‘undefined’. If that isn’t possible, the compiler should indicate the expected behaviour is not what you’re going to get.

To fix this i need to change ‘setInterval(this.setIntervalCallback, 400);’ to ‘setInterval(() => this.setIntervalCallback(), 400);’ so ‘this’ is correctly scoped in the callback.

[RyanCavanaugh]

See “something about this” in https://github.com/Microsoft/TypeScript/wiki/FAQ#common-feature-requests

You can have the “correct” code generated by using an arrow function:

    private setIntervalCallback = () => {
      console.log(this.someNumber);
    };

[zijianhuang]

@leewinder ,

setInterval(()=>this.setIntervalCallback(), 400);

should be just working fine so that the callback can see this.