TypeScript is not able to infer the type narrowing from method

[iRealNirmal]

Bug Report

🔎 Search Terms

Type narrowing

🕗 Version & Regression Information

TS - 4.3.2
(Regression not aware)

💻 Code

  private _enabled(): boolean {
    return this.maxlength ? true : false;
  }

  private _createValidator(): void {
    const maxlength = this.maxlength;
    this._validator = (this._enabled()) ? validator(toInteger(maxlength)) : null;
  }

🙁 Actual behavior

When using method getting error.

Argument of type 'string | number | null' is not assignable to parameter of type 'string | number'. while same thing is working when using variable directly.

🙂 Expected behavior

Narrowing should work whether we use method or property.

[MartinJohns]

This just sounds like an issue with the library you use. Without a reproducible example we can't tell you what's wrong, but most likely TypeScript works as it should.

[wangweixuan]

To make this narrowing, the compiler has to look into _enabled method and perform interprocedural analysis. That might be too costly and hard to implement. Currently type narrowing applies mostly to local variables.
For this problem I would recommend you to use non-null assertion on maxlength.

[MartinJohns]

Performing analysis for every method would be way too costly. I think the most close would be something like #38390, which aims to automatically infer type guards for simple cases.

[MartinJohns]

If it's costly process at least can we suggest to either use non-null assertion ?

I don't understand the question. You're free to use the non-null assertion operator whenever in your code.

[RyanCavanaugh]

We don't inline method bodies for CFA purposes, and even if we did, we don't retroactively apply narrowings on the basis of assuming non-mutation of the initializing reference.

We don't automatically suggest ! because we want you to actually think about whether something might be null or not 🙂