Declaring as object or undefined has problems in ts4.3

[casperno]

Bug Report

🔎 Search Terms

Searched for 'undefined' in issues after 4.3

🕗 Version & Regression Information

• This changed between versions 4.2.3, and 4.3.2

⏯ Playground Link

Note that the error is show for all 4.1, 4.2 and 4.3. But pasting the same code into a project using 4.1.5 does NOT show the error.

https://www.typescriptlang.org/play?ts=4.3.2&ssl=4&ssc=4&pln=17&pc=6#code/GYVwdgxgLglg9mABMOcAUBKRBvAUIgxAGwFMpEAHAJzgCNSBbALhwEMmwQHaSqBfRAB9E4ACYlgMMCVGIAvCLDjJ00QG5c+QqEiwEiAM5kAaqyIgSmPIRuUa9EgwXZ2ARgAMfDTb6abAen9EKhIjKC0CajpGeUVlKRkNCMRAxAgzIkQ4KAALXkQGMhy4WVzWcnSkUThEVjAAT1ypAHNksNNzSwwkmxhgRDQohwYsa1sCVIg4BgoYUipDVnqDRDLyAHIhxnXEGBWxCQTZOtkABRoKXih6xHXWHerQxDBsxBIADz3yfWvL2+kAG68dbJGxTMAGOCkAB0RDgzUG9kY0NYGGSvj4QA

💻 Code

 let problem: {a:number} | undefined;

 function setValue(){
        problem= {a:10};
    }

    // reset 
    // _this causes compiler to belive that 'problem' can only be undefined from now on
    problem = undefined;

    // call other method that can do anything
    setValue();

    if (problem) {
        _// compiler says that 'problem' is undefined and Property 'a' does not exist on type 'never'_
        console.log(problem.a)
    }

🙁 Actual behavior

compiler says that 'problem' is undefined and Property 'a' does not exist on type 'never'

🙂 Expected behavior

Expected compiler to know that 'problem could be both undefined and {a:number}

[MartinJohns]

You forgot to fill out the issue template for bug reports.

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Works in 4.2.3

Doesn't work in 4.2.3 either: Playground link

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Essentially a duplicate of #9998.

[casperno]

Sorry about the missing template, it's my first report. And thanks for the quick response

I too see that the Playground shows errors for 4.3, 4.2 and even 4.1.

But if I paste this code into a project that is set up with 4.1, it works.
And I've just refactored a lot of code that didn't fail with 4.2, but fails with 4.3.

So there seems to have been a change in 4.3 that takes it beyond #9998

[fatcerberus]

Your other project may not have strictNullChecks enabled.

[casperno]

Neither project has strictNullChecks enabled, but strict is true for the 4.2 project.

Setting strict = true for 4.1 project give the same error,
strict = true, strictNullChecks = false removes it.

Looks like this is the cause.

Has default behaviour of strict = true changed from 4.2 to 4.3?

[MartinJohns]

You see this behavior due to a fixed bug. See #44025.

[casperno]

Thank you both for the swift response and for clearing up my confusion.

Typescript rocks, keep up the excellent work!