Unexpected "Object is possibly undefined" error

[DashwoodIce9]

Bug Report

Hi, I stumbled upon this weird behavior that I couldn't wrap my head around. I searched with "object is possibly undefined bug typescript github" on Google and went through the first 5-6 bugs but didn't find any which quite resembled this case. I am new to Typescript so it is entirely possible that this is expected behavior. If that's the case I apologize and humbly ask for an explanation. Thanks!

🔎 Search Terms

"Object is possibly undefined"

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about "common-bugs-that-arent-bugs."

⏯ Playground Link

Playground link with relevant code

💻 Code

interface TestInterface {
    optionalParam?: {
        [key: string]: string;
    };
}

const result: TestInterface = {
    optionalParam: {
        Key1: "Value1",
    },
};

result.optionalParam["Key2"] = "Value2"; // 'result.optionalParam' is possibly 'undefined'.

🙁 Actual behavior

'result.optionalParam' is possibly 'undefined' error on the last line.

🙂 Expected behavior

Code compiles without any errors. optionalParam is typed as though it can be undefined however, in the example above result.optionalParam is clearly defined. There is no code in between result initialization and result.optionalParam which could set it to undefined. So, this should've worked I think.

[MartinJohns]

Duplicate of #50357.

[DashwoodIce9]

Hi Martin, thanks for pointing me to the duplicate. I've read through that and #47920 several times over the past 2 days but I couldn't make sense of it. The discussion at #47920 is unfortunately a bit too complicated for me. Could you please let me know (or point me to) if this functionality should be implemented some other way and/or is this not supported by Typescript yet?

The comment at #50357 (comment) says to use -

const anObject: Required<Pick<typeof anotherVar, LongListOfKeys>> = {
  // object initializer with keys from LongListOfKeys
}

But, what are Required and Pick? I didn't get that.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.