Typescript does not automatically infer existence & validity of a variable

[aghArdeshir]

Bug Report

When I want to check existence of a variable and then do something with it, I have to do it right before the usage, I cannot move it somewhere else and this makes refactoring hard/impossible

🔎 Search Terms

infer, infer automatically

🕗 Version & Regression Information

Typescript: 4.3.5

⏯ Playground Link

Playground link with relevant code

Typescript Nightly produced more weird results: Results in TSNext (Note the string and number should give error, but they don't)

💻 Code

function iReceiveNumber1(a: number, b: number) {
    console.log(a + b);
}


function produceNumber(): number | undefined {
    return Math.random() > 0.5 ? 1 : undefined;
}

/**
 * Example 1 that works, but lots of code duplicates
 */

const e1: number | undefined = produceNumber();
const e2: number | undefined = produceNumber();

typeof e1 === 'number' && typeof e2 === 'number' && iReceiveNumber1(e1, e2);

/**
 * Example 2 that does not work, but I expect it to
 */

const e3: number | undefined = produceNumber();
const e4: number | undefined = produceNumber();

const e3AndE4Exist = typeof e3 === 'number' &&  typeof e4 === 'number';
e3AndE4Exist && iReceiveNumber1(e3, e4);

// I receive error in above line, but I did not expect error

🙁 Actual behavior

I receive error that says Type 'undefined' is not assignable to type number. Although I checked the existence in a variable (e3AndE4Exist

🙂 Expected behavior

I expected TypeScript to automatically infer that the type is not undefined as I checked it in e3AndE4Exist variable right before I call the iReceiveNumber function.

I also had hard times opening Typescript Playground, some requests failed. I don't know if that is my connection problem or not.

[MartinJohns]

This already works in TypeScript 4.4.0 due to #44730, at least in v4.4.0-beta. This is a duplicate of #12184.

[RyanCavanaugh]

Typescript Nightly produced more weird results: Results in TSNext

I don't see anything weird in the nightly build, can you explain what you find to be wrong?

[aghArdeshir]

I'm sorry. I checked again today, and in nightly version everything was still weird. I tried a simple const a: number = 'a' and again, I did not see any error. I think there is something wrong with my connection. By the way all requests from https://typescript.azureedge.net/cdn/4.4.0-dev.20210727/... fail.

I tried with version 4.4.0 locally and the issue is solved and everything was fine. Sorry for the inconvenience. 👍

I think this issue can be closed

[RyanCavanaugh]

Thanks!