throw error: Unable to compile TypeScript - when check data type by function

[ngvcanh]

Expected Behavior

Detect the previously returned undefined instance and the other is string

Actual Behavior

Can't understand checked data case it is undefined previously and throw error

Steps to reproduce the problem

Throw error at line 89 in index.ts file.

Minimal reproduction

git clone https://github.com/ngvcanh/node-ts-error.git
cd node-ts-error
npm i
npm start

Specifications

• ts-node version: 9.0.0
• node version: 12.16.1
• TypeScript version: 4.0.3
  tsconfig.json, if you're using one:

{
  "compilerOptions": {
    "target": "es5",
    "module": "commonjs",
    "strict": true,
    "outDir": "dist",
    "sourceMap": true,
    "baseUrl": "src"
  }
}

• Operating system and version: Windows 10 x64 - 19041.572
• If Windows, are you using WSL or WSL2?:

[cspotcode]

What error(s) do you get when you compile your code with tsc?

[ngvcanh]

What error(s) do you get when you compile your code with tsc?

@cspotcode
I compiled with tsc and no error in there

[cspotcode]

What do you think is the difference between the two? Something to do with not using --files? Given that we use typescript to do the typechecking, what would have changed? Where does a bugfix need to happen?

[ngvcanh]

@cspotcode

I would like to describe a bit of what I have tried.

That's when I was using the map() method to traverse an array of data and that required returning a value.

Inside the callback function passed to the map() method, I call a defined function that returns a string or undefined.

I have tried the following 3 methods of checking data:

let value = format(...);

// Option 1
if (value === undefined){
    return undefined;
}
else{
    return value + "any";
}

// Option 2
if (value === undefined){
    return undefined;
}
return value + "any";

// Option 3
if (isUndefined(value)) return undefined;
return value + "any";

So, when I run the code without compile, Option 3 has an error while options 1 and 2 have no errors. The isUndefined() function simply returns the result of the condition like options 1 and 2.

function isUndefined(obj: any): boolean {
  return obj === undefined;
}

I think TypeScript does not understand that undefined type is checked inside the function.

I'm new to TypeScript and I really do not understand the difference between the two.

[cspotcode]

@ngvcanh
I'm getting the same error from tsc, according to this reproduction: https://github.com/TypeStrong/ts-node-repros/runs/1306068270#step:5:38

You can see what I'm doing in run.sh. I clone your reproduction, then run tsc.

[ngvcanh]

@cspotcode
That error is what I mean. I think TypeScript ignores conditions that are listed inside a conditional function.

I'm new to TypeScript so I got a bit confused with them. I don't know what I should do to overcome that error.

And you've tagged needs more info but I don't really know what to add as well. Can you tell me what more information I should provide?

Thank you and regards for your support.

[cspotcode]

"needs more info" was a signal to myself that I was waiting for your response.

This is the ts-node issue tracker, which is intended for reporting bugs or enhancement requests for ts-node.

In this case, this is a TypeScript diagnostic, and ts-node is correctly reporting the diagnostic to the user. So this is not a ts-node bug.

To ask questions about TypeScript, I recommend the TypeScript Discord server, which is one of our suggested links when you open an issue.

[ngvcanh]

I just want to comment on this as a solution for someone to come later and need it. I have no intention of reopening this issue.

The function declaration is as follows so that TypeScript understands the function used to check the data type:

function isUndefined (obj: any): obj is undefined {
    return obj === undefined;
}

The way of declaring functions to check other data types is similar to the above declaration