TypeScript : assign Type 'string | undefined' to Type 'string'

Question

I'm a new comer in typescript, and I learnt that we can not assign Type 'string | undefined' to Type 'string' from internet tutorials like https://linguinecode.com/post/how-to-solve-typescript-possibly-undefined-value. But when I write a demo on WebStorm, I found it can be compiled, without any errors, and also can run. This totally confused me.

Here is my TS code:

function validateToken(token: string) {
    return token;
}

function run() {
    const token = 'kjadj' as string | undefined;

    let a = validateToken(token);
    console.log(a)
}

after compile , it became below JS code:

function validateToken(token) {
    return token;
}
function run() {
    var token = 'kjadj';
    var a = validateToken(token);
    console.log(a);
}

My typescript version is 4.7.4, WebStorm version is Build #WS-221.5921.27, built on June 22, 2022.

Thanks for you guys' help. Followed suggestions in comment area, so far my tsconfig.json is:

{
  "compilerOptions": {
    "module": "commonjs",
    "target": "es5",
    "sourceMap": true,
    "strictNullChecks": true,
    "strict": true,
  },
  "exclude": [
    "node_modules"
  ]
}

strictNullChecks option does work in TS playground, but it doesn't work in my local WebStorm either, also, I can compile my ts file via tsc demo.ts command. BTW this's my file constructor:

.
├── demo.js.map
├── index.html
├── demo.ts
├── demo.js
├── tsconfig.json

Throws as expected using a normal strict config

CertainPerformance
– CertainPerformance

2022-07-07 06:32:40 +00:00
Commented Jul 7, 2022 at 6:32 — CertainPerformance
– CertainPerformance, Commented Jul 7, 2022 at 6:32

Mattstir · Accepted Answer · 2022-07-11 12:37:46Z

2

You need to activate strictNullChecks

To test out how differnt typescript configs behave you can use your sample in typescriptPlayground. Under the "TS Config" dropdown you can easily swap settings ;)

Update: As you updated the question and added a commend: The reason for tsc somefile to still work fine is likely:

When input files are specified on the command line, tsconfig.json files are ignored.

Explained in the typescript handbook

So the easiest solution for you is probably to set the files in the tsconfig file, or even better the include and just use tsc as command ;)

edited Jul 11, 2022 at 12:37

answered Jul 7, 2022 at 11:54

Mattstir

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5 Comments

Aaron Hao Over a year ago

Thanks, dude. I tried it, but found although the web page will mark line 8 with red underline, but it still can work. When i click 'run' button, it will output 'kjadj' into Logs area.

Ashirbad Panigrahi Over a year ago

Dear Mattstir without strictNullChecks option it also shows the error but when you try to compile the .ts file it simply compiles into .js file without showing any error in terminal

Aaron Hao Over a year ago

Thank you Mattstir, I think you 99% resolve my confusion. When I enable strictNullChecks in my tsconfig.json, and follow your suggestion, use tsc instead of tsc demo.ts, the compiler throw an error. But finally it compile a js file, and it's runnable.

Aaron Hao Over a year ago

Why are there such strange results? Is the compiler's error just a guideline?

Mattstir Over a year ago

As normal javascript is also valid Typescript depending on your settings it really comes down to what you configure (how strict, etc..) ;) If my answer answered your original question it would be nice if you accept it ;) thx

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