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

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.

CodePudding user response：

Inside the compiler options you should set "strict": true. You can check the docs here.

CodePudding user response：

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 ;)