Good evening! I am newbie in javascript and typescript.
I can't understand something. I carefully typed my code and the situation appeared. I can't imagine, why, through it's something basic. Look:
class A {
id: number;
constructor(id:number){
this.id=4;
}
isIdZero?():boolean{
if(this.id===0)
return true;
else
return false;
}
}
let a : A= new A(0);
let b : A={
id:9
}
alert(b.isIdZero());
This code makes an error "Cannot invoke an object which is possibly 'undefined'. ts(2722)"
Why? Function equals to what typed in {}, isn't it?
How can I correct it?
My question is probably so primitive that I didn't find anything.
isIdZero?():boolean{
The question mark here means you're defining isIdZero so it may be undefined, instead of being a function. Since it might be undefined, typescript won't let you call it unless you check it first. Eg:
if (b.isIdZero) {
alert(b.isIdZero());
}
If you want isIdZero to be a mandatory property, then remove the question mark. However, this change will cause the following code to start showing a type error, since the object doesn't have all the properties it needs to be an A:
let b: A = {
id:9
}
You can either add the function to the object you're creating
let b: A = {
id: 9,
isIdZero: () => false,
}
Or you can stop trying to call this an A, and just let it be an object with an id that's a number:
let b = {
id: 9,
}
the ? operator you are using after isIdZero? tells typescript that you expect a possibility of this property being undefined.
It doesn't seem like that's necessary, so you can remove that.