What is the reason that TypeScript 4 transpiler failed to recognize the incorrect type being passed to the function?

After finding a different question, I ended up here pondering over a new one.

The original query that brought me here was: Why is form: FormGroup showing 'controls' as undefined in an Angular11 component?

The solution seemed to involve changing this snippet (ngSubmit)="send(newMaterialFormGroup.value)"> to (ngSubmit)="send(newMaterialFormGroup)">

By passing the object itself, I was able to access controls successfully.

However, the fundamental question still lingers...

This is how it looks on the component side:

send(form: FormGroup): void{
    let jsonModel = {
      materialNum: form.controls['materialNum'].value,
    };

    let jsonModelRaw: string = JSON.stringify(jsonModel);
    //...
}

I'm working with "typescript": "~4.1.5"

Despite explicitly stating that the parameter was of type FormGroup and having strict typing enabled in TypeScript settings, why did TS fail to detect this issue?

angulartypescripttypescript-generics

Answer №1

The issue appears to stem from the fact that the data type of newMaterialFormGroup.value was initially set as any. This explains why there were no errors during compilation when it was utilized in a context where a FormGroup was expected - since any can be assigned to any type, it circumvents the type checking system.

It is advisable to utilize unknown instead of any whenever feasible. While similar in concept because it acts as a wildcard type that can be assigned to anything just like any, unknown cannot be directly assigned:

let a: any = 4;    // OK
let b: number = a; // OK

let x: unknown = 4; // OK
let y: number = x;  // Error

Playground Link

An unknown value must be explicitly checked before assignment is allowed:

let x: unknown = 4; // OK

let y: number = -1;
if (typeof x === "number")
    y = x; // OK

Playground Link

Utilizing unknown proves beneficial as it retains its "wildcard" nature without compromising the integrity of the type system.

For further insight, visit: 'unknown' vs. 'any'

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