Can the automatic casting feature of TypeScript be turned off when dealing with fields that have identical names?

Imagine you have a class defined as follows:

Class Flower {
    public readonly color: string;
    public readonly type: string;
    constructor(color: string, type: string) {
    this.color = color;
    this.type = type;
}

Now, let's introduce another class called FlowerDto, which mirrors the Flower class in structure but is intended to represent a Data Transfer Object (Dto). The field names of FlowerDto are subject to change based on external factors, but for now they align with those of Flower.

Class FlowerDto {
    public readonly color: string;
    public readonly type: string;
    constructor(color: string, type: string) {
    this.color = color;
    this.type = type;
}

In Typescript, it is possible to define a function like this:

function getFlower(): Flower {
    return new FlowerDto("red", "rose");
}

Is there a way to prevent Typescript from allowing this behavior? Such inconsistencies can be detected at compile time and may lead to confusion and costly errors if not addressed promptly.

I have considered the possibility of a compiler option in tsconfig that disables implicit casting (casting without specifying the "any" keyword), but my search has been futile so far. Any assistance on this matter would be greatly appreciated.

Answer №1

In short, the answer is a resounding no - it's not possible, as far as my knowledge goes.

Within typescript, a class consists of two components: the javascript class (the actual implementation) and the interface (the definition). By specifying Coordinate as a return type, you are essentially informing typescript that the function must output something that aligns with the interface of the Coordinate class. Given that both classes have virtually identical interfaces, a CoordinateDto can indeed fulfill said interface criteria and pass all type checks.

A somewhat unconventional workaround involves incorporating some form of flag property to render the two interfaces incompatible. For example, assigning public readonly type = 'dto'; to CoordinateDto, while allocating public readonly type = 'raw'; to Coordinate. Since these are immutable string literals, despite being strings, they will mismatch. As a result, an error would be triggered in your sample function:

Type 'CoordinateDto' is not compatible with type 'Coordinate'.
  Property types are conflicting.
    Type '"dto"' cannot be assigned to type '"raw"'.

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