Can we implement more stringent type checking for union types?

Question

Can we implement more stringent type checking for union types?

Following the guidelines outlined in section 3.4 of the specification, there is an issue with compilation when trying to assign a boolean value to a variable declared as a string or number:

let s: string | number;
s = false; // Error - not a string or a number

Given classes A, B, and C, it would be logically consistent to expect similar compilation errors when assigning different types to a variable declared as A or B. However, surprisingly, this code compiles without any issues:

let a: A | B;
a = new C();
a = "hi";
a = {}; // No error - a can be assigned anything!

Is there a workaround to enforce stricter type checking in this scenario? Furthermore, is this behavior intentional or a bug in TypeScript's design? If intended, the reasoning behind this design decision seems unclear.

typescript

Answer 1

Answer №1

The definitions for A, B, or C were not provided, but we can make an educated guess:

class A {}
class B {}
class C {}
let a: A | B;
a = new C();
a = "hi";
a = {}; // Here, 'a' can take any form!

TypeScript operates on a structural type system (refer to the FAQ) where these empty classes are exchangeable.
From the FAQ explanation:

Types with no members can be replaced by any other type. Typically, you wouldn't create an interface without properties.

But if you introduce properties to your classes, like in real-world scenarios, you'll encounter errors:

class A { a: number }
class B { b: string }
class C { c: boolean }
let a: A | B;
a = new C(); // results in an error
a = "hi";     // results in an error
a = {};       // results in an error

Answer 2

The definitions for A, B, or C were not provided, but we can make an educated guess:

class A {}
class B {}
class C {}
let a: A | B;
a = new C();
a = "hi";
a = {}; // Here, 'a' can take any form!

TypeScript operates on a structural type system (refer to the FAQ) where these empty classes are exchangeable.
From the FAQ explanation:

Types with no members can be replaced by any other type. Typically, you wouldn't create an interface without properties.

But if you introduce properties to your classes, like in real-world scenarios, you'll encounter errors:

class A { a: number }
class B { b: string }
class C { c: boolean }
let a: A | B;
a = new C(); // results in an error
a = "hi";     // results in an error
a = {};       // results in an error

Can we implement more stringent type checking for union types?

Answer №1

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