What sets apart `[boolean, string?]` from `[boolean, string] | [boolean]` in typescript?

Question

What sets apart `[boolean, string?]` from `[boolean, string] | [boolean]` in typescript?

When it comes to TypeScript, a tuple with the type [boolean, string?] is not interchangeable with an interface or type that expects either [boolean, string] | [boolean]. I believe this distinction arises because in the latter case, you are explicitly specifying that the tuple can be either [boolean, string] or [boolean], rather than allowing for a more generic tuple type that could support both. Even though this explanation makes sense logically, my question remains: since the compiler ultimately interprets it as [boolean, string] | [boolean] and acknowledges that it could be either type at runtime, why not treat these types as equivalent? Is this decision based on random choice to avoid simplification, or is there a potential issue that would arise if such unification were allowed?

typescript

Answer 1

Answer №1

Here are two examples showing the difference between these types:

// Valid
const example1: [boolean, string?] = [true, undefined];
// Invalid
const example2: [boolean, string] | [boolean] = [true, undefined];

Answer 2

Here are two examples showing the difference between these types:

// Valid
const example1: [boolean, string?] = [true, undefined];
// Invalid
const example2: [boolean, string] | [boolean] = [true, undefined];

Answer 3

Answer №2

In one response, it was mentioned that when the set of compiler options known as the --strict suite is activated without using the --exactOptionalPropertyTypes flag, it is possible to assign undefined to any optional property, even within optional tuple elements. However, the scenario changes when dealing with missing or undeclared properties where assigning undefined might not be accepted. This creates a distinction between [boolean, string?] and [boolean, string] | [boolean] especially when setting a value of undefined for the second element.

If --exactOptionalPropertyTypes is turned on, the ability to assign undefined to optional properties diminishes significantly, resulting in stricter rules:

// --exactOptionalPropertyTypes enabled

const p1: [boolean, string?] = [true, undefined]; // error!
// ---------------------------------> ~~~~~~~~~
// Type 'undefined' is not assignable to type 'string'

const p2: [boolean, string] | [boolean] = [true, undefined]; // error!
// -> ~~
// Type 'undefined' is not assignable to type 'string'

...

Answer 4

In one response, it was mentioned that when the set of compiler options known as the --strict suite is activated without using the --exactOptionalPropertyTypes flag, it is possible to assign undefined to any optional property, even within optional tuple elements. However, the scenario changes when dealing with missing or undeclared properties where assigning undefined might not be accepted. This creates a distinction between [boolean, string?] and [boolean, string] | [boolean] especially when setting a value of undefined for the second element.

If --exactOptionalPropertyTypes is turned on, the ability to assign undefined to optional properties diminishes significantly, resulting in stricter rules:

// --exactOptionalPropertyTypes enabled

const p1: [boolean, string?] = [true, undefined]; // error!
// ---------------------------------> ~~~~~~~~~
// Type 'undefined' is not assignable to type 'string'

const p2: [boolean, string] | [boolean] = [true, undefined]; // error!
// -> ~~
// Type 'undefined' is not assignable to type 'string'

...

What sets apart `[boolean, string?]` from `[boolean, string] | [boolean]` in typescript?

Answer №1

Answer №2

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