interface I1 {
x: number;
y: string;
}
interface I2 {
x?: number;
y?: string;
}
const tmp1: Partial<I1> = {}, tmp2: I2 = {};
Can you spot a clear distinction between these two entities, as demonstrated in the above code snippet?
interface I1 {
x: number;
y: string;
}
interface I2 {
x?: number;
y?: string;
}
const tmp1: Partial<I1> = {}, tmp2: I2 = {};
Can you spot a clear distinction between these two entities, as demonstrated in the above code snippet?
Expressing differences in types.
The variables tmp1 and tmp2 hold the same type, as Partial does this:
type MyPartial<T> = { [K in keyof T]+?: T[K] }
Where +? refers to a modifier making a property optional.
Despite their structural compatibility, these types signify distinct concepts:
I1 indicates an object with mandatory properties. Altering it using Partial makes it deviate from expectations.I2 is inherently optional - all properties can be missing.An example clarifies the distinction:
interface User {
age: number;
name: string;
}
This enforces that every User must have both age and name. However, a Partial<User> could exist temporarily without full info for functions like
function saveUser(user: User) { /* ... */ }
which expects complete user data.
In contrast, fully optional types fit scenarios where any subset suffices:
interface SurveyResult {
score?: number;
comment?: string;
}
This accommodates any survey outcome, whether only one field or none are filled. Even empty results may contribute.
The response provided tackles the immediate query you raised, but you may be curious as to why there are multiple methods for representing optional type fields like this.
There are actually several rationales behind opting for Partial<I1> over I2 (though this is not an exhaustive compilation). Firstly, and most prominently, if a new field is added to I1, it will automatically be included in Partial<I1>. This proves particularly beneficial in extensive projects or situations where the type of I1 originates from an external source. Moreover, Partial<I1> offers enhanced readability for other developers who may later tweak the code (or even for yourself when troubleshooting bugs months down the line). It can also significantly condense the syntax, especially when dealing with numerous fields within I1. Not to mention the potential pitfalls of typos and other errors that may stem from duplicating code across multiple locations. As a general guideline, it's advisable to employ Partial<Type1> rather than manually crafting a distinct Type2 in scenarios akin to the one posed initially.
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