问题
I know that, we can use Readonly< T> to redeclare all fields of T as readonly, as in: [Typescript: extending an interface and redeclaring the existing fields as readonly
What about nested fields? For example:
interface School {
teachers: Array<Teacher>;
students: Array<Student>;
}
interface Teacher {
teacherId: number;
personInfo: PersonInfo;
}
interface Student {
studentId: number;
personInfo: PersonInfo;
}
interface PersonInfo {
name: string;
age: number
}
How to create a SchoolReadonly type, in which all nested fields are readonly.
A simple test case:
var s: SchoolReadonly = {
teachers: [{teacherId: 1, personInfo: {name: "John", age: 40}}],
students: [{studentId: 1, personInfo: {name: "Dan", age: 20}}]
}
s.teachers[0].personInfo.name = "John2"; //should produce readonly error
s.students[0].personInfo.age = 22; //should produce readonly error
As a constraint, I do not want to add Readonly directly to the School, Teacher, Student and PersonInfo interfaces.
回答1:
You can use generics and type aliases, it's a lot of code, but it will do the trick:
interface School<T extends Teacher<PersonInfo>, S extends Student<PersonInfo>> {
teachers: Array<T>;
students: Array<S>;
}
type EditableSchool = School<EditableTeacher, EditableStudent>;
type ReadonlySchool = Readonly<School<ReadonlyTeacher, ReadonlyStudent>>;
interface Teacher<P extends PersonInfo> {
teacherId: number;
personInfo: P;
}
type EditableTeacher = Teacher<PersonInfo>;
type ReadonlyTeacher = Readonly<Teacher<Readonly<PersonInfo>>>;
interface Student<P extends PersonInfo> {
studentId: number;
personInfo: P;
}
type EditableStudent = Student<PersonInfo>;
type ReadonlyStudent = Readonly<Student<Readonly<PersonInfo>>>;
interface PersonInfo {
name: string;
age: number
}
var s: ReadonlySchool = {
teachers: [{teacherId: 1, personInfo: {name: "John", age: 40}}],
students: [{studentId: 1, personInfo: {name: "Dan", age: 20}}]
}
s.teachers = null;
s.teachers[0].personInfo.name = "John2"; // Cannot assign to 'name' because it is a constant or a read-only property
s.students[0].personInfo.age = 22; // Cannot assign to 'age' because it is a constant or a read-only property
(code in playground)
Edit
If you want s.teachers[0] = nul
to fail then you need to also change the Array
to ReadonlyArray
, so:
interface School<T extends Teacher<PersonInfo>, Ta extends ArrayLike<T>, S extends Student<PersonInfo>, Ts extends ArrayLike<S>> {
teachers: Ta;
students: Ts;
}
type EditableSchool = School<EditableTeacher, Array<EditableTeacher>, EditableStudent, Array<EditableStudent>>;
type ReadonlySchool = Readonly<School<ReadonlyTeacher, ReadonlyArray<ReadonlyTeacher>, ReadonlyStudent, ReadonlyArray<ReadonlyStudent>>>;
The rest is the same, but then:
s.teachers = null; // Cannot assign to 'teachers' because it is a constant or a read-only property
s.teachers[0] = null; // Index signature in type 'ReadonlyArray<Readonly<Teacher<Readonly<PersonInfo>>>>' only permits reading
True, it's not a simple solution and is very verbose, but I don't think that a simple generic solution exists, at least for now, for the problem.
Consider:
type ReadyonlyDeep<T> = {
readonly [P in keyof T]: ReadyonlyDeep<T[P]>;
}
This works great as long as the object has only other simple objects:
interface A {
b: B
}
interface B {
str: string;
}
let a: ReadyonlyDeep<A>;
a.b.str = "fe"; // Cannot assign to 'str' because it is a constant or a read-only property
But if you introduce an array:
interface B {
str: string;
moreB: B[];
}
Then:
a.b.moreB = []; // Cannot assign to 'moreB' because it is a constant or a read-only property
a.b.moreB[0].str = "hey"; // this is fine though
It probably isn't an easy problem to solve.
来源:https://stackoverflow.com/questions/41237887/make-all-fields-in-an-interface-readonly-in-a-nested-way