Is there a way to initialize an object literal and declare its interface with read-only property in-hand at the same time ?
For example
let a = { readonly b: 2, readonly c: 3 }
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3 Answers
You can use a as const assertion:
let a = { b: 2, c: 3 } as const // typed as { readonly b: 2; readonly c: 3; }
a.b = 2 //Cannot assign to 'b' because it is a read-only property.
If you want only some props to be readonly, that is not really possible, best you can do is use an Object.assign with one part containing readonly properties and the other containing the mutable properties:
let a = Object.assign({ b: 2, c: 3 } as const, {
d: 0
});
a.b = 2 // err
a.d = 1 //ok
answered Jun 11, 2019 at 10:07
Titian Cernicova-Dragomir
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10 Comments
Juraj Kocan
but you can still change b and c property
Roberto Zvjerković
No you can not. "Cannot assign to 'b' because it is a read-only property."
Titian Cernicova-Dragomir
@JurajKocan wasn't the point to make the properties readonly ?
Juraj Kocan
@ritaj what ts version are u using? i can still write this a.b = 3 with no error in ts 3.5.1
Quoc-Hao Tran
indeed it works ! I got an error writing a.b = 3. Using typescript 3.4.5
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"as const" is just a compile time error, not runtime error, have to use getter only property.
const a = Object.assign({ get b() { return 2 }, get c() { return 3 } }, {
d: 0
});
a.b = 2 // err
a.d = 1 //ok
alert(JSON.stringify(a))
FYI, Object.Freeze make whole object can't asssign new propeties
TS Playground, see transformed code, and run it
Comments
you can write it with custom type
type Point = {
readonly x: number;
readonly y: number;
};
const a: Point = {
x: 1,
y: 2,
};
a.x = 2; // ERROR Cannot assign to 'x' because it is a constant or a read-only property.
