We can declare a typed tuple in TypeScript, for example, with the type annotation [string, number]
. This means an array of 2 elements where the first element needs to be a string and the second a number.
We can also declare read-only arrays with ReadonlyArray<string>
which means a read-only array of strings.
Now I want to have a read-only tuple like in the first example, but I want it to be read-only like in the second example. How would I declare that?
Since the type [string, number]
already is an Array
, you can simply use:
Readonly<[string, number]>
Example:
let tuple: Readonly<[string, number]> = ['text', 3, 4, 'another text'];
tuple[0] = 'new text'; //Error (Readonly)
let string1: string = tuple[0]; //OK!
let string2: string = tuple[1]; //Error (Type number)
let number1: number = tuple[0]; //Error (Type string)
let number2: number = tuple[1]; //OK!
let number3: number = tuple[2]; //Error (Type any)
The accepted answer leaves array mutation methods unaffected, which can cause unsoundness in the following way:
const tuple: Readonly<[number, string]> = [0, ''];
tuple.shift();
let a = tuple[0]; // a: number, but at runtime it will be a string
The code below fixes this issue, and includes Sergey Shandar's destructuring fix. You'll need to use --noImplicitAny
for it to work properly.
type ArrayItems<T extends ReadonlyArray<any>> = T extends ReadonlyArray<infer TItems> ? TItems : never;
type ExcludeProperties<TObj, TKeys extends string | number | Symbol> = Pick<TObj, Exclude<keyof TObj, TKeys>>;
type ArrayMutationKeys = Exclude<keyof any[], keyof ReadonlyArray<any>> | number;
type ReadonlyTuple<T extends any[]> = Readonly<ExcludeProperties<T, ArrayMutationKeys>> & {
readonly [Symbol.iterator]: () => IterableIterator<ArrayItems<T>>;
};
const tuple: ReadonlyTuple<[number, string]> = [0, ''];
let a = tuple[0]; // a: number
let b = tuple[1]; // b: string
let c = tuple[2]; // Error when using --noImplicitAny
tuple[0] = 1; // Error
let [d, e] = tuple; // d: number, e: string
let [f, g, h] = tuple; // Error
It looks like there will be a clean solution for this requirement coming with TypeScript 3.4 version:
With so-called const contexts , the compiler can be told to treat an array or an object as immutable, meaning that their properties are read-only. This also allows the creation of literal tuple types with narrower type inference (ie your ["a", "b"]
can for the first time be of type ["a", "b"]
, not string[]
without specifiying the whole thing as a contextual type)
The syntax will look like this:
let foo = ["text", 1] as const
or
let foo = <const> ["text", 1]
Here is the extended information of the corresponding PR. As of now, the feature should be available in typescript@next
.
Readonly<[string, T]>
doesn't allow destruction. For example
const tuple: Readonly<[string, number]> = ["text", 4]
const [n, v] = tuple // error TS2488: Type 'Readonly<[string, number]>' must have a '[Symbol.iterator]()' method that returns an iterator.
So, it's better to use a custom interface
export interface Entry<T> {
readonly [0]: string
readonly [1]: T
readonly [Symbol.iterator]: () => IterableIterator<string|T>
}
For example
const tuple: Entry<number> = ["text", 4]
const [name, value] = tuple // ok
const nameCheck: string = name
const valueCheck: number = value
From Typescript version 3.4 you can just prefix tuple type with readonly
keyword ( source ).
TypeScript 3.4 also introduces new support for
readonly
tuples. We can prefix any tuple type with thereadonly
keyword to make it areadonly
tuple, much like we now can with array shorthand syntax. As you might expect, unlike ordinary tuples whose slots could be written to,readonly
tuples only permit reading from those positions.function foo(pair: readonly [string, string]) { console.log(pair[0]); // okay pair[1] = "hello!"; // error }
As of v3.2.2, there's no perfect way of making a readonly tuple type without converting it to an object that looks like an array, but is not.
The lead architect of TypeScript has said this on the topic of combining Readonly<T>
with tuple types.
Here is the best solution I've come up with:
type ReadonlyTuple<T extends any[]> = {
readonly [P in Exclude<keyof T, keyof []>]: T[P]
} & Iterable<T[number]>
The technical post webpages of this site follow the CC BY-SA 4.0 protocol. If you need to reprint, please indicate the site URL or the original address.Any question please contact:yoyou2525@163.com.