TypeScript's robustly-typed rest parameters

Question

TypeScript's robustly-typed rest parameters

Is there a way to properly define dynamic strongly typed rest parameters using TypeScript 3.2? Let's consider the following scenario:

function execute<T, Params extends ICommandParametersMapping, Command extends keyof Params, Args extends Params[Command]>(command: Command, ...rest: Args): Args{
    return;
}

execute('cmd2', true, 1, 'hello');

interface ICommandParametersMapping {
    ['cmd1']: [string];
    ['cmd2']: [boolean, number, string];
    ['cmd3']: [boolean, boolean];
}

Initially, everything appears to function correctly. When passing arguments to execute with the command cmd2, TypeScript provides type information for the 3 arguments and the return value is also accurate...

However, an issue arises when declaring ...rest: Args as the rest parameter.

The error message received is:

A rest parameter must be of an array type.

typescript typescript-generics

Answer 1

Answer №1

Issue

The problem arises with the type of U, which must be in array form. While U is a value of P, it cannot be assumed that all values within P are arrays. This uncertainty stems from the fact that the function exec relies on an unspecified P which may deviate from the structure outlined by the interface ICommandNameArgumentTypeMapping. Therefore, there is a risk that this unknown entity could introduce properties that are not arrays.

Resolution

To address this issue, it is essential to ensure that all current and future values maintain an array format.

interface ICommandNameArgumentTypeMapping {
    ['cmd1']: [string];
    ['cmd2']: [boolean, number, string];
    ['cmd3']: [boolean, boolean];
    [index: string]: any[]
}

This additional feature is referred to as an index signature.

A more precise approach would involve substituting (string | number | boolean)[] for any[].

Additional Considerations

Your code contains several other errors:

The computed property name cmd2 is repeated
The type parameter T remains unused
The type parameter P is misapplied (not describing parameters or return types)
Although exec pledges to return U, it currently yields undefined

The rectified solution:

function exec<P extends ICommandNameArgumentTypeMapping, E extends keyof P, U extends P[E]>(mapping: P, command: E, ...rest: U): U{
    return rest;
}

interface ICommandNameArgumentTypeMapping {
    ['cmd1']: [string];
    ['cmd2']: [boolean, number, string];
    ['cmd3']: [boolean, boolean];
    [index: string]: any[]
}

declare const mapping: ICommandNameArgumentTypeMapping;

exec(mapping, 'cmd2', true, 1, 'hello');

Answer 2

Issue

The problem arises with the type of U, which must be in array form. While U is a value of P, it cannot be assumed that all values within P are arrays. This uncertainty stems from the fact that the function exec relies on an unspecified P which may deviate from the structure outlined by the interface ICommandNameArgumentTypeMapping. Therefore, there is a risk that this unknown entity could introduce properties that are not arrays.

Resolution

To address this issue, it is essential to ensure that all current and future values maintain an array format.

interface ICommandNameArgumentTypeMapping {
    ['cmd1']: [string];
    ['cmd2']: [boolean, number, string];
    ['cmd3']: [boolean, boolean];
    [index: string]: any[]
}

This additional feature is referred to as an index signature.

A more precise approach would involve substituting (string | number | boolean)[] for any[].

Additional Considerations

Your code contains several other errors:

The computed property name cmd2 is repeated
The type parameter T remains unused
The type parameter P is misapplied (not describing parameters or return types)
Although exec pledges to return U, it currently yields undefined

The rectified solution:

function exec<P extends ICommandNameArgumentTypeMapping, E extends keyof P, U extends P[E]>(mapping: P, command: E, ...rest: U): U{
    return rest;
}

interface ICommandNameArgumentTypeMapping {
    ['cmd1']: [string];
    ['cmd2']: [boolean, number, string];
    ['cmd3']: [boolean, boolean];
    [index: string]: any[]
}

declare const mapping: ICommandNameArgumentTypeMapping;

exec(mapping, 'cmd2', true, 1, 'hello');

Answer 3

Answer №2

The issue lies in the declaration of

P extends ICommandNameArgumentTypeMapping

. This essentially means that the exec() function can accept any mapping that includes all the properties defined in the interface, potentially allowing non-array types to be passed through. By removing this constraint and addressing any potential typos, you will notice that the error messages disappear.

interface ICommandNameArgumentTypeMapping {
    ['cmd1']: [string];
    ['cmd2']: [boolean, number, string];
    ['cmd3']: [boolean, boolean];
}

type P = ICommandNameArgumentTypeMapping;

function exec<T, E extends keyof P, U extends P[E]>(command: E, ...rest: U): U{
    return rest;
}

exec('cmd2', true, 1, 'hello');

Answer 4

The issue lies in the declaration of

P extends ICommandNameArgumentTypeMapping

. This essentially means that the exec() function can accept any mapping that includes all the properties defined in the interface, potentially allowing non-array types to be passed through. By removing this constraint and addressing any potential typos, you will notice that the error messages disappear.

interface ICommandNameArgumentTypeMapping {
    ['cmd1']: [string];
    ['cmd2']: [boolean, number, string];
    ['cmd3']: [boolean, boolean];
}

type P = ICommandNameArgumentTypeMapping;

function exec<T, E extends keyof P, U extends P[E]>(command: E, ...rest: U): U{
    return rest;
}

exec('cmd2', true, 1, 'hello');

Answer 5

Answer №3

When using the syntax ...rest, remember that rest will be treated as an array:

const newFunction = (...params) => console.log(params);

newFunction('goodbye', 'universe');

Therefore, it is recommended to use the following format instead:

(action: A, ...rest: V[])

You can adjust this to suit the specific requirements of your program.

Answer 6

When using the syntax ...rest, remember that rest will be treated as an array:

const newFunction = (...params) => console.log(params);

newFunction('goodbye', 'universe');

Therefore, it is recommended to use the following format instead:

(action: A, ...rest: V[])

You can adjust this to suit the specific requirements of your program.

TypeScript's robustly-typed rest parameters

Answer №1

Answer №2

Answer №3

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