Incorrect type inference for array rest assignment and inability to add annotation

TypeScript Version: 2.7.1-insiders.20180127

Search Terms: array destructuring rest assignment

Code

// A *self-contained* demonstration of the problem follows...
// Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp, Webpack, etc.
const [stringA, ...numbers] = ['string', 1, 2];
// const [stringA, ...numbers] = <[string, number, number]>['string', 1, 2];
// const [stringA, ...numbers]: [string, number[]] = ['string', 1, 2];
// const [stringA, ...numbers]: [string, [number, number]] = ['string', 1, 2];

Expected behavior: Expect numbers to be of type number[] rather than (string | number)[].
TS also does not allow annotating as a tuple

Actual behavior: const numbers: (string | number)[]

Playground Link: http://www.typescriptlang.org/play/index.html#src=const%20%5BstringA%2C%20...numbers%5D%20%3D%20%5B'string'%2C%201%2C%202%5D%3B%0A%2F%2F%20const%20%5BstringA%2C%20...numbers%5D%20%3D%20%3C%5Bstring%2C%20number%2C%20number%5D%3E%5B'string'%2C%201%2C%202%5D%3B%0A%2F%2F%20const%20%5BstringA%2C%20...numbers%5D%3A%20%5Bstring%2C%20number%5B%5D%5D%20%3D%20%5B'string'%2C%201%2C%202%5D%3B%0A%2F%2F%20const%20%5BstringA%2C%20...numbers%5D%3A%20%5Bstring%2C%20%5Bnumber%2C%20number%5D%5D%20%3D%20%5B'string'%2C%201%2C%202%5D%3B

Related Issues:

Duplicate of #3369. A tuple type is not inferred in the absence of an explicit type annotation.

you can find related discussions in #16656

Mohamed Hegazy (@mhegazy)

A tuple type is not inferred in the absence of an explicit type annotation

Mohamed Hegazy (@mhegazy) so how would the numbers rest assignment be annotated as a tuple? I believe I tried to this in the last line:

const [stringA, ...numbers]: [string, [number, number]] = ['string', 1, 2];

The right type should be const [stringA, ...numbers]: [string, number, number]. and numbers should be inferred correctly. that is a bug we should fix.

I have run into this issue too with overload function

// I have tried this but it does not work
interface OperatorTuple extends Array<OperatorFunction<any, any>> {
  0: OperatorFunction<any, any>;
  1: OperatorFunction<any, any>;
  2: OperatorFunction<any, any>;
  3: OperatorFunction<any, any>;
  4: OperatorFunction<any, any>;
  5: OperatorFunction<any, any>;
  6: OperatorFunction<any, any>;
  7: OperatorFunction<any, any>;
  length: 8;
}

// this is an overload function
export function Select<A, B>(mapFn: ((state: A) => B), ...operators: OperatorFunction<any, any>[]) {
  return function(target: any, name: string): void {
    if (typeof mapFn !== 'function') {
      throw new TypeError(
        `Unexpected type '${typeof mapFn}' in select operator,` + ` expected 'function'`
      );
    }

    // trying to cast type tuple
    operators = [...operators] as OperatorTuple;

    const descriptor = Object.getOwnPropertyDescriptor(target, name);

    if (delete target[name]) {
      Object.defineProperty(target, name, {
        get() {
          const source$ = NgrxSelect.store;

          if (!source$) {
            throw new Error('NgrxSelect not connected to store!');
          }

         // I have tried this too but have failed
         if (operators.length > 8) {
            throw new Error()
         }
          
          // Error: Expect 0-9 arguments but received 1 or more
          return source$.pipe(select(mapFn), ...operators);
        },
        ...descriptor
      });
    }
  };
}

The issue in the opening post is fixed in typescript@next, maybe even 3.0.0-rc.
numbers is now of type number[].

It's now [number, number], even better (?)

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Incorrect type inference for array rest assignment and inability to add annotation #21519

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Incorrect type inference for array rest assignment and inability to add annotation #21519

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mhegazy commented on Jan 31, 2018

theseyi commented on Feb 1, 2018

mhegazy commented on Feb 1, 2018

sandangel commented on Feb 8, 2018

ajafff commented on Jul 27, 2018

RyanCavanaugh commented on Oct 29, 2025

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