TypeScript does not perform type checking on arrays created using the Array() constructor and filled with the fill method

Question

TypeScript does not perform type checking on arrays created using the Array() constructor and filled with the fill method

Using TypeScript version 2.4.2, compiled with the --target ES6 option has interesting results.

For example, when using this line of code:

var coins: { coin: number}[] = [1,1,1]

TypeScript throws an error:

Error TS2322: Type 'number[]' is not assignable to type '{ coin: number; }[]'

Surprisingly, when using this line:

var coins: { coin: number}[] = Array(3).fill(1)

The code compiles successfully without any errors.

This raises the question - is this a bug in TypeScript or is it actually intended behavior? And if so, why does TypeScript allow this kind of array declaration without performing type checking?

javascript typescript issue-tracking typescript2.4

Answer 1

Answer №1

The issue arises when using Array(3) to create an array with a length of 3 and type any[], followed by applying the fill method on that array. Because any can be assigned to any type, the array generated as any[] can then be assigned to arrays of other types (such as { coin: number}).

If you were to explicitly provide the type parameter for the Array function, you would receive an error:

var coins: { coin: number}[] = Array<{ coin: number}>(3).fill(1) // 1 cannot be assigned to type { coin: number}

Unfortunately, there is no foolproof way to enforce the inclusion of the type argument for arrays in this scenario.

Answer 2

The issue arises when using Array(3) to create an array with a length of 3 and type any[], followed by applying the fill method on that array. Because any can be assigned to any type, the array generated as any[] can then be assigned to arrays of other types (such as { coin: number}).

If you were to explicitly provide the type parameter for the Array function, you would receive an error:

var coins: { coin: number}[] = Array<{ coin: number}>(3).fill(1) // 1 cannot be assigned to type { coin: number}

Unfortunately, there is no foolproof way to enforce the inclusion of the type argument for arrays in this scenario.

TypeScript does not perform type checking on arrays created using the Array() constructor and filled with the fill method

Answer №1

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