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?

javascripttypescriptissue-trackingtypescript2.4

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.

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