为什么数字/字符串/。。。可赋值为空?

export type ArrayWithNumberOrString = Array<number | string>;
export type ArrayWithNumberOrNull = Array<number | null>;


function doSomethingWithNumbersOnly(array: number[]): number[] {
  return array;
};

const numbersAndString: ArrayWithNumberOrString = [1, 2, 3, 'string'];
const numbersAndNull: ArrayWithNumberOrNull = [1, 2, 3, null];

doSomethingWithNumbersOnly(numbersAndString);
doSomethingWithNumbersOnly(numbersAndNull);