題目
中文
Block，Element，Modifier 方法（BEM）是 CSS 中類的流行命名約定。
例如，塊組件用 btn 表示，依賴於塊的元素用 btn__price 表示，更改塊樣式的修飾符將用 btn--big 或者 btn__price--warning表示。
實現從這三個參數生成字符串聯合的 BEM<B, E, M>。其中 B 是字符串字面量，E 和 M 是字符串數組（可以爲空）。
English
The Block, Element, Modifier methodology (BEM) is a popular naming convention for classes in CSS.
For example, the block component would be represented as btn, element that depends upon the block would be represented as btn__price, modifier that changes the style of the block would be represented as btn--big or btn__price--warning.
Implement BEM<B, E, M> which generate string union from these three parameters. Where B is a string literal, E and M are string arrays (can be empty).
答案
type BEM<
B extends string,
E extends string[],
M extends string[]
> = `${B}${E extends [] ? '' : `__${E[number]}`}${M extends []
? ''
: `--${M[number]}`}`;