BEM style string

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题目

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).

解答

type BEM<
  B extends string,
  E extends string[],
  M extends string[]
> = `${B}${E extends [] ? "" : `__${E[number]}`}${M extends []
  ? ""
  : `--${M[number]}`}`;