非空断言运算符（后缀 !）

TypeScript 还具有一种特殊的语法，可以在不进行任何显式检查的情况下从类型中删除 null 和 undefined。 在任何表达式之后写 ! 实际上是一个类型断言，该值不是 null 或 undefined：

TypeScript also has a special syntax for removing null and undefined from a type without doing any explicit checking. Writing ! after any expression is effectively a type assertion that the value isn't null or undefined:

function liveDangerously(x?: number | null) {
  // No error
  console.log(x!.toFixed());
}

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