TypeScript 内置了一些类型，包括 ES5，ES6 语法 API 的类型声明，很多地方看起来很有意思。我也不知道学习 TypeScript 从哪里入手，索性看着写声明文件学习一下，遇到有趣的就记录一下。因此未有定序，不似别人脉络清晰，偶有所得，翻用自喜。
is 使用
Array 的 every 方法，类型声明很有意思：
interface ReadonlyArray<T> {
...
/**
* Determines whether all the members of an array satisfy the specified test.
* @param predicate A function that accepts up to three arguments. The every method calls
* the predicate function for each element in the array until the predicate returns a value
* which is coercible to the Boolean value false, or until the end of the array.
* @param thisArg An object to which the this keyword can refer in the predicate function.
* If thisArg is omitted, undefined is used as the this value.
*/
every<S extends T>(predicate: (value: T, index: number, array: readonly T[]) => value is S, thisArg?: any): this is readonly S[];
/**
* Determines whether all the members of an array satisfy the specified test.
* @param predicate A function that accepts up to three arguments. The every method calls
* the predicate function for each element in the array until the predicate returns a value
* which is coercible to the Boolean value false, or until the end of the array.
* @param thisArg An object to which the this keyword can refer in the predicate function.
* If thisArg is omitted, undefined is used as the this value.
*/
every(predicate: (value: T, index: number, array: readonly T[]) => unknown, thisArg?: any): boolean;
}
第二个类型声明很简单，就是 every 返回一个 boolean 判断 every 是否全部通过测试。第一个类型声明重点在于 is，is 在 TypeScript 中叫做类型谓词，简单理解就是可以收缩类型。这个类型声明的意思就是，假设 predicate 可以将数据类型收缩到 S，可以认为通过 every
