{
"articleTitle": "实现First",
"date": "2022-10-22 21:18:15",
"tags": [
"typescript",
"First"
],
"categories": "ts",
"timestamp": 1666473495000,
"readingTime": 0,
"outline": "ts类型体操，实现First过程中的知识点"
}
readingTime: <1min
First
实现一个通用First<T>，它接受一个数组T并返回它的第一个元素的类型。
例如：
type arr1 = ['a', 'b', 'c']
type arr2 = [3, 2, 1]
type head1 = First<arr1> // expected to be 'a'
type head2 = First<arr2> // expected to be 3
infer
TODO: infer相关
infer关键字用于推断泛型类型的类型参数，可以用于条件类型中。
answer
type First<T extends any[]> = T extends [infer R, ...any[]] ? R : never