reactjs中的提升功能

Question

const updateSearchTopStoriesState = (hits, page) => (prevState) => {
const { searchKey, results } = prevState

有人可以向我解释以上内容的作用吗？为什么有2个箭头功能？

这是整个代码

const updateSearchTopStoriesState = (hits, page) => (prevState) => {
const { searchKey, results } = prevState;
const oldHits = results && results[searchKey]
? results[searchKey].hits
: [];
const updatedHits = [
...oldHits,
...hits
];
return {
results: {
...results,
[searchKey]: { hits: updatedHits, page }
},
isLoading: false
};
};
class App extends Component {
...

Answer 1

连续两个箭头功能被称为thunk。

简单来说，以这种格式可以更容易理解：

function updateSearchTopStoriesState = (hits, page) {
   function updateState (prevState) {
    ...code
   }
   return updateState;
}

Answer 2

箭头功能意味着隐式返回。因此，如果看到两个箭头紧挨着，则意味着第一个函数返回一个函数。这不是React，而是ES2016。上面的答案是正确的：

function updateSearchTopStoriesState = (hits, page) {
   return function updateState(prevState) {
    ...code
   }
//Anything you put here would be disregarded since it is after the return statement.
}

此为基本示例：

const add = num1 => num2 => num3 => num1+num2+num3;

console.log(add(3)(4)(5));