javascript - 这个 Array.prototype.find() 是如何工作的？

Question

我不明白的是该函数及其参数fruit，因为它没有传递任何内容。

var inventory = [
  {name: 'apples', quantity: 2},
  {name: 'bananas', quantity: 0},
  {name: 'cherries', quantity: 5}
];

function findCherries(fruit) { 
  return fruit.name === 'cherries';
}

console.log(inventory.find(findCherries));

Answer 1

...because it's not being passed anything.

它正在传递一些东西，你只是看不到执行此操作的代码，它位于Array.prototype.find内部方法的代码。请记住，这一行:

console.log(inventory.find(findCherries));

...将 findCherries(函数引用)的值传递给find，它不调用 findCherries。 find 会为数组中的每个条目执行一次，并提供三个参数(findCherries 仅使用其中一个参数)。第一个参数是该调用的数组条目。

Array.prototype.find 的完整 polyfill 会使阅读变得棘手，所以让我们做一些类似但更简单的事情，以便您可以看到调用:

// A bit like Array.prototype.find, but **not** the same; much simplified
function fakeFind(callback) {
  for (var index = 0; index < this.length; ++index) {
    var entry = this[index];
    //  vvvvvvvvvvvvvvvvvvvvvvvvvvvv---- This is where findCherries is called
    if (callback(entry, index, this)) {
      return entry;
    }
  }
  return undefined;
}

实时复制:

// A bit like Array.prototype.find, but **not** the same; much simplified
function fakeFind(callback) {
  for (var index = 0; index < this.length; ++index) {
    var entry = this[index];
    //  vvvvvvvvvvvvvvvvvvvvvvvvvvvv---- This is where findCherries is called
    if (callback(entry, index, this)) {
      return entry;
    }
  }
  return undefined;
}
Object.defineProperty(Array.prototype, "fakeFind", {
  value: fakeFind
});
var inventory = [
  {name: 'apples', quantity: 2},
  {name: 'bananas', quantity: 0},
  {name: 'cherries', quantity: 5}
];

function findCherries(fruit) { 
  return fruit.name === 'cherries';
}

console.log(inventory.fakeFind(findCherries));