在创建 ._each 函数时如何编写 'context' 变量 (underscore.js)

Question

我正在尝试从 underscore.js 重新创建 _.each() 函数，但是无法使 'context' 变量正确。我明白它的意思，但似乎在绕圈子试图实施它。我已经用注释标记了下面的代码区域。任何提示表示赞赏，谢谢！

_.each = function (collection, iteratee, context) {
  //-----context--------
  if (context) {
    if (Array.isArray(collection)) {
      for (let i = 0; i < collection.length; i++) {
        iteratee.call(context, collection[i], collection);
      }
    } else {
    for (const key in collection) {
      iteratee.call(context, collection[key], collection);
    }
  }
    return collection;
  }
  //-----end of context----
  //check if array or object
  if (Array.isArray(collection)) {
    for (let i = 0; i < collection.length; i++) {
      iteratee(collection[i], i, collection);
    }
  } else {
    for (const key in collection) {
      iteratee(collection[key], key, collection);
    }
  }
  return collection;
};

Answer 1

_.each(
 [1, 2, 3],
 function(value){
   // get context by this
   console.log(this.times * value);
 },
 { times: 10 } // iteratee context,
)

Answer 2

您根本不需要 if (context) {。从根本上讲，无论是否提供上下文，您都需要进行相同的操作，因此您可以将代码留在if之外，并始终执行.call(context, /* other arguments */。 Function#call will always execute the function and the first parameter will be the this value of that invocation. If the value undefined is provided, then that's not an error - in non-strict mode the value of this will be assigned to the global object but in strict mode it How does the "this" keyword work?

因此，如果未提供上下文，那么您通常无需执行任何特殊操作。

const _ = {};

_.each = function (collection, iteratee, context) {
  //check if array or object
  if (Array.isArray(collection)) {
    for (let i = 0; i < collection.length; i++) {
      iteratee.call(context, collection[i], collection);;
    }
  } else {
    for (const key in collection) {
      iteratee.call(context, collection[key], collection);
    }
  }
  return collection;
};


const arr = [1, 2, 3];
const obj = {
  foo: "hello",
  bar: "world"
};

function print(value) {
  console.log(value);
}

console.log("--- basic foreach ---");

_.each(arr, print);
_.each(obj, print);

console.log("--- foreach with context ---");

function contextPrint(value) {
  console.log(this[value]);
}

_.each(arr, contextPrint, { 1: "one", 2: "two", 3: "three" });
_.each(obj, contextPrint, { hello: "this", world: "works" });

console.log("--- no context provided ---");

_.each(arr, contextPrint);
_.each(obj, contextPrint);

与 Underscore 比较，行为相同：

const arr = [1, 2, 3];
const obj = {
  foo: "hello",
  bar: "world"
};

function print(value) {
  console.log(value);
}

console.log("--- basic foreach ---");

_.each(arr, print);
_.each(obj, print);

console.log("--- foreach with context ---");

function contextPrint(value) {
  console.log(this[value]);
}

_.each(arr, contextPrint, { 1: "one", 2: "two", 3: "three" });
_.each(obj, contextPrint, { hello: "this", world: "works" });

console.log("--- no context provided ---");

_.each(arr, contextPrint);
_.each(obj, contextPrint);

<script src="https://cdnjs.cloudflare.com/ajax/libs/underscore.js/1.11.0/underscore-min.js" integrity="sha512-wBiNJt1JXeA/ra9F8K2jyO4Bnxr0dRPsy7JaMqSlxqTjUGHe1Z+Fm5HMjCWqkIYvp/oCbdJEivZ5pLvAtK0csQ==" crossorigin="anonymous"></script>

在创建 ._each 函数时如何编写 'context' 变量 (underscore.js)

How would you write the 'context' variable while creating the ._each function (underscore.js)

javascript

underscore.js