Question

对于此对象数组：

var contacts = [
  {
      "firstName": "Akira",
      "lastName": "Laine",
      "number": "0543236543",
      "likes": ["Pizza", "Coding", "Brownie Points"]
  },
  {
      "firstName": "Harry",
      "lastName": "Potter",
      "number": "0994372684",
      "likes": ["Hogwarts", "Magic", "Hagrid"]
  },
  {
      "firstName": "Sherlock",
      "lastName": "Holmes",
      "number": "0487345643",
      "likes": ["Intriguing Cases", "Violin"]
  },
  {
      "firstName": "Kristian",
      "lastName": "Vos",
      "number": "unknown",
      "likes": ["JavaScript", "Gaming", "Foxes"]
  }
];

然后有一个for循环，该循环遍历对象数组以及用于针对对象进行查找的参数。我知道我们正在迭代，并且该函数将在returns之后停止运行。我添加了console.log(i);来查看迭代器变量的值，以查看循环发生了多少次。

function lookUpProfile(name, prop) {  

    for (var i = 0; i < contacts.length; i++) {  
      console.log(i);
      if (contacts[i].firstName === name) {  
        if (contacts[i].hasOwnProperty(prop)) {         
          return contacts[i][prop]   
        } else {
          return "No such property"; 
        }
      }

    } //end of loop
  return "No such contact"; 
  }


   console.log(lookUpProfile("Kristian", "lastName")); // Vos

因此，我运行代码并查看控制台输出并获得期望的结果，我们进行4次迭代，然后return当前对象的属性。

为什么当我重构函数时代码不起作用，所以我没有逻辑嵌套的if，而是使用了逻辑AND运算符&&，它将两个测试条件合并为一个if语句-像这样：

function lookUpProfile(name, prop) { 
  for (var i = 0; i < contacts.length; i++) {  
    console.log(i);
    if (contacts[i].firstName === name && contacts[i].hasOwnProperty(prop)) {                 
      return contacts[i][prop]   
    } else {
      return "No such property"; 
    }

  } //end of loop
  return "No such contact"; 
}

 console.log(lookUpProfile("Kristian", "lastName")); // Expected output: Vos

现在，当我运行代码时，似乎没有迭代发生，因为console.log(i);似乎一次undefined输出了一次，而不是多次输出，所以我们似乎没有在循环，我也不明白为什么。其次，即使满足我们的测试条件，我们也将返回“无此类属性”。

如果这个简单的重构为什么破坏了功能，我需要了解什么。我知道一旦函数返回某些结果，函数执行将结束并且迭代将停止，我认为此问题与循环外和控制流中断有关，但是我不明白为什么。请有人可以清楚地说明为什么在我的特定情况下会发生这种情况。

Answer 1

如果您嵌套了if条件

if (contacts[i].firstName === name)

这将检查名称，如果名称不匹配则不执行任何操作

但是在第二个例子中，如果名称不匹配，则在else语句后返回else语句

 if (contacts[i].firstName === name && contacts[i].hasOwnProperty(prop)) {                 
      return contacts[i][prop]   
    } else {                           // problem is here
  return "No such property"; 
 }

只需从循环中删除else条件

var contacts = [{"firstName": "Akira","lastName": "Laine","number": "0543236543","likes": ["Pizza", "Coding", "Brownie Points"]},{"firstName": "Harry","lastName": "Potter","number": "0994372684","likes": ["Hogwarts", "Magic", "Hagrid"]},{"firstName": "Sherlock","lastName": "Holmes","number": "0487345643","likes": ["Intriguing Cases", "Violin"]},{"firstName": "Kristian","lastName": "Vos","number": "unknown","likes": ["JavaScript", "Gaming", "Foxes"]}];


function lookUpProfile(name, prop) {
  for (var i = 0; i < contacts.length; i++) {
    if (contacts[i].firstName === name && contacts[i].hasOwnProperty(prop)) {
      return contacts[i][prop]
    }
  } //end of loop
  return "No such contact";
}

console.log(lookUpProfile("Kristian", "lastName"));

Answer 2

因为在您之前的代码中，仅当"No such property"为contacts[i].hasOwnProperty(prop)时才返回false。现在，如果contacts[i].firstName === name为假，也将返回它。您希望将这两个if块分开。

此外，您的console.log(i)打印undefined的原因是，它位于for变量声明之前的i循环之上。将其向下移动1行。

Answer 3

return将停止循环。

在您的情况下，由于名称或属性不匹配，循环在第一次迭代后停止

var contacts = [{"firstName":"Akira","lastName":"Laine","number":"0543236543","likes":["Pizza","Coding","Brownie Points"]},{"firstName":"Harry","lastName":"Potter","number":"0994372684","likes":["Hogwarts","Magic","Hagrid"]},{"firstName":"Sherlock","lastName":"Holmes","number":"0487345643","likes":["Intriguing Cases","Violin"]},{"firstName":"Kristian","lastName":"Vos","number":"unknown","likes":["JavaScript","Gaming","Foxes"]}];

function lookUpProfile(name, prop) {
  for (var i = 0; i < contacts.length; i++) {
    if (contacts[i].firstName === name && contacts[i].hasOwnProperty(prop)) {
      return contacts[i][prop]
    }
  } //end of loop
  return "No such contact";
}

console.log(lookUpProfile("Kristian", "lastName"));
console.log(lookUpProfile("Kristian1", "lastName"));

带for循环的数组迭代-使用条件语句和返回值

3 个答案: