javascript - 为什么 Prop 显示未定义？

Question

所以我有在 Freecodecamp 上运行的代码。无法理解为什么未定义其显示属性。

prop 实际上是在调用函数时从函数中获取值。与 prop 不同，名称不会因为未定义而出现。

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){
// Only change code below this line

switch(name) {
  case "Akira":
  case "Harry":
  case "Sherlock":
  case "Kristian":
  for ( var j=0; j<contacts.length;j++) {
  if(contacts[j].firstName == name) {
    return contacts[j][prop];
  } else {
     console.log("No such contact");
  }
}break;
}
}

if (!contacts.hasOwnProperty(prop)) {
  console.log("No such property");
// Only change code above this line
}

// Change these values to test your function
lookUpProfile("Kristian", "lastName");`

Answer 1

这就是为什么正确的代码缩进很重要。您的 if 语句位于您的函数之外，因此您尝试使用它的范围内不存在 prop。将您的 if 语句移至函数 lookUpProfile 中，然后它不会是未定义的。

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) {
  // Only change code below this line
  if (!contacts.hasOwnProperty(prop)) {
    console.log("No such property");
    // Only change code above this line
  }
  switch(name) {
    case "Akira":
    case "Harry":
    case "Sherlock":
    case "Kristian":
      for (var j = 0; j < contacts.length; j++) {
        if(contacts[j].firstName === name) {
          return contacts[j][prop];
        } else {
          console.log("No such contact");
        }
      } break;
  }
}

// Change these values to test your function
lookUpProfile("Kristian", "lastName");