javascript - 如何让我通过 forEach 传递的函数引用其他范围内的变量？

Question

我不知道为什么我的 findDroids 函数无法在我的 droids 函数中引用 result 变量。当我运行这段代码时，我得到“结果未定义”。非常感谢任何提示/指导。我是 Javascript 的新手，所以请放轻松 :)

function droids(arr) {
  let result = '';
  arr.forEach(findDroids);
  return result;
}

function findDroids(value, index, arr){
  if (arr[index] == "Droids"){
    result = "Found Droids!";
  } else{
    result = "These are not the droids you're looking for."
  }
}

// Uncomment these to check your work! 
const starWars = ["Luke", "Finn", "Rey", "Kylo", "Droids"] 
const thrones = ["Jon", "Danny", "Tyrion", "The Mountain", "Cersei"] 
console.log(droids(starWars)) // should log: "Found Droids!"
console.log(droids(thrones)) //should log: "These are not the droids you're looking for."

Answer 1

因为在 JS 中，let 变量的作用域是它最近的函数。在这种情况下，result 仅在 droids 级别可用。使变量成为全局变量应该可行:

let result = ''; // Available everywhere

function droids(arr) {
  //let result = ''; // Available only at `droids` level
  arr.forEach(findDroids);
  return result;
}

function findDroids(value, index, arr){
  if (arr[index] == "Droids"){
    result = "Found Droids!";
  } else{
    result = "These are not the droids you're looking for."
  }
}

// Uncomment these to check your work! 
const starWars = ["Luke", "Finn", "Rey", "Kylo", "Droids"] 
const thrones = ["Jon", "Danny", "Tyrion", "The Mountain", "Cersei"] 
console.log(droids(starWars)) // should log: "Found Droids!"
console.log(droids(thrones)) //should log: "These are not the droids you're looking for."

话虽如此，使用全局变量可能不是最好的选择。您可以使用 haystack.includes(needle) 在 haystack 中找到 needle 以轻松检查数组是否包含您要查找的值:

const arr = ["qqq", "www", "eee"]
console.log(arr.includes("qqq") ? "Found droids" : "Not found")
console.log(arr.includes("zzz") ? "Found droids" : "Not found")