為什么不使用箭頭函式獲得輸出？

當我嘗試使用普通函式的 buildGraph 函式時，它可以正常作業，但箭頭函式將空 obj 作為輸出。需要幫助。

var roads = [
    "Alice's House-Bob's House", "Alice's House-Cabin",
    "Alice's House-Post Office", "Bob's House-Town Hall",
    "Daria's House-Ernie's House", "Daria's House-Town Hall",
    "Ernie's House-Grete's House", "Grete's House-Farm",
    "Grete's House-Shop", "Marketplace-Farm",
    "Marketplace-Post Office", "Marketplace-Shop",
    "Marketplace-Town Hall", "Shop-Town Hall"
   ];

let buildGraph = (edges) => {
    let obj = {}
    let addEdge = (from, to) => (
        obj[from] === null ? obj[from] = [to] : obj[from]?.push(to)
    )
    edges.map(r => r.split("-")).map(([from, to]) => addEdge(from, to))
    return obj;
}
console.log(buildGraph(roads))   // Output: {}

uj5u.com熱心網友回復：

替換null為undefined。如果key在物件中找不到 a ，則回傳undefinednot null。

var roads = [
  "Alice's House-Bob's House",
  "Alice's House-Cabin",
  "Alice's House-Post Office",
  "Bob's House-Town Hall",
  "Daria's House-Ernie's House",
  "Daria's House-Town Hall",
  "Ernie's House-Grete's House",
  "Grete's House-Farm",
  "Grete's House-Shop",
  "Marketplace-Farm",
  "Marketplace-Post Office",
  "Marketplace-Shop",
  "Marketplace-Town Hall",
  "Shop-Town Hall",
];

let buildGraph = (edges) => {
  let obj = {};
  let addEdge = (from, to) =>
    obj[from] === undefined ? (obj[from] = [to]) : obj[from]?.push(to);
  edges.map((r) => r.split("-")).map(([from, to]) => addEdge(from, to));
  return obj;
};

console.log(buildGraph(roads));

uj5u.com熱心網友回復：

這不是箭頭函式，而是這個比較：

obj[from] === null

運算子執行精確的===型別比較，對于物件上不存在的屬性，屬性的確切型別不是null，而是undefined. 因此，比較是false并且該屬性永遠不會添加到物件中。

將其與以下內容進行比較undefined：

var roads = [
    "Alice's House-Bob's House", "Alice's House-Cabin",
    "Alice's House-Post Office", "Bob's House-Town Hall",
    "Daria's House-Ernie's House", "Daria's House-Town Hall",
    "Ernie's House-Grete's House", "Grete's House-Farm",
    "Grete's House-Shop", "Marketplace-Farm",
    "Marketplace-Post Office", "Marketplace-Shop",
    "Marketplace-Town Hall", "Shop-Town Hall"
   ];

let buildGraph = (edges) => {
    let obj = {}
    let addEdge = (from, to) => (
        obj[from] === undefined ? obj[from] = [to] : obj[from]?.push(to)
    )
    edges.map(r => r.split("-")).map(([from, to]) => addEdge(from, to))
    return obj;
}
console.log(buildGraph(roads))   // Output: {}

uj5u.com熱心網友回復：

你也可以使用.reduce(). 如果您想將某些東西組合在一起，通常會使用它

var roads = [
    "Alice's House-Bob's House", "Alice's House-Cabin",
    "Alice's House-Post Office", "Bob's House-Town Hall",
    "Daria's House-Ernie's House", "Daria's House-Town Hall",
    "Ernie's House-Grete's House", "Grete's House-Farm",
    "Grete's House-Shop", "Marketplace-Farm",
    "Marketplace-Post Office", "Marketplace-Shop",
    "Marketplace-Town Hall", "Shop-Town Hall"
   ];

let result = roads.reduce((obj, road) => {
  let [from, to] = road.split("-")
  obj[from] ? obj[from].push(to) : obj[from] = [to]
  return obj
},{})

console.log(result)

uj5u.com熱心網友回復：

因為你的obj[from] === null比較

有時“雙重相等”仍然有意義：

var roads = [
    "Alice's House-Bob's House", "Alice's House-Cabin",
    "Alice's House-Post Office", "Bob's House-Town Hall",
    "Daria's House-Ernie's House", "Daria's House-Town Hall",
    "Ernie's House-Grete's House", "Grete's House-Farm",
    "Grete's House-Shop", "Marketplace-Farm",
    "Marketplace-Post Office", "Marketplace-Shop",
    "Marketplace-Town Hall", "Shop-Town Hall"
   ];

let buildGraph = (edges) => {
    let obj = {}
    let addEdge = (from, to) => 
        (obj[from] == null ? obj[from] = [to] : obj[from]?.push(to))
    
    edges.map(r => r.split("-")).map(([from, to]) => addEdge(from, to))
    return obj;
}
console.log(buildGraph(roads))   // Output: {}

這樣你就可以一步比較 undefined 或 null

標籤：javascript ecmascript-6 箭头函数

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