Why not getting output with arrow function?

Question

When i'm trying buildGraph function with normal function its working properly but with arrow function getting empty obj as output. Help needed.

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: {}

Som Shekhar Mukherjee · Accepted Answer · 2022-04-21 11:17:31Z

2

Replace null with undefined. If a key is not found in an object it returns undefined not 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));

answered Apr 21, 2022 at 11:17

Som Shekhar Mukherjee

8,1982 gold badges16 silver badges34 bronze badges

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Comments

David · Accepted Answer · 2022-04-21 11:17:54Z

It's not the arrow function, it's this comparison:

obj[from] === null

The === operator performs an exact type-comparison, and for properties which don't exist on an object the exact type of the property is not null, it's undefined. So that comparison is false and the property is never added to the object.

Compare it to undefined instead:

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: {}

Ilijanovic · Accepted Answer · 2022-04-21 11:23:01Z

2

You could also use .reduce(). Its often used if you want to group something together

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)

answered Apr 21, 2022 at 11:23

Ilijanovic

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Comments

malarres · Accepted Answer · 2022-04-21 11:22:10Z

It's because your obj[from] === null comparison

Sometimes the 'double equal' still makes sense:

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: {}

That way you're comparing to undefined or null in one step

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