遍历JavaScript中的分层对象

Question

I have an object like this:

var shoppers = {
  "Bob": {favoriteApple: "Honeycrisp"},
  "Sue": {favoriteApple: "Braeburn"},
  "Derek": {favoriteApple: "Honeycrisp"}
}

I want to iterate through this object and count how many people prefer each kind of apple. How can I do this?

On a related note, why does this log as undefined three times instead of logging the actual apple name?

for (var person in shoppers) {
    console.log(person.favoriteApple); // undefined
}

Answer 1

You can use following

1. Iterate over the keys of main object by using Object.keys and forEach
2. Create an empty object to store the various items with count
3. Inside loop, check if the item is already present in the countObj, if yes then increment the count, else set the count as 1

 var shoppers = { "Bob": { favoriteApple: "Honeycrisp" }, "Sue": { favoriteApple: "Braeburn" }, "Derek": { favoriteApple: "Honeycrisp" } }; // Create empty object to store item count var countObj = {}; // Get the array of keys in object using `Object.keys` // Iterate over keys array using `forEach` Object.keys(shoppers).forEach(function(key) { // Get the value of favoriteApple var favApple = shoppers[key].favoriteApple; // If count exists, increment it else add count as 1 countObj[favApple] = countObj[favApple] ? countObj[favApple] + 1 : 1; }); console.log(countObj); document.getElementById('result').innerHTML = JSON.stringify(countObj, 0, 4); 

 <pre id="result"></pre> 

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Regarding the second question

why does console.log(person.favoriteApple); log as undefined three times instead of logging the actual apple name?

for (var person in shoppers) {
    console.log(person.favoriteApple);
}

person in the for is the key of the object shoppers . To access the actual value of the key use shoppers[person] .

Code:

for (var key in shoppers) {
    // To stop showing the inherited/prototyped properties
    if (shoppers.hasOwnProperty(key)) {
        console.log(shoppers[key].favoriteApple);
    }
}

Answer 2

When you use

for (var prop in obj)

It iterates over keys of this object. So, in your case person will be Bob , Sue , Derek .

All you need is to use this key to access a value of a property:

for (var person in shoppers)
{
    console.log(shoppers[person]);
}

Answer 3

you can keep a tally like so:

var favoriteApples = {};
for(var name in shoppers) {
  var person = shoppers[name];
  if(!favoriteApples[person.favoriteApple]) {
    favoriteApples[person.favoriteApple] = 1;
  }else{
    favoriteApples[person.favoriteApple]++;
  }
}

console.log(favoriteApples);
// prints Object {Honeycrisp: 2, Braeburn: 1}

Answer 4

 var shoppers = { "Bob": {favoriteApple: "Honeycrisp"}, "Sue": {favoriteApple: "Braeburn"}, "Derek": {favoriteApple: "Honeycrisp"} } $.each(shoppers,function(key,val){ console.log(key); $.each(val,function(k,v){ console.log(k + " " + v) }); }); 

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