Javascript分配和增加Obj的键值（使用Reduce）

Question

Im following a course online and one of the challenges is this:

Write a function called vowelCount that accepts a string and returns an object with each key being the vowel and the value being the number of times the vowel occurs in the string (the order of keys in the object does not matter).

vowelCount('incredible');
// {i:2, e: 2}
vowelCount('awesome');
// {a:1, e:2, o:1}

So far, I've come up with the following code, using Javascript's reduce:

function vowelCount(word) {
  var vowels = ['a', 'e', 'i', 'o', 'u'];

  var final = word.split('').reduce(function(obj, val, index) {
    if (vowels.indexOf(val) > -1) {
      //obj[val] = 0
      obj[val]++;
    }
    return obj
  }, {})
  console.log(final)
}

I think I'm close but I'm having trouble wrapping my head on how to assign and increment the numerical value of the vowel key at point of checking if its a vowel. I tried instantiating the value to 0, but that only keeps the value at 1.

Answer 1

Use short circuit evaluation to check if the value exists, and if not use 0 instead:

 console.log(vowelCount('incredible')); // {i:2, e: 2} console.log(vowelCount('awesome')); // {a:1, e:2, o:1} function vowelCount(word) { var vowels = ['a', 'e', 'i', 'o', 'u']; return word.split('').reduce(function(obj, val, index) { if (vowels.indexOf(val) > -1) { obj[val] = (obj[val] || 0) + 1; } return obj; }, {}); }

In addition, instead of using an array of vowels and the Array.indexOf() check, you can initialize the result object with the vowels, and increment them directly:

 console.log(vowelCount('incredible')); // {i:2, e: 2} console.log(vowelCount('awesome')); // {a:1, e:2, o:1} function vowelCount(word) { return word.split('').reduce(function(obj, val) { if(val in obj) { obj[val]++; } return obj; }, { a: 0, e: 0, i: 0, o: 0, u: 0 }); }

Answer 2

I think this is actually a bad usecase for reduce. Might just use a regular for loop and use the or operator ( || ) to replace undefined with 0 :

 function vowelCount(word){
   const vowels = "aeiou";
   const result = {};
   for(const char of word)
     if(vowels.includes(char))
       result[char] = (result[char] || 0) + 1;
   return result;
}

If that is too spooky you could just check if char already exists in result and set it otherwise:

function vowelCount(word){
   const vowels = "aeiou";
   const result = {};
   for(const char of word){
     if(vowels.includes(char)){
       if(!result[char]) result[char] = 0;
       result[char]++;
     }
  }
   return result;
}

Answer 3

You were very close! It was because you were trying to increment undefined. You must set that value equal to a number on the accumulator before you increment it.

function vowelCount(word) {
    var vowels = ['a', 'e', 'i', 'o', 'u'];

    var final = word.split('').reduce(function(obj, val, index) {
    if (vowels.indexOf(val) > -1) {
        // if we have seen the vowel, we increment it. Otherwise, it is the first time.
        obj[val] ? obj[val]++ : obj[val] = 1;
    }
    return obj
    }, {})
    console.log(final)
}