如何在Javascript中使用'in'关键字

Question

I have a question here. I know my code have plenty of problems and I need your help. The problem is, To return an object which have numbers of repetition information of the string. The given string is

var r = countFreq(["a", "b", "b", "c", "c", "c", "d"]);

and the result have to be

{"a":1, "b":2, "c":3, "d":1 }

by console.log(r);

All that I know is, key(properties) have to be element and value(value of property) have to be the number of repetition. AND, I must use 'in' key world to solve this problem.

Like, 
if('property' in 'object')  {
 //...
}else {
 //...
}

(if there's no property initialize as 1, and if there's a same property, add 1 each time)

I really appreciate your help. (This post may have grammatical errors. I really feel sorry about that...)

      function countFreq(array) {
        var i;
        for(i=0; i<array.length; i++)
        {
          if(array[i] in array)
          {
            return i += 1;
          }else
          {
            return i = 1;
          }
          console.log(array[i]+": "+i+", ");
        }
      }
      var r = countFreq(["a","b","c","c","c","d"]);
      console.log(r);

Answer 1

According to MDN - The 'in' operator returns true if the specified property is in the specified object or its prototype chain. Prop is a string or symbol representing a property name or array index (non-symbols will be coerced to strings). Object is to check if it (or its prototype chain) contains the property with specified name.

So in your case, it depends what your object is? if you object is an array, you need to use prop as properties of array. All index values up to length of array will return true. MDN Example of arrays

var trees = ['redwood', 'bay', 'cedar', 'oak', 'maple'];
0 in trees        // returns true
3 in trees        // returns true
6 in trees        // returns false
'bay' in trees    // returns false (you must specify the 
                  // index number, not the value at that index)
'length' in trees // returns true (length is an Array property)
Symbol.iterator in trees // returns true (arrays are iterable, works only in ES2015+)

Answer 2

I think you are misunderstanding the in operator. In can be used in 2 cases, as a boolean operator to check for the presence of an index in an array or to iterate over the indexes of an array with a for loop. You are using it to check for the presence of a value in an array directly, which you cannot do. Also you are returning from the function after each iteration so you will only ever get 1 or 0.

I presume you want something like the following:

countFreq(array) {
    var results = { a: 0, b: 0, c: 0, d: 0 };
    for (var index in array) {
        results[array[index]] ++;
    }
    return results;
}

Now you can access each result with results['a'] for instance, after calling countFreq . I think you need to read up on return and loops in JavaScript.