I'm just beginning to learn JavaScript and this is a part of my Take-home exam, and the requirement is quite tricky for me.

We have many ways to counting instances of values in an object in JS, but in here I have to achieve it by calling reduce() and return an ES6 map object.

Here is the requirement:

/**
* Takes an array of items, and returns a Map with the
* frequencies (counts) of each item in the array.
*
* Must do this by using a single call to reduce.
*
* For example,
*  freqs([1, 2, 3, 2, 7, 2, 1]) returns Map {1 => 2, 2 => 3, 3 => 1, 7 => 1}
*  freqs("One fish two fish red fish blue fish".split(' '))
*  returns Map {"One" => 1, "fish" => 4, "two" => 1, "red" => 1, "blue" => 1}
*/

And here's my code so far:

function freqs(items) {
  var map = new Map();
  return items.reduce(function(allkey, key) {
    if (map.has(key)) {
      var value = map.get(key);
      map.set(key, value++);
    } else {
      map.set(key, 1);
    }
    return map;
  }, {})
}

I used debugger and found out that on map.set(key, value++) is not overwriting the new value in the map when it iterates. Why?

So when I input C = [1,2,3,2,7,2,1]; the output is always like this:

Map(4) {1 => 1, 2 => 1, 3 => 1, 7 => 1}

Any suggestions?

I'm just beginning to learn JavaScript and this is a part of my Take-home exam, and the requirement is quite tricky for me.

We have many ways to counting instances of values in an object in JS, but in here I have to achieve it by calling reduce() and return an ES6 map object.

Here is the requirement:

/**
* Takes an array of items, and returns a Map with the
* frequencies (counts) of each item in the array.
*
* Must do this by using a single call to reduce.
*
* For example,
*  freqs([1, 2, 3, 2, 7, 2, 1]) returns Map {1 => 2, 2 => 3, 3 => 1, 7 => 1}
*  freqs("One fish two fish red fish blue fish".split(' '))
*  returns Map {"One" => 1, "fish" => 4, "two" => 1, "red" => 1, "blue" => 1}
*/

And here's my code so far:

function freqs(items) {
  var map = new Map();
  return items.reduce(function(allkey, key) {
    if (map.has(key)) {
      var value = map.get(key);
      map.set(key, value++);
    } else {
      map.set(key, 1);
    }
    return map;
  }, {})
}

I used debugger and found out that on map.set(key, value++) is not overwriting the new value in the map when it iterates. Why?

So when I input C = [1,2,3,2,7,2,1]; the output is always like this:

Map(4) {1 => 1, 2 => 1, 3 => 1, 7 => 1}

Any suggestions?

• javascript

Share Improve this question Follow edited Oct 12, 2017 at 0:58 Travis Su asked Oct 12, 2017 at 0:44 Travis SuTravis Su 70922 gold badges77 silver badges1818 bronze badges 22

• 5 FWIW, this is a weird application of reduce. The correct way would be to pass the map as initial value (instead of the object literal) and rename the first argument from allkey to map. – Felix Kling Commented Oct 12, 2017 at 0:46
• @Rafael OH dude I should delete that – Travis Su Commented Oct 12, 2017 at 0:49
• You also need to do a table lookup on every iteration, asking if the map entry exists and initialize the entry to 0 otherwise, and increment every time. – Rafael Commented Oct 12, 2017 at 0:49
• @Rafael: They are doing that. if (map.has(key)) { – Felix Kling Commented Oct 12, 2017 at 0:50
• 3 Hint to solve your actual problem: Look at what value++ does (developer.mozilla/en-US/docs/Web/JavaScript/Reference/…) – Felix Kling Commented Oct 12, 2017 at 0:50

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1 Answer 1

Sorted by: Reset to default 5

The problem with your code is that a++ returns the value of a before adding 1:

var a = 1;
console.log(a++);
console.log(a);

Since Map#set returns the map after being modified, you can simplify your code like this:

function freqs(items) {
  return items.reduce(
    (map, e) => map.set(e, (map.get(e) || 0) + 1),
    new Map()
  );
}

Alternatively, you may use the somewhat obscure and rarely seen increment operator prefix, which also increments the variable, but returns the incremented new version of it:

var a = 1;
console.log(++a);
console.log(a);