So I am trying to map the number of times a char appears in a string. I know that in C++ it would be.

std::string str = "AbBAaaaa";
std::unordered_map<char, int> myMap;

for(auto i = str)
{
   ++mymap[i];
}

How would I translate this to JavaScript?

So I am trying to map the number of times a char appears in a string. I know that in C++ it would be.

std::string str = "AbBAaaaa";
std::unordered_map<char, int> myMap;

for(auto i = str)
{
   ++mymap[i];
}

How would I translate this to JavaScript?

• javascript
• c++
• hashmap
• unordered-map

2 Answers 2

I would reduce the string into an object indexed by character. The function passed to reduce is called for each element in the input, where the first argument (the a) is the accumulator, which is either the initial value ({} here) or what the last iteration returned. The second argument (the char) is the current character being iterated over.

const str = "AbBAaaaa";
const charCounts = Array.prototype.reduce.call(str, (a, char) => {
  a[char] = (a[char] || 0) + 1;
  return a;
}, {});
console.log(charCounts);

You could also use

const charCounts = [...str].reduce((a, char) => // ...

which is shorter and probably a bit easier to understand at a glance, but unnecessarily creates an intermediate array from the str.

The imperative version of this, with a for loop, would look like:

const str = "AbBAaaaa";
const charCounts = {};
for (let i = 0; i < str.length; i++) {
  const char = str[i];
  charCounts[char] = (charCounts[char] || 0) + 1;
}
console.log(charCounts);

Javascript already has map and you can achieve same result of your C++ app as shown in this snippet

function charOccurances(str) {
  var myMap = {};
  if(str.length!==0){
    for (let i = 0; i < str.length; i++) {
      myMap[str[i]] = (myMap[str[i]] || 0) + 1;
    }
  }
  return myMap;
}

const str = "AbABaaaa";
console.log(charOccurances(str));