Count the number of data types in an array - JavaScript

We are required to write a JavaScript function that takes in an array that contains elements of different data types and the function should return a map representing the frequency of each data type.

Let's say the following is our array:

const arr = [23, 'df', undefined, null, 12, {
   name: 'Rajesh'
}, [2, 4, 7], 'dfd', null, Symbol('*'), 8];

Understanding Data Types in JavaScript

JavaScript's typeof operator returns string representations of data types. Note that arrays and null both return "object", which is a known quirk of JavaScript.

Example

Following is the code:

const arr = [23, 'df', undefined, null, 12, {
   name: 'Rajesh'},
   [2, 4, 7], 'dfd', null, Symbol('*'), 8];

const countDataTypes = arr => {
   return arr.reduce((acc, val) => {
      const dataType = typeof val;
      if(acc.has(dataType)){
         acc.set(dataType, acc.get(dataType)+1);
      }else{
         acc.set(dataType, 1);
      };
      return acc;
   }, new Map());
};

console.log(countDataTypes(arr));

Output

Following is the output in the console:

Map(5) {
   'number' => 3,
   'string' => 2,
   'undefined' => 1,
   'object' => 4,
   'symbol' => 1
}

How It Works

The function uses reduce() to iterate through the array and build a Map. For each element, it gets the data type using typeof and either increments the existing count or sets it to 1 for new types.

Alternative Approach Using Object

You can also use a plain object instead of a Map:

const countDataTypesObj = arr => {
   return arr.reduce((acc, val) => {
      const dataType = typeof val;
      acc[dataType] = (acc[dataType] || 0) + 1;
      return acc;
   }, {});
};

console.log(countDataTypesObj(arr));

{
  number: 3,
  string: 2,
  undefined: 1,
  object: 4,
  symbol: 1
}

Key Points

• Arrays, objects, and null all return "object" from typeof
• The Map approach provides better key handling than plain objects
• Both approaches use reduce() for efficient single-pass counting

Conclusion

This solution efficiently counts data types using reduce() and Map. The typeof operator groups arrays, objects, and null together as "object" type.