Given an array of integers, return the number of pairs whose sum is K

FindSum.js

const _ = require('underscore');

class FindSum {
  
  // O(n)
  find(arr, totalSum) {    
    const m = new Map();
    
    // O(n)
    arr.forEach((value) => {
      if (m.get(value) !== undefined) {
        m.set(value, m.get(value) + 1);
      } else {
        m.set(value, 1);
      }
    })
    
    let result = 0;
    
    // O(n)
    arr.forEach((value) => {      
      if (m.get(totalSum - value) !== undefined) {
        result += m.get(totalSum - value);
      }
      
      if ((totalSum - value) === value) {
        result--;
      }
    })
    
    return result / 2;
  }
  
}

const test = (currentResult, expectedResult) => {
  if (!_.isEqual(currentResult, expectedResult)) {
    console.log('Error!');
    console.log('Current Result:');
    console.log(currentResult);
    console.log('Expected Result:');
    console.log(expectedResult);
  } else {
    console.log('Success.');
  }
} 

var obj = new FindSum();

test(obj.find([9,1,3,11,8,7,5,5], 10), 3);