JavaScript - Array Methods

count-duplicates

const countDuplicates = arr => arr.reduce((acc, el) => {
  acc[el] = (acc[el] || 0) + 1;
  return acc;
}, {});

countDuplicates(['a', 'a', 'b', 'c', 'd', 'd', 'd') // {a: 2, b: 1, c: 2, d: 3}

diff-arrays

const diffArrays = (arr1, arr2) => [...arr1.filter(val => arr2.indexOf(val) === -1), ...arr2.filter(val => arr1.indexOf(val) === -1)];

diffArray(['a', 'b', 'c', 'e'], ['a', 'b', 'c', 'd', 'e']); //['d']

matrix-rotation

// Returns a reverse array
const reverse = arr => [...arr].reverse();

// flips rows with columns
const flip = matrix => matrix[0].map((column, index) => matrix.map(row => row[index]));

const rotate90 = matrix => flip(matrix).map(reverse);
const rotate180 = matrix => reverse(matrix).map(reverse);
const rotate270 = matrix => flip(matrix.map(reverse));

rotate90([[1,2,3],[4,5,6],[7,8,9]]); // [[7,4,1],[8,5,2],[9,6,3]]
rotate180([[1,2,3],[4,5,6],[7,8,9]]); // [[9,8,7],[6,5,4],[3,2,1]]
rotate270([[1,2,3],[4,5,6],[7,8,9]]); // [[3,6,9],[2,5,8],[1,4,7]]

pascals-triangle

const pascalsTriangle = height => {
    return Array(height)
      .fill()
      .reduce(
      (acc, current, rowIdx) => {
        acc.push(
          Array(rowIdx+1)
            .fill()
              .map((val, colIdx) => (colIdx === 0 || colIdx === rowIdx) ? 1 :
                acc[rowIdx-1][colIdx-1] + acc[rowIdx-1][colIdx]
              )
        );
      return acc;
    },[]);
  }
  
  pascalsTriangle(5); // [[1], [1, 1], [1, 2, 1], [1, 3, 3, 1], [1, 4, 6, 4, 1]]