safe-math-avg.js ~ safer numeric average computation

safer-math-avg.js

/*
 * Update 17 August 2020:
 * Now part of safe math blog post at https://dfkaye.com/posts/2020/08/17/safer-math-operations-in-javascript-using-tdd/
 * Test suite at https://dfkaye.com/demos/safe-math-test-suite/
 */


// 14 August 2020

// Returns the average of a list of functionally numeric values.
// Params may be a single array of values, or any number of value params.
// Assumes booleans and numeric strings are functionally numeric, but ignores other non-numerics.
// Includes safer math helper against 0.1 + 0.2 (0.30000000000000004) from gist at
//  https://gist.github.com/dfkaye/c2210ceb0f813dda498d22776f98d48a

function avg() {
  var args = Array.isArray(arguments[0])  
    ? arguments[0]
    : arguments;
  
  var size = 0;

  return [].slice.call(args).reduce(function(sum, next) {
    // Expand values to { left, right, by }.
    var e = expand(+sum, +next);

    return +e.right === +e.right
      // If next is a number, add the values and increment the set size.
      ? (size += 1, (e.left + e.right) / e.by)
      // Else just return current sum.
      : sum
  }, size) / size;
}

/*
 * eExpand() is a helper carved from safer math ops gist at
 * https://gist.github.com/dfkaye/c2210ceb0f813dda498d22776f98d48a
 */
function expand(a, b) {
  // remove formatting commas
  var reCommas = /[\,]/g
  var left = (a).toString().replace(reCommas, '');
  var right = (b).toString().replace(reCommas, '');

  // expand to integer values based on largest mantissa length
  var reDecimal = /[\.]/;
  var ml = reDecimal.test(left) && left.split('.')[1].length;
  var mr = reDecimal.test(right) && right.split('.')[1].length;
  var pow = ml > mr ? ml : mr;
  var by = Math.pow(10, pow);

  // left & right number pair, plus the expansion factor.
  // The multiplication operator, *, coerces non-numerics to their equivalent,
  // e.g., {} => NaN, true => 1, [4] => '4' => 4      
  return { left: left * by, right: right * by, by };
}

/* Test it out... */

console.log( avg(1, 2, 3, 4) );
// 2.5

console.log( avg([1, 2, , 3, 4]) );
// 2.5

console.log( avg(1, 2, 3, 'bonk', 4) );
// 2.5

console.log( avg([1, 2, 'bonk' , 3, 4]) );
// 2.5

console.log( avg(1, 2, '3.0', 'bonk', 4) );
// 2.5

console.log( avg([1, 2, 'bonk' , '3.0', 4]) );
// 2.5

// false evaluates to 0
console.log( avg(1, 2, 3, false, 4) );
// 2.5

// true evaluates to 1 
console.log( avg([1, true, 2, 3, 4]) );
// 2.5

/* 0.1 + 0.2 */

console.log( avg(0.1, 0.2) );
// 0.15

console.log( avg(0.01, 0.02) );
// 0.015