Javascript function to convert length, weight, area, and volume units with ease

unitConv.js

/**
 * unitConv - converts length, weight, area and volume units
 * 
 * Usage: unitConv.type(value, unitIn, unitOut)
 *   type: length | weight | area | volume
 *   v: numeric value
 *   unitIn: name of the input unit
 *   unitOut: name of the output unit
 * 
 * Supported units:
 *   length: mm, cm, m, km, in, ft, yd, mi, nm
 *   weight: g, kg, lb, oz, ct (carat), st (short ton), lt (long ton), ton, gr (grains)
 *   area: cm2, m2, ha, km2, in2, ft2, yd2, mi2, ac (acre)
 *   volume: ml, l, m3, in3, ft3, pt (pint), qt (quart), gal, bbl (barrel)
 * 
 */

var unitConv = (function(){
  var r = {};
  
  ['length', 'weight', 'area', 'volume'].forEach(function(type){
    r[type] = function(value, unitIn, unitOut) {
      return this.conv(type, value, unitIn, unitOut);
    };
  })

  r.conv = function(type, value, unitIn, unitOut) {
    // List of factors
    let factors = {
      length: {mm: 1000, cm: 100, m:1, km: 0.001, in: 39.370078740157, ft: 3.2808398950131, yd: 1.0936132983377, mi: 0.00062137119223733, nm: 0.00053995680345572},
      weight: {g: 1000, kg: 1, lb: 2.2046226218488, oz: 35.27396194958, ct: 5000, st: 0.0011023113109244, lt: 0.00098420652761106, ton: 0.001, gr: 15432.358352941},
      area: {cm2: 10000, m2: 1, ha: 0.0001, km2: 0.000001, in2: 1550.0031000062, ft2: 10.76391041671, yd2: 1.1959900463011, mi2: 0.00000038610215854781, ac: 0.00024710538146717},
      volume: {ml: 1000, l: 1, m3: 0.001, in3: 61.023744094732, ft3: 0.035314666721489, pt: 2.1133764188652, qt: 1.0566882094326, gal: 0.26417205235815, bbl: 0.0083864143605761}
    };

    // unitIn to base then base to unitOut
    return (value / factors[type][unitIn]) * factors[type][unitOut];
  }

  return r;
})();