Fluent API for performing length conversions

length.js

const validUnits = ['km', 'm', 'cm', 'mm', 'in', 'ft', 'mile'];

const isLengthObj = (lo) =>
  '_val' in lo &&
  '_unit' in lo &&
  typeof lo._val === 'number' &&
  typeof lo._unit === 'string' &&
  validUnits.includes(lo._unit);

const formatUnit = (unit, val) => {
  switch(unit) {
    case 'km':
    case 'm':
    case 'cm':
    case 'mm':
    case 'in':
    case 'ft': return unit;
    case 'mile': return (val !== 1) ? 'miles' : unit;
  }
}

const toCm = ({_val, _unit}) => {
  switch(_unit) {
    case 'km':
      return _val * 100000;
    case 'm':
      return _val * 100;
    case 'cm':
      return _val;
    case 'mm':
      return 0.1 * _val;
    case 'in':
      return _val * 2.54;
    case 'ft':
      return _val * 30.48;
    case 'mile':
      return _val * 160934;
    default:
      throw new Error(`Unit ${_unit} is not valid. Supported units: ${validUnits.join(', ')}`);
  }
};

const toUnit = (lo, toUnit) => {
  const std = toCm(lo);
  switch(toUnit) {
    case 'km':
      return std / 100000;
    case 'm':
      return std / 100;
    case 'cm':
      return std;
    case 'mm':
      return std * 10;
    case 'in':
      return std / 2.54;
    case 'ft':
      return std / 30.48;
    case 'mile':
      return std / 160934;
    default:
      throw new Error(`Unit ${lo._unit} is not valid. Supported units: ${validUnits.join(', ')}`);
  }
}

const length = (val, unit) => {
  if (!validUnits.includes(unit)) {
    throw new Error(`Unit ${unit} is not valid. Supported units: ${validUnits.join(', ')}`);
  }
  if (!(typeof val === 'number')) {
    throw new Error(`Non number value provided: ${val}`);
  }

  return {
    _val: val,
    _unit: unit,
    add: (nVal, nUnit) => {
      if (typeof nVal === 'object') {
        if (isLengthObj(nVal)) {
          return length(val + toUnit(nVal, unit), unit);
        } else {
          throw new Error(`Invalid legnth object: ${nVal}`);
        }
      } else {
        return length(val + toUnit({_val: nVal, _unit: nUnit}, unit), unit);
      }
    },
    subtract: (nVal, nUnit) => {
      if (typeof nVal === 'object') {
        if (isLengthObj(nVal)) {
          return length(val - toUnit(nVal, unit), unit);
        } else {
          throw new Error(`Invalid legnth object: ${nVal}`);
        }
      } else {
        return length(val - toUnit({_val: nVal, _unit: nUnit}, unit), unit);
      }
    },
    to: (nUnit) => {
      if (!validUnits.includes(nUnit)) {
        throw new Error(`Unit ${nUnit} is not valid. Supported units: ${validUnits.join(', ')}`);
      }
      return length(toUnit({_val: val, _unit: unit}, nUnit), nUnit);
    },
    toString: () => `${val.toFixed(3)} ${unit}${unit === 'mile' ? 's' : ''}`,
    rawValue: () => val,
    rawUnit: () => unit
  }
};

module.exports = length;