Express middleware for coercing form-urlencoded data into primitive counterparts

clean-form-data.js

'use strict';

/**
 * This file is useful for x-www-form-urlencoded requests where all data is a string.
 * If the request uses application/json, this middleware is not needed.
 * This loops over all of the data and converts strings to their primitive form,
 * basically normalizing numbers, booleans, and null/undefined/'' values.
 * This could potentially have side effects, though none were encountered after
 * months of using this middleware on a busy project. 
 */

const REG_METHOD_WITH_BODY = /^(?:put|patch|post)$/;

function isObject(val) {
  return val.constructor === Object;
}

function isNumber(val) {
  return !isNaN(parseFloat(val)) && isFinite(val);
}

function isBoolean(val) {
  return val === 'false' || val === 'true';
}

function isArray(val) {
  return Array.isArray(val);
}

function parseObject(obj) {
  var result = {};
  var key;
  for (key in obj) {
    result[key] = parseValue(obj[key]);
  }
  return result;
}

function parseArray(arr) {
  var result = [];
  for (var i = 0; i < arr.length; i++) {
    result[i] = parseValue(arr[i]);
  }
  return result;
}

function parseNumber(val) {
  return Number(val);
}

function parseBoolean(val) {
  return val === 'true';
}

function parseValue(val) {
  if (typeof val === 'undefined' || ('' + val) === 'null' || val === '') {
    return null;
  } else if (isBoolean(val)) {
    return parseBoolean(val);
  } else if (isArray(val)) {
    return parseArray(val);
  } else if (isObject(val)) {
    return parseObject(val);
  } else if (isNumber(val)) {
    return parseNumber(val);
  } else {
    return val;
  }
}

module.exports = () => {
	return function(req, res, next) {
		if (REG_METHOD_WITH_BODY.test(req.method.toLowerCase()) && req.get('Content-Type') !== 'application/json') {
			req.body = parseValue(req.body);
		}
		next();
	};
};