JSON object to query string (using underscore/lodash)

Object2QS.js

var objectToQueryString = function(obj) {
	var qs = _.reduce(obj, function(result, value, key) {
			return (!_.isNull(value) && !_.isUndefined(value)) ? (result += key + '=' + value + '&') : result;
	}, '').slice(0, -1);
	return qs;
};

Using _.forOwn instead of _.reduce (null checks removed):

const obj = {
    a: 1,
    b: 2,
    c: [3, 4, 5, null, 7]
};

const objectToQueryString = obj => {
  const results = [];
  _.forOwn(obj, (value, key) => {
    if (Array.isArray(value)) {
    	_.forOwn(value, value => {
      	results.push(`${key}=${value}`);
      });
    } else {
    	results.push(`${key}=${value}`);
    }
  });
  return results.join('&');
};

console.log(objectToQueryString(obj));

https://jsfiddle.net/ddaanxfr/1/

_.map(obj, (value, key) => `${key}=${value}`).join('&')

Here's some other options that don't require lodash.

'use strict'
// Other ways APIs implement URLSearchParams. Not my own, but I can't find the original source.
// https://developer.mozilla.org/en-US/docs/Web/API/URLSearchParams/URLSearchParams

var obj = {
  a: 1,
  b: 2,
  c: [3, 4, 5, null, 7],
  foo: {bar: 'baz'}
};

function URLSearchParamsWithArrayBrackets(obj, prefix) {
  let str = [],
    p;
  for (p in obj) {
    if (obj.hasOwnProperty(p)) {
      let k = prefix ? prefix + "[]" : p,
        v = obj[p];
      str.push(
        v !== null && typeof v === "object"
          ? URLSearchParamsWithArrayBrackets(v, k)
          : encodeURIComponent(k) + "=" + encodeURIComponent(v)
      );
    }
  }
  return str.join("&");
}

function URLSearchParamsWithArrayIndices(obj, prefix) {
  let str = [],
    p;
  for (p in obj) {
    if (obj.hasOwnProperty(p)) {
      let k = prefix ? prefix + "[" + p + "]" : p,
        v = obj[p];
      str.push(
        v !== null && typeof v === "object"
          ? URLSearchParamsWithArrayIndices(v, k)
          : encodeURIComponent(k) + "=" + encodeURIComponent(v)
      );
    }
  }
  return str.join("&");
}

var params;
params = new URLSearchParams(obj).toString(); // 'a=1&b=2&c=3%2C4%2C5%2C%2C7&foo=%5Bobject+Object%5D'
params = URLSearchParamsWithArrayBrackets(obj); // 'a=1&b=2&c%5B%5D=3&c%5B%5D=4&c%5B%5D=5&c%5B%5D=null&c%5B%5D=7&foo%5B%5D=baz'
params = URLSearchParamsWithArrayIndices(obj); // 'a=1&b=2&c%5B0%5D=3&c%5B1%5D=4&c%5B2%5D=5&c%5B3%5D=null&c%5B4%5D=7&foo%5Bbar%5D=baz'

From https://gist.github.com/3ch01c/37b7a292c744628f189eeabc917a309b/

Object.entries(someobject).reduce((result, item)=>{return result += ${item[0]}=${item[1]}&}, '').slice(0, -1)