How to build query string with Javascript

The URLSearchParams API is available in all modern browsers. For example:

const params = new URLSearchParams({
 var1: "value",
 var2: "value2",
 arr: "foo",
});
console.log(params.toString());
//Prints "var1=value&var2=value2&arr=foo"

2k20 update: use Josh's solution with URLSearchParams.toString().

Old answer:

Without jQuery

var params = {
 parameter1: 'value_1',
 parameter2: 'value 2',
 parameter3: 'value&3' 
};
var esc = encodeURIComponent;
var query = Object.keys(params)
 .map(k => esc(k) + '=' + esc(params[k]))
 .join('&');

For browsers that don't support arrow function syntax which requires ES5, change the .map... line to

 .map(function(k) {return esc(k) + '=' + esc(params[k]);})

If you're using jQuery you might want to check out jQuery.param() http://api.jquery.com/jQuery.param/

Example:

var params = {
 parameter1: 'value1',
 parameter2: 'value2',
 parameter3: 'value3' 
};
var query = $.param(params);
console.log(query);

This will print out:

parameter1=value1&parameter2=value2&parameter3=value3

This doesn't directly answer your question, but here's a generic function which will create a URL that contains query string parameters. The parameters (names and values) are safely escaped for inclusion in a URL.

function buildUrl(url, parameters){
 var qs = "";
 for(var key in parameters) {
 var value = parameters[key];
 qs += encodeURIComponent(key) + "=" + encodeURIComponent(value) + "&";
 }
 if (qs.length > 0){
 qs = qs.substring(0, qs.length-1); //chop off last "&"
 url = url + "?" + qs;
 }
 return url;
}
// example:
var url = "http://example.com/";
var parameters = {
 name: "George Washington",
 dob: "17320222"
};
console.log(buildUrl(url, parameters));
// => http://www.example.com/?name=George%20Washington&dob=17320222

Create an URL object and append the values to seachParameters

let stringUrl = "http://www.google.com/search";
let url = new URL(stringUrl);
let params = url.searchParams;
params.append("q", "This is seach query");
console.log(url.toString());

The output will be

http://www.google.com/search?q=This+is+seach+query

With jQuery you can do this by $.param

$.param({ action: 'ship', order_id: 123, fees: ['f1', 'f2'], 'label': 'a demo' })
// -> "action=ship&order_id=123&fees%5B%5D=f1&fees%5B%5D=f2&label=a+demo"

ES2017 (ES8)

Making use of Object.entries(), which returns an array of object's [key, value] pairs. For example, for {a: 1, b: 2} it would return [['a', 1], ['b', 2]]. It is not supported (and won't be) only by IE.

Code:

const buildURLQuery = obj =>
 Object.entries(obj)
 .map(pair => pair.map(encodeURIComponent).join('='))
 .join('&');

Example:

buildURLQuery({name: 'John', gender: 'male'});

Result:

"name=John&gender=male"

querystring can help.

So, you can

const querystring = require('querystring')
url += '?' + querystring.stringify(parameters)

No, I don't think standard JavaScript has that built in, but Prototype JS has that function (surely most other JS frameworks have too, but I don't know them), they call it serialize.

I can reccomend Prototype JS, it works quite okay. The only drawback I've really noticed it it's size (a few hundred kb) and scope (lots of code for ajax, dom, etc.). Thus if you only want a form serializer it's overkill, and strictly speaking if you only want it's Ajax functionality (wich is mainly what I used it for) it's overkill. Unless you're careful you may find that it does a little too much "magic" (like extending every dom element it touches with Prototype JS functions just to find elements) making it slow on extreme cases.

If you don't want to use a library, this should cover most/all of the same form element types.

function serialize(form) {
 if (!form || !form.elements) return;
 var serial = [], i, j, first;
 var add = function (name, value) {
 serial.push(encodeURIComponent(name) + '=' + encodeURIComponent(value));
 }
 var elems = form.elements;
 for (i = 0; i < elems.length; i += 1, first = false) {
 if (elems[i].name.length > 0) { /* don't include unnamed elements */
 switch (elems[i].type) {
 case 'select-one': first = true;
 case 'select-multiple':
 for (j = 0; j < elems[i].options.length; j += 1)
 if (elems[i].options[j].selected) {
 add(elems[i].name, elems[i].options[j].value);
 if (first) break; /* stop searching for select-one */
 }
 break;
 case 'checkbox':
 case 'radio': if (!elems[i].checked) break; /* else continue */
 default: add(elems[i].name, elems[i].value); break;
 }
 }
 }
 return serial.join('&');
}

You can do that nowadays with FormData and URLSearchParams without the need to loop over anything.

const formData = new FormData(form);
const searchParams = new URLSearchParams(formData);
const queryString = searchParams.toString();

Older browsers will need a polyfill, though.

Might be a bit redundant but the cleanest way i found which builds on some of the answers here:

const params: {
 key1: 'value1',
 key2: 'value2',
 key3: 'value3',
}
const esc = encodeURIComponent;
const query = Object.keys(params)
 .map(k => esc(k) + '=' + esc(params[k]))
 .join('&');
return fetch('my-url', {
 method: 'POST',
 headers: {'Content-Type': 'application/x-www-form-urlencoded'},
 body: query,
})

Source

The UrlSearchParams API is a great suggestion, but I can't believe nobody mentioned the incredibly useful .get and .set methods. They can be used to manipulate the query string and not only they're very easy to use, they also solve a number of issues you might encounter. For example, in my case I wanted to build a query string without duplicate keys. .set solves this problem for you. Quoting from the MDN docs:

URLSearchParams.set() Sets the value associated with a given search parameter to the given value. If there are several values, the others are deleted.

Example (from MDN):

let url = new URL('https://example.com?foo=1&bar=2');
let params = new URLSearchParams(url.search);
// Add a third parameter
params.set('baz', 3);
params.toString(); // "foo=1&bar=2&baz=3"

Alternative, shorter syntax:

let url = new URL('https://example.com?foo=1&bar=2');
// Add a third parameter
url.searchParams.set('baz', 3);
url.searchParams.toString(); // "foo=1&bar=2&baz=3"

I'm not entirely certain myself, I recall seeing jQuery did it to an extent, but it doesn't handle hierarchical records at all, let alone in a php friendly way.

One thing I do know for certain, is when building URLs and sticking the product into the dom, don't just use string-glue to do it, or you'll be opening yourself to a handy page breaker.

For instance, certain advertising software in-lines the version string from whatever runs your flash. This is fine when its adobes generic simple string, but however, that's very naive, and blows up in an embarrasing mess for people whom have installed Gnash, as gnash'es version string happens to contain a full blown GPL copyright licences, complete with URLs and <a href> tags. Using this in your string-glue advertiser generator, results in the page blowing open and having imbalanced HTML turning up in the dom.

The moral of the story:

 var foo = document.createElement("elementnamehere"); 
 foo.attribute = allUserSpecifiedDataConsideredDangerousHere; 
 somenode.appendChild(foo); 

Not:

 document.write("<elementnamehere attribute=\"" 
 + ilovebrokenwebsites 
 + "\">" 
 + stringdata 
 + "</elementnamehere>");

_{Google need to learn this trick. I tried to report the problem, they appear not to care.}

I know this is very late answer but works very well...

var obj = {
a:"a",
b:"b"
}
Object.entries(obj).map(([key, val])=>`${key}=${val}`).join("&");

note: object.entries will return key,values pairs

output from above line will be a=a&b=b

Hope its helps someone.

Happy Coding...

As Stein says, you can use the prototype javascript library from http://www.prototypejs.org. Include the JS and it is very simple then, $('formName').serialize() will return what you want!

For those of us who prefer jQuery, you would use the form plugin: http://plugins.jquery.com/project/form, which contains a formSerialize method.

Using URL and URLSearchParams

let url = new URL("https://foo.bar");
url.search = new URLSearchParams({
 customerid: 12345,
 lang: "en"
}).toString();
console.log(url.toString());

You don't actually need a form to do this with Prototype. Just use Object.toQueryString function:

Object.toQueryString({ action: 'ship', order_id: 123, fees: ['f1', 'f2'], 'label': 'a demo' })
// -> 'action=ship&order_id=123&fees=f1&fees=f2&label=a%20demo'

Is is probably too late to answer your question.
I had the same question and I didn't like to keep appending strings to create a URL. So, I started using $.param as techhouse explained.
I also found a URI.js library that creates the URLs easily for you. There are several examples that will help you: URI.js Documentation.
Here is one of them:

var uri = new URI("?hello=world");
uri.setSearch("hello", "mars"); // returns the URI instance for chaining
// uri == "?hello=mars"
uri.setSearch({ foo: "bar", goodbye : ["world", "mars"] });
// uri == "?hello=mars&foo=bar&goodbye=world&goodbye=mars"
uri.setSearch("goodbye", "sun");
// uri == "?hello=mars&foo=bar&goodbye=sun"
// CAUTION: beware of arrays, the following are not quite the same
// If you're dealing with PHP, you probably want the latter...
uri.setSearch("foo", ["bar", "baz"]);
uri.setSearch("foo[]", ["bar", "baz"]);`

Remove undefined params 💪😃

 urlParams = obj =>{
 const removeUndefined = JSON.parse(JSON.stringify(obj))
 const result = new URLSearchParams(removeUndefined).toString();
 return result ? `?${result}`: '';
 }
 console.log(urlParams({qwe: undefined, txt: 'asd'})) // '?txt=asd'
 console.log(urlParams({qwe: undefined})) // ''

These answers are very helpful, but i want to add another answer, that may help you build full URL. This can help you concat base url, path, hash and parameters.

var url = buildUrl('http://mywebsite.com', {
 path: 'about',
 hash: 'contact',
 queryParams: {
 'var1': 'value',
 'var2': 'value2',
 'arr[]' : 'foo'
 }
 });
 console.log(url);

You can download via npm https://www.npmjs.com/package/build-url

Demo:

;(function () {
 'use strict';
 var root = this;
 var previousBuildUrl = root.buildUrl;
 var buildUrl = function (url, options) {
 var queryString = [];
 var key;
 var builtUrl;
 var caseChange; 
 
 // 'lowerCase' parameter default = false, 
 if (options && options.lowerCase) {
 caseChange = !!options.lowerCase;
 } else {
 caseChange = false;
 }
 if (url === null) {
 builtUrl = '';
 } else if (typeof(url) === 'object') {
 builtUrl = '';
 options = url;
 } else {
 builtUrl = url;
 }
 if(builtUrl && builtUrl[builtUrl.length - 1] === '/') {
 builtUrl = builtUrl.slice(0, -1);
 } 
 if (options) {
 if (options.path) {
 var localVar = String(options.path).trim(); 
 if (caseChange) {
 localVar = localVar.toLowerCase();
 }
 if (localVar.indexOf('/') === 0) {
 builtUrl += localVar;
 } else {
 builtUrl += '/' + localVar;
 }
 }
 if (options.queryParams) {
 for (key in options.queryParams) {
 if (options.queryParams.hasOwnProperty(key) && options.queryParams[key] !== void 0) {
 var encodedParam;
 if (options.disableCSV && Array.isArray(options.queryParams[key]) && options.queryParams[key].length) {
 for(var i = 0; i < options.queryParams[key].length; i++) {
 encodedParam = encodeURIComponent(String(options.queryParams[key][i]).trim());
 queryString.push(key + '=' + encodedParam);
 }
 } else { 
 if (caseChange) {
 encodedParam = encodeURIComponent(String(options.queryParams[key]).trim().toLowerCase());
 }
 else {
 encodedParam = encodeURIComponent(String(options.queryParams[key]).trim());
 }
 queryString.push(key + '=' + encodedParam);
 }
 }
 }
 builtUrl += '?' + queryString.join('&');
 }
 if (options.hash) {
 if(caseChange)
 builtUrl += '#' + String(options.hash).trim().toLowerCase();
 else
 builtUrl += '#' + String(options.hash).trim();
 }
 } 
 return builtUrl;
 };
 buildUrl.noConflict = function () {
 root.buildUrl = previousBuildUrl;
 return buildUrl;
 };
 if (typeof(exports) !== 'undefined') {
 if (typeof(module) !== 'undefined' && module.exports) {
 exports = module.exports = buildUrl;
 }
 exports.buildUrl = buildUrl;
 } else {
 root.buildUrl = buildUrl;
 }
}).call(this);
var url = buildUrl('http://mywebsite.com', {
		path: 'about',
		hash: 'contact',
		queryParams: {
			'var1': 'value',
			'var2': 'value2',
			'arr[]' : 'foo'
		}
	});
	console.log(url);