How to serialize an Object into a list of URL query parameters?

Question

One line with no dependencies:

new URLSearchParams(obj).toString();
// OUT: param1=something&param2=somethingelse&param3=another&param4=yetanother

Use it with the URL builtin like so:

let obj = { param1: 'something', param2: 'somethingelse', param3: 'another' }
obj['param4'] = 'yetanother';
const url = new URL(`your_url.com`);
url.search = new URLSearchParams(obj);
const response = await fetch(url);

[Edit April 4, 2020]: null values will be interpreted as the string 'null'.

If you use jQuery, this is what it uses for parameterizing the options of a GET XHR request:

$.param( obj )

http://api.jquery.com/jQuery.param/

An elegant one: (assuming you are running a modern browser or node)

var str = Object.keys(obj).map(function(key) {
  return key + '=' + obj[key];
}).join('&');

And the ES2017 equivalent: (thanks to Lukas)

let str = Object.entries(obj).map(([key, val]) => `${key}=${val}`).join('&');

Note: You probably want to use encodeURIComponent() if the keys/values are not URL encoded.

var str = "";
for (var key in obj) {
    if (str != "") {
        str += "&";
    }
    str += key + "=" + encodeURIComponent(obj[key]);
}

Example: http://jsfiddle.net/WFPen/

ES2017 approach

Object.entries(obj).map(([key, val]) => `${key}=${encodeURIComponent(val)}`).join('&')

ES6:

function params(data) {
  return Object.keys(data).map(key => `${key}=${encodeURIComponent(data[key])}`).join('&');
}

console.log(params({foo: 'bar'}));
console.log(params({foo: 'bar', baz: 'qux$'}));