How to build query string with Javascript

Just wondering if there is anything built-in to Javascript that can take a Form and return the query parameters, eg: "var1=value&var2=value2&arr[]=foo&arr[]=bar..."

I've been wondering this for years.

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