How to create query parameters in Javascript?

Is there any way to create the query parameters for doing a GET request in JavaScript?

Just like in Python you have urllib.urlencode(), which takes in a dictionary (or list of two tuples) and creates a string like 'var1=value1&var2=value2'.

Here you go:

function encodeQueryData(data) {
   const ret = [];
   for (let d in data)
     ret.push(encodeURIComponent(d) + '=' + encodeURIComponent(data[d]));
   return ret.join('&');
}

Usage:

const data = { 'first name': 'George', 'last name': 'Jetson', 'age': 110 };
const querystring = encodeQueryData(data);

URLSearchParams has increasing browser support.

const data = {
  var1: 'value1',
  var2: 'value2'
};

const searchParams = new URLSearchParams(data);

// searchParams.toString() === 'var1=value1&var2=value2'

Node.js offers the querystring module.

const querystring = require('querystring');

const data = {
  var1: 'value1',
  var2: 'value2'
};

const searchParams = querystring.stringify(data);

// searchParams === 'var1=value1&var2=value2'

functional

function encodeData(data) {
    return Object.keys(data).map(function(key) {
        return [key, data[key]].map(encodeURIComponent).join("=");
    }).join("&");
}   

Zabba has provided in a comment on the currently accepted answer a suggestion that to me is the best solution: use jQuery.param().

If I use jQuery.param() on the data in the original question, then the code is simply:

const params = jQuery.param({
    var1: 'value',
    var2: 'value'
});

The variable params will be

"var1=value&var2=value"

For more complicated examples, inputs and outputs, see the jQuery.param() documentation.

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"

We've just released arg.js, a project aimed at solving this problem once and for all. It's traditionally been so difficult but now you can do:

var querystring = Arg.url({name: "Mat", state: "CO"});

And reading works:

var name = Arg("name");

or getting the whole lot:

var params = Arg.all();

and if you care about the difference between ?query=true and #hash=true then you can use the Arg.query() and Arg.hash() methods.

This should do the job:

const createQueryParams = params => 
      Object.keys(params)
            .map(k => `${k}=${encodeURI(params[k])}`)
            .join('&');

Example:

const params = { name : 'John', postcode: 'W1 2DL'}
const queryParams = createQueryParams(params)

Result:

name=John&postcode=W1%202DL

If you are using Prototype there is Form.serialize

If you are using jQuery there is Ajax/serialize

I do not know of any independent functions to accomplish this, though, but a google search for it turned up some promising options if you aren't currently using a library. If you're not, though, you really should because they are heaven.

Just like to revisit this almost 10 year old question. In this era of off-the-shelf programming, your best bet is to set your project up using a dependency manager (npm). There is an entire cottage industry of libraries out there that encode query strings and take care of all the edge cases. This is one of the more popular ones -

https://www.npmjs.com/package/query-string

A little modification to typescript:

  public encodeData(data: any): string {
    return Object.keys(data).map((key) => {
      return [key, data[key]].map(encodeURIComponent).join("=");
    }).join("&");
  }

I have improved the function of shog9`s to handle array values

function encodeQueryData(data) {
    const ret = [];
    for (let d in data) {
        if (typeof data[d] === 'object' || typeof data[d] === 'array') {
            for (let arrD in data[d]) {
                ret.push(`${encodeURIComponent(d)}[]=${encodeURIComponent(data[d][arrD])}`)
            }
        } else if (typeof data[d] === 'null' || typeof data[d] === 'undefined') {
            ret.push(encodeURIComponent(d))
        } else {
            ret.push(`${encodeURIComponent(d)}=${encodeURIComponent(data[d])}`)
        }

    }
    return ret.join('&');
}

Example

let data = {
  user: 'Mark'
  fruits: ['apple', 'banana']
}

encodeQueryData(data) // user=Mark&fruits[]=apple&fruits[]=banana

By using queryencoder, you can have some nice-to-have options, such custom date formatters, nested objects and decide if a val: true will be just value or value=true.

const { encode } = require('queryencoder');

const object = {
    date: new Date('1999-04-23')
};

// The result is 'date=1999-04-23'
const queryUrl = encode(object, {
    dateParser: date => date.toISOString().slice(0, 10)
});

Here is an example:

let my_url = new URL("https://stackoverflow.com")
my_url.pathname = "/questions"

const parameters = {
    title: "just",
    body: 'test'
}

Object.entries(parameters).forEach(([name, value]) => my_url.searchParams.set(name, value))

console.log(my_url.href)

This thread points to some code for escaping URLs in php. There's escape() and unescape() which will do most of the work, but the you need add a couple extra things.

function urlencode(str) {
str = escape(str);
str = str.replace('+', '%2B');
str = str.replace('%20', '+');
str = str.replace('*', '%2A');
str = str.replace('/', '%2F');
str = str.replace('@', '%40');
return str;
}

function urldecode(str) {
str = str.replace('+', ' ');
str = unescape(str);
return str;
}