How to create query parameters in Javascript?

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'

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"

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.

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)

The built-in URL class provides a convenient interface for creating and parsing URLs.

There are no networking methods that require exactly a URL object, strings are good enough. So technically we don’t have to use URL. But sometimes it can be really helpful.

Example

let url = new URL("https://google.com/search");
url.searchParams.set('var1', "value1");
url.searchParams.set('var2', "value2");
url.searchParams.set('var3', "value3");

console.log(url)

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;
}