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'