URL Parameters

Introduction

URL parameters (also called “query strings” or “URL query parameters”) are ways to send information through a URL.

How they work

URL parameters begin with a question mark (?). After that, URL parameters contain key/value pairs separated by an ampersand (&). The key and value have an equal sign (=) between them.

URL parameters can be beneficial within your own applications. An example of where they could be helpful is a dynamic product page (e.g. https://www.example.com/product/?id=1). This page would check the URL parameters for an ID and then use this ID to do an API call to fetch a product with a specific ID. In this example, it would get a product with an ID of 1.

Adding URL parameters

Adding URL parameters is very straightforward. You add it to the URL that you are navigating users to.

In the example below, we are adding:

• the firstName key with a value of ola.
• the lastName key with the value of nordmann.

	<a href="https://www.example.com/?firstName=ola&lastName=nordmann">Submit</a>

The above example is hardcoded and added into our HTML, which isn’t of much use. We typically need to add dynamic values, such as different first and last names.

Dynamic URL parameters

We will now look at how we can dynamically add URL parameter values:

Generating an anchor element with createElement

In this example, we are generating HTML and are using variables to set our values:

	// Get the container
	const container = document.getElementById('container');
	
	// These values are hardcoded here but they could come from another
	// source such as an API
	const firstName = 'Ola';
	const lastName = 'Nordmann';
	
	// Create a new <a> anchor link
	const newLink = document.createElement('a');
	
	// Here we use a template literal/template string to add our values
	newLink.href = `https://www.example.com/?firstName=${firstName}&lastName=${lastName}`;
	
	// We set the text content of our anchor link
	newLink.textContent = 'Submit';
	
	// We add our anchor link to the parent container
	container.appendChild(newLink);

Generating an anchor element with innerHTML

The following example is the same as the above, except it is setting the innerHTML:

	// Get the container
	const container = document.getElementById('container');
	
	// These values are hardcoded here, but they could come from another
	// source such as an API
	const firstName = 'Ola';
	const lastName = 'Nordmann';
	
	// Setting the HTML for the container element
	container.innerHTML = `<a href="https://www.example.com/?firstName=${firstName}&lastName=${lastName}">Submit</a>`;

Note: Setting innerHTML can be considered dangerous. innerHTML will parse JavaScript, meaning malicious JavaScript could be added in some cases. Please see the Additional resources on this page for more information on this.

Navigating programmatically

We can use JavaScript to navigate programmatically, i.e. navigate only using code. In this example, we make use of window.location.replace to programmatically change the URL:

	// Get the button with id of "btn"
	const button = document.getElementById('btn');
	
	// These values are hardcoded here, but they could come from another
	// source such as an API
	const firstName = 'Ola';
	const lastName = 'Nordmann';
	
	// Add a "click" event listener that calls "onButtonClick"
	button.addEventListener('click', onButtonClick);
	
	// This function will fire each time the button is clicked
	function onButtonClick() {
	  // Replace the URL with our newly created URL that contains our
	  // dynamic URL parameters
	  window.location.replace(
	    `https://www.example.com/?firstName=${firstName}&lastName=${lastName}`,
	  );
	}

Consuming URL parameters

We have looked at how we add URL parameters. We will now look at how we can make use of URL parameters.

Parameter string

We can get the whole parameter string with window.location.search, the query that starts with the question mark (?).

In this example, we are navigating to http://localhost:8000/?firstName=ola&lastName=nordmann, which has the following code in a script:

	// Navigated to: http://localhost:8000/?firstName=ola&lastName=nordmann
	
	// Get the parameter string from the URL
	const parameterString = window.location.search;
	console.log(parameterString);
	
	// Logs:
	// ?firstName=ola&lastName=nordmann
	//

Parsing the parameter string with URLSearchParams

We saw how we could obtain the parameter string; however, this is not of much use on its own. It would still require much work to extract the values out of it.

URLSearchParams() is a function which will parse the query string for us and allow us to get specific values, among other features.

	// Navigated to: http://localhost:8000/?firstName=ola&lastName=nordmann
	
	const parameterString = window.location.search;
	const searchParameters = new URLSearchParams(parameterString);
	
	// Iterate through the search parameters
	for (const parameter of searchParameters) {
	  console.log(parameter);
	}
	// Logs:
	// (2) ['firstName', 'ola']
	// (2) ['lastName', 'nordmann']

Note: You will see that the parameters are output as two values in an array, e.g. ['firstName', 'ola']. This is an ideal situation to do array destructuring.

get()

We can use the get() method to obtain a specific value for a specific key.

	// Navigated to: http://localhost:8000/?firstName=ola&lastName=nordmann
	
	const parameterString = window.location.search;
	const searchParameters = new URLSearchParams(parameterString);
	
	console.log(searchParameters.get('firstName'));
	// Logs:
	// ola

set()

You can use the set() method to add a new key/value pair. It takes two arguments, the key and value you want to set.

In this example, we are setting a new key/value pair:

	// Navigated to: http://localhost:8000/?firstName=ola&lastName=nordmann
	
	const parameterString = window.location.search;
	const searchParameters = new URLSearchParams(parameterString);
	
	searchParameters.set('country', 'Norway');
	
	console.log(searchParameters.get('country'));
	// Logs:
	// Norway

toString()

You can use the toString() method on your URLSearchParams object to convert the search parameters back into a string that can be used in a URL.

In this example, you will see that we have a search string logged:

	// Navigated to: http://localhost:8000/?firstName=ola&lastName=nordmann
	
	const parameterString = window.location.search;
	const searchParameters = new URLSearchParams(parameterString);
	
	console.log(searchParameters.toString());
	// Logs:
	// firstName=ola&lastName=nordmann

Other methods

There are additional methods worth looking into, which you can find in the MDN web docs for URLSearchParams.

Lesson task

You will likely come across the need to use URL parameters as a front-end web developer.

Goal

You will practise using URL parameters.

Brief

Complete the Level 1 process.

NOTE: Lesson tasks do not get submitted on Moodle and are not assessed by tutors. They are mainly there for you to practise what you have learned in the lesson.

Level 1 process

1. Create a index.html and add a hyperlink to a product.html page. Pass in the follow key/value pairs into this URL:

1.1 firstName key with a value of Ola.

1.2 lastName key with a value of Nordmann.

2. Create aproduct.html page and import a new file called product.js.

3. Obtain the values from the URL and log them out. You should end up with Ola and Nordmann being logged out.

Additional resources

MDN Web Docs: WindowClient.navigate()

How to use .innerHTML safely

DOM Manipulation and the Dangers of ‘innerHTML’

MDN Web Docs: URLSearchParams