«   Previous tip Next tip   »

JavaScript: Conditional JavaScript, only download when it is appropriate to do so

Last updated: 16th December 2020
Conditional JavaScript, only download when it is appropriate to do so


In this post, I'd like to share a few techniques which you can use for selectively downloading/executing resources such as JavaScript.

As an example, if the users device does not have a lot a RAM, you could decide to skip the downloading (and in turn, the parse + execution costs) of a particular JavaScript resource.

Note: I won't be specifying the exact times you should minimise usage of JavaScript. It ultimately depends on your use case.

Load JavaScript if the device has enough RAM

Let's start out with this:

if (navigator.deviceMemory > 1) {
    await import('./costly-module.js');
Getting the device memory from JavaScript

Browser support is limited to Chromium based browsers, which according to caniuse is 70% of the global usage stats. If navigator.deviceMemory is undefined, maybe consider loading the JavaScript regardless.

Detecting device memory on the server

The Web Platform is full of hidden features. Turns out you can get this information on your server!

Getting the device memory from a network request header

The browser can send a network request header which means your server can decide on the most appropriate resource to send back down. Use this <meta> tag in your HTML to activate this feature:

<meta http-equiv="Accept-CH" content="Device-Memory">

Load JavaScript when the device has enough CPU

You can get the number of logical processors on the users device with hardwareConcurrency.

if (navigator.hardwareConcurrency > 4) {
    await import('./costly-module.js');

MDN has an explanation:

The number of logical processor cores can be used to measure the number of threads which can effectively be run at once without them having to context switch.

Logical processors

This feature can be especially useful when creating new Workers - part of the Web Workers API.

Browser support is very good.

Load JavaScript when the user has enough battery left

We've all been there. Low battery 🔋️, no charger, and you start clearing apps in the hope that your 4% lasts on the journey home.

A page with JavaScript will utilise more battery than a page without JavaScript - and yes, the same applies to CSS and images.

The Network Download, Parse and Execution costs will defintely use battery, but don't forget the variety of tasks which JavaScript is capable of e.g. creating long-running timers.

// { level: 0.53 (53%), charging: true...}
const {level, charging} = await navigator.getBattery();

// If the device is currently charging
// Or the battery level is more than 20%
if (charging || level > 0.2) {
        await import('./costly-module.js');

You also get charging times and discharging times which in some scenarios could be even more valuable than what the current level is.

Battery Status

Browser support seems to be limited to Chromium based browsers. Interestingly it was removed in Firefox due to privacy concerns (a.k.a. fingerprinting 🖐🏼️).

Load JavaScript when the device has enough storage

This one really depends on what the JavaScript is doing. For example if the JavaScript is:

Then maybe consider the idea of using storage availability to influence downloading of resources.

const {quota} = await navigator.storage.estimate();
const fiftyMegabytesInBytes = 50 * 1e+6;

if (quota > fiftyMegabytesInBytes) {
    await import('./costly-module.js');

Browser support is decent

Storage quota

Load JavaScript when the device has a good network connection

Tip: Network Connection types can be simulated in DevTools. Learn DevTools in my ModernDevTools.com course.

At this point, you can argue it's also effective to skip the download of any optional resource - not just JavaScript.

// ⚠️ 4g does not mean fast!
if (navigator.connection.effectiveType === '4g') {
    await import('./costly-module.js');

The navigator.connection object gives some information on the network. E.g. the downlink property gives you the bandwidth in megabits per second. The round-trip time is in milliseconds

Network Info

How you use this API is highly dependant on your use case. But as an example, if navigator.connection.downlink is low, you can download smaller resources instead. In an ideal scenario, this setting would be customisable however (via your webpage UI).

Note: Being on WiFi doesn't mean fast, and being on 3G doesn't mean slow. For example, think about tethering!

Browser support seems to be limited to Chromium based browsers.

Getting the network information on the server

You can also get some of that network information on your server, in the form of a network request header.

Network downlink as a request header

To achieve this, use the following <meta> tag in your main HTML file:

<meta http-equiv="Accept-CH" content="Downlink">

Subsequent requests now have the request header: downlink: 6 for example. You can also use content="ECT" to get the Effective Connection Type as a network request header.


While you can infer context based on network information like roundtrip times and connection types, the saveData property states a device preference for reduced data usage.

if (navigator.connection.saveData === false) {
    await import('./costly-module.js');

This is available server side (as a client hint), and at some point it should be available as a CSS Media Feature.


It's not all about JavaScript, you can also use these various device details to:


Thank you for taking the time to read this! Hope you learnt something new!

«   Previous tip Next tip   »

Sign up to receive a developer tip, in the form of a gif, in your inbox each week