How to simulate slow lazy chunk-loading in React

25 March 2021   0 comments   JavaScript, React

Suppose you have one of those React apps that lazy-load some chunk. It just basically means it injects a .js static asset URL into the DOM and once it's downloaded by the browser, it carries on the React rendering with the new code loaded. Well, what if the network is really slow? In local development, it can be hard to simulate this. You can mess with the browser's Devtools to try to slow down the network, but even that can be too fast sometimes.

What I often do is, I take this:

const SettingsApp = React.lazy(() => import("./app"));

...and change it to this:

const SettingsApp = React.lazy(() =>
  import("./app").then((module) => {
    return new Promise((resolve) => {
      setTimeout(() => {
        resolve(module as any);
      }, 10000);

Now, it won't load that JS chunk until 10 seconds later. Only temporarily, in local development.

I know it's admittedly just a hack but it's nifty. Just don't forget to undo it when you're done simulating your snail-speed web app.

PS. That resolve(module as any); is for TypeScript. You can just change that to resolve(module); if it's regular JavaScript.


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