276 lines
11 KiB
Markdown
276 lines
11 KiB
Markdown
---
|
|
title: 'Using Preload Scripts'
|
|
description: 'This guide will step you through the process of creating a barebones Hello World app in Electron, similar to electron/electron-quick-start.'
|
|
slug: tutorial-preload
|
|
hide_title: false
|
|
---
|
|
|
|
:::info Follow along the tutorial
|
|
|
|
This is **part 3** of the Electron tutorial.
|
|
|
|
1. [Prerequisites][prerequisites]
|
|
1. [Building your First App][building your first app]
|
|
1. **[Using Preload Scripts][preload]**
|
|
1. [Adding Features][features]
|
|
1. [Packaging Your Application][packaging]
|
|
1. [Publishing and Updating][updates]
|
|
|
|
:::
|
|
|
|
## Learning goals
|
|
|
|
In this part of the tutorial, you will learn what a preload script is and how to use one
|
|
to securely expose privileged APIs into the renderer process. You will also learn how to
|
|
communicate between main and renderer processes with Electron's inter-process
|
|
communication (IPC) modules.
|
|
|
|
## What is a preload script?
|
|
|
|
Electron's main process is a Node.js environment that has full operating system access.
|
|
On top of [Electron modules][modules], you can also access [Node.js built-ins][node-api],
|
|
as well as any packages installed via npm. On the other hand, renderer processes run web
|
|
pages and do not run Node.js by default for security reasons.
|
|
|
|
To bridge Electron's different process types together, we will need to use a special script
|
|
called a **preload**.
|
|
|
|
## Augmenting the renderer with a preload script
|
|
|
|
A BrowserWindow's preload script runs in a context that has access to both the HTML DOM
|
|
and a limited subset of Node.js and Electron APIs.
|
|
|
|
:::info Preload script sandboxing
|
|
|
|
From Electron 20 onwards, preload scripts are **sandboxed** by default and no longer have access
|
|
to a full Node.js environment. Practically, this means that you have a polyfilled `require`
|
|
function that only has access to a limited set of APIs.
|
|
|
|
| Available API | Details |
|
|
| ------------------ | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
|
|
| Electron modules | Renderer process modules |
|
|
| Node.js modules | [`events`](https://nodejs.org/api/events.html), [`timers`](https://nodejs.org/api/timers.html), [`url`](https://nodejs.org/api/url.html) |
|
|
| Polyfilled globals | [`Buffer`](https://nodejs.org/api/buffer.html), [`process`](../api/process.md), [`clearImmediate`](https://nodejs.org/api/timers.html#timers_clearimmediate_immediate), [`setImmediate`](https://nodejs.org/api/timers.html#timers_setimmediate_callback_args) |
|
|
|
|
For more information, check out the [Process Sandboxing](./sandbox.md) guide.
|
|
|
|
:::
|
|
|
|
Preload scripts are injected before a web page loads in the renderer,
|
|
similar to a Chrome extension's [content scripts][content-script]. To add features to your renderer
|
|
that require privileged access, you can define [global][] objects through the
|
|
[contextBridge][contextbridge] API.
|
|
|
|
To demonstrate this concept, you will create a preload script that exposes your app's
|
|
versions of Chrome, Node, and Electron into the renderer.
|
|
|
|
Add a new `preload.js` script that exposes selected properties of Electron's `process.versions`
|
|
object to the renderer process in a `versions` global variable.
|
|
|
|
```js title="preload.js"
|
|
const { contextBridge } = require('electron')
|
|
|
|
contextBridge.exposeInMainWorld('versions', {
|
|
node: () => process.versions.node,
|
|
chrome: () => process.versions.chrome,
|
|
electron: () => process.versions.electron
|
|
// we can also expose variables, not just functions
|
|
})
|
|
```
|
|
|
|
To attach this script to your renderer process, pass its path to the
|
|
`webPreferences.preload` option in the BrowserWindow constructor:
|
|
|
|
```js {2,8-10} title="main.js"
|
|
const { app, BrowserWindow } = require('electron')
|
|
const path = require('node:path')
|
|
|
|
const createWindow = () => {
|
|
const win = new BrowserWindow({
|
|
width: 800,
|
|
height: 600,
|
|
webPreferences: {
|
|
preload: path.join(__dirname, 'preload.js')
|
|
}
|
|
})
|
|
|
|
win.loadFile('index.html')
|
|
}
|
|
|
|
app.whenReady().then(() => {
|
|
createWindow()
|
|
})
|
|
```
|
|
|
|
:::info
|
|
|
|
There are two Node.js concepts that are used here:
|
|
|
|
- The [`__dirname`][dirname] string points to the path of the currently executing script
|
|
(in this case, your project's root folder).
|
|
- The [`path.join`][path-join] API joins multiple path segments together, creating a
|
|
combined path string that works across all platforms.
|
|
|
|
:::
|
|
|
|
At this point, the renderer has access to the `versions` global, so let's display that
|
|
information in the window. This variable can be accessed via `window.versions` or simply
|
|
`versions`. Create a `renderer.js` script that uses the [`document.getElementById`][]
|
|
DOM API to replace the displayed text for the HTML element with `info` as its `id` property.
|
|
|
|
```js title="renderer.js" @ts-nocheck
|
|
const information = document.getElementById('info')
|
|
information.innerText = `This app is using Chrome (v${versions.chrome()}), Node.js (v${versions.node()}), and Electron (v${versions.electron()})`
|
|
```
|
|
|
|
Then, modify your `index.html` by adding a new element with `info` as its `id` property,
|
|
and attach your `renderer.js` script:
|
|
|
|
```html {18,20} title="index.html"
|
|
<!DOCTYPE html>
|
|
<html>
|
|
<head>
|
|
<meta charset="UTF-8" />
|
|
<meta
|
|
http-equiv="Content-Security-Policy"
|
|
content="default-src 'self'; script-src 'self'"
|
|
/>
|
|
<meta
|
|
http-equiv="X-Content-Security-Policy"
|
|
content="default-src 'self'; script-src 'self'"
|
|
/>
|
|
<title>Hello from Electron renderer!</title>
|
|
</head>
|
|
<body>
|
|
<h1>Hello from Electron renderer!</h1>
|
|
<p>👋</p>
|
|
<p id="info"></p>
|
|
</body>
|
|
<script src="./renderer.js"></script>
|
|
</html>
|
|
```
|
|
|
|
After following the above steps, your app should look something like this:
|
|
|
|
|
|
|
|
And the code should look like this:
|
|
|
|
```fiddle docs/fiddles/tutorial-preload
|
|
|
|
```
|
|
|
|
## Communicating between processes
|
|
|
|
As we have mentioned above, Electron's main and renderer process have distinct responsibilities
|
|
and are not interchangeable. This means it is not possible to access the Node.js APIs directly
|
|
from the renderer process, nor the HTML Document Object Model (DOM) from the main process.
|
|
|
|
The solution for this problem is to use Electron's `ipcMain` and `ipcRenderer` modules for
|
|
inter-process communication (IPC). To send a message from your web page to the main process,
|
|
you can set up a main process handler with `ipcMain.handle` and
|
|
then expose a function that calls `ipcRenderer.invoke` to trigger the handler in your preload script.
|
|
|
|
To illustrate, we will add a global function to the renderer called `ping()`
|
|
that will return a string from the main process.
|
|
|
|
First, set up the `invoke` call in your preload script:
|
|
|
|
```js {1,7} title="preload.js"
|
|
const { contextBridge, ipcRenderer } = require('electron')
|
|
|
|
contextBridge.exposeInMainWorld('versions', {
|
|
node: () => process.versions.node,
|
|
chrome: () => process.versions.chrome,
|
|
electron: () => process.versions.electron,
|
|
ping: () => ipcRenderer.invoke('ping')
|
|
// we can also expose variables, not just functions
|
|
})
|
|
```
|
|
|
|
:::caution IPC security
|
|
|
|
Notice how we wrap the `ipcRenderer.invoke('ping')` call in a helper function rather
|
|
than expose the `ipcRenderer` module directly via context bridge. You **never** want to
|
|
directly expose the entire `ipcRenderer` module via preload. This would give your renderer
|
|
the ability to send arbitrary IPC messages to the main process, which becomes a powerful
|
|
attack vector for malicious code.
|
|
|
|
:::
|
|
|
|
Then, set up your `handle` listener in the main process. We do this _before_
|
|
loading the HTML file so that the handler is guaranteed to be ready before
|
|
you send out the `invoke` call from the renderer.
|
|
|
|
```js {1,15} title="main.js"
|
|
const { app, BrowserWindow, ipcMain } = require('electron')
|
|
const path = require('node:path')
|
|
|
|
const createWindow = () => {
|
|
const win = new BrowserWindow({
|
|
width: 800,
|
|
height: 600,
|
|
webPreferences: {
|
|
preload: path.join(__dirname, 'preload.js')
|
|
}
|
|
})
|
|
win.loadFile('index.html')
|
|
}
|
|
app.whenReady().then(() => {
|
|
ipcMain.handle('ping', () => 'pong')
|
|
createWindow()
|
|
})
|
|
```
|
|
|
|
Once you have the sender and receiver set up, you can now send messages from the renderer
|
|
to the main process through the `'ping'` channel you just defined.
|
|
|
|
```js title='renderer.js' @ts-expect-error=[2]
|
|
const func = async () => {
|
|
const response = await window.versions.ping()
|
|
console.log(response) // prints out 'pong'
|
|
}
|
|
|
|
func()
|
|
```
|
|
|
|
:::info
|
|
|
|
For more in-depth explanations on using the `ipcRenderer` and `ipcMain` modules,
|
|
check out the full [Inter-Process Communication][ipc] guide.
|
|
|
|
:::
|
|
|
|
## Summary
|
|
|
|
A preload script contains code that runs before your web page is loaded into the browser
|
|
window. It has access to both DOM APIs and Node.js environment, and is often used to
|
|
expose privileged APIs to the renderer via the `contextBridge` API.
|
|
|
|
Because the main and renderer processes have very different responsibilities, Electron
|
|
apps often use the preload script to set up inter-process communication (IPC) interfaces
|
|
to pass arbitrary messages between the two kinds of processes.
|
|
|
|
In the next part of the tutorial, we will be showing you resources on adding more
|
|
functionality to your app, then teaching you distributing your app to users.
|
|
|
|
<!-- Links -->
|
|
|
|
[content-script]: https://developer.chrome.com/docs/extensions/mv3/content_scripts/
|
|
[contextbridge]: ../api/context-bridge.md
|
|
[`document.getelementbyid`]: https://developer.mozilla.org/en-US/docs/Web/API/Document/getElementById
|
|
[dirname]: https://nodejs.org/api/modules.html#modules_dirname
|
|
[global]: https://developer.mozilla.org/en-US/docs/Glossary/Global_object
|
|
[ipc]: ./ipc.md
|
|
[modules]: ../api/app.md
|
|
[node-api]: https://nodejs.org/dist/latest/docs/api/
|
|
[path-join]: https://nodejs.org/api/path.html#path_path_join_paths
|
|
|
|
<!-- Tutorial links -->
|
|
|
|
[prerequisites]: tutorial-1-prerequisites.md
|
|
[building your first app]: tutorial-2-first-app.md
|
|
[preload]: tutorial-3-preload.md
|
|
[features]: tutorial-4-adding-features.md
|
|
[packaging]: tutorial-5-packaging.md
|
|
[updates]: tutorial-6-publishing-updating.md
|