Uncaught (in promise) ReferenceError: client is not defined [duplicate]

This is because require() does not exist in the browser/client-side JavaScript.

Now you're going to have to make some choices about your client-side JavaScript script management.

You have three options:

1. Use the <script> tag.
2. Use a CommonJS implementation. It has synchronous dependencies like Node.js
3. Use an asynchronous module definition (AMD) implementation.

CommonJS client side-implementations include (most of them require a build step before you deploy):

1. Browserify - You can use most Node.js modules in the browser. This is my personal favorite.
2. Webpack - Does everything (bundles JavaScript code, CSS, etc.). It was made popular by the surge of React, but it is notorious for its difficult learning curve.
3. Rollup - a new contender. It leverages ES6 modules and includes tree-shaking abilities (removes unused code).

You can read more about my comparison of Browserify vs (deprecated) Component.

AMD implementations include:

1. RequireJS - Very popular amongst client-side JavaScript developers. It is not my taste because of its asynchronous nature.

Note, in your search for choosing which one to go with, you'll read about Bower. Bower is only for package dependencies and is unopinionated on module definitions like CommonJS and AMD.

I am coming from an Electron environment, where I need IPC communication between a renderer process and the main process. The renderer process sits in an HTML file between script tags and generates the same error.

The line

const {ipcRenderer} = require('electron')

throws the Uncaught ReferenceError: require is not defined

I was able to work around that by specifying Node.js integration as true when the browser window (where this HTML file is embedded) was originally created in the main process.

function createAddItemWindow() {

    // Create a new window
    addItemWindown = new BrowserWindow({
        width: 300,
        height: 200,
        title: 'Add Item',

        // The lines below solved the issue
        webPreferences: {
            nodeIntegration: true,
            contextIsolation: false
        }
})}

That solved the issue for me. The solution was proposed here.

ES6: In HTML, include the main JavaScript file using attribute type="module" (browser support):

<script type="module" src="script.js"></script>

And in the script.js file, include another file like this:

import { hello } from './module.js';
...
// alert(hello());

Inside the included file (module.js), you must export the function/class that you will import:

export function hello() {
    return "Hello World";
}

A working example is here. More information is here.