build.onEnd is not a function - in custom plugin setup() {...}

[rw3iss]

I'm trying to make a quick plugin to basically do sipel template-parsing/replacement in specific types of files (ie. HTML files - in order to add a hash string to the end of static assets urls).

Anyway, I'm trying to utilize the onBuild hook to manually generator the output file, which the docs refer to here:
https://esbuild.github.io/plugins/#caching-your-plugin

However, when running, it's saying it doesn't exist on the build object:

transform setup { onResolve: [Function: onResolve], onLoad: [Function: onLoad] }
 > C:\Users\rw3is\Sites\adcloud\app\scripts\build.js:48:14: error: build.onEnd is not a function
    48 │         build.onEnd(result => {
       ╵               ^
    at setup (C:\Users\rw3is\Sites\adcloud\app\scripts\build.js:48:15)
    at handlePlugins (C:\Users\rw3is\Sites\adcloud\app\node_modules\esbuild\lib\main.js:659:7)

1 error
Error building html: Build failed with 1 error:

The code is essentially:

let transformHtmlTemplatePlugin = {
    name: 'transform',
    setup(build) {
        let fs = require('fs')

        console.log('transform setup', build)

        // Load ".txt" files and return an array of words
        build.onLoad({ filter: /\.html$/ }, async (args) => {
            let text = await fs.promises.readFile(args.path, 'utf8');
            console.log('html file', text);
            text = text.replace('{TEST}', 'THIS IS TEST');
            return {
                contents: text,
                loader: 'text',
            }
        })
        
       //... more ommitted
}

And the build process is:

function buildHtml(entryFile, outFile) {
    esbuild.build({
        entryPoints: [entryFile],
        outfile: outFile,
        target: TARGET,
        logLevel: 'verbose',
        // loader: { // built-in loaders: js, jsx, ts, tsx, css, json, text, base64, dataurl, file, binary
        //     '.html': 'file'
        // },
        define,
        plugins: [transformHtmlTemplatePlugin]
    })
        .then(r => { console.log(`Build html ${entryFile} to ${outFile} succeeded.`) })
        .catch((e) => {
            console.log("Error building html:", e.message);
            process.exit(1)
        })
}

Is there anything wrong with this approach, or is there a reason why onBuild doesn't exist on the plugin's build argument?