Why can't I access 'window' in an exposeFunction() function with Puppeteer?

Question

I have a very simple Puppeteer script that uses exposeFunction() to run something inside headless Chrome.

(async function(){

    var log = console.log.bind(console),
        puppeteer = require('puppeteer');


    const browser = await puppeteer.launch();
    const page = await browser.newPage();

    var functionToInject = function(){
        return window.navigator.appName;
    }

    await page.exposeFunction('functionToInject', functionToInject);

    var data = await page.evaluate(async function(){
        console.log('woo I run inside a browser')
        return await functionToInject();
    });

    console.log(data);

    await browser.close();

})()

This fails with:

ReferenceError: window is not defined

Which refers to the injected function. How can I access window inside the headless Chrome?

I know I can do evaluate() instead, but this doesn't work with a function I pass dynamically:

(async function(){

    var log = console.log.bind(console),
        puppeteer = require('puppeteer');

    const browser = await puppeteer.launch();
    const page = await browser.newPage();

    var data = await page.evaluate(async function(){
        console.log('woo I run inside a browser')
        return window.navigator.appName;
    });

    console.log(data);

    await browser.close();

})()

Answer 1

exposeFunction() isn't the right tool for this job.

From the Puppeteer docs

page.exposeFunction(name, puppeteerFunction)

puppeteerFunction Callback function which will be called in Puppeteer's context.

'In puppeteer's context' is a little vague, but check out the docs for evaluate():

page.evaluateHandle(pageFunction, ...args)

pageFunction Function to be evaluated in the page context

exposeFunction() doesn't expose a function to run inside the page, but exposes a function to be be run in node to be called from the page.

I have to use evaluate():