How can I dynamically inject functions to evaluate using Puppeteer?

Question

I am using Puppeteer for headless Chrome. I wish to evaluate a function inside the page that uses parts of other functions, defined dynamically elsewhere.

The code below is a minimal example / proof. In reality functionToInject() and otherFunctionToInject() are more complex and require the pages DOM.

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

var functionToInject = function(){
    return 1+1;
}

var otherFunctionToInject = function(input){
    return 6
}

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

return data

When I run the code, I get:

Error: Evaluation failed: TypeError: functionToInject is not a function

Which I understand: functionToInject isn't being passed into the page's JS context. But how do I pass it into the page's JS context?

Answer 1

You can add function to page context with addScriptTag:

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

function functionToInject (){
    return 1+1;
}

function otherFunctionToInject(input){
    return 6
}

await page.addScriptTag({ content: `${functionToInject} ${otherFunctionToInject}`});

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

console.log(data);

await browser.close();

This example is a dirty way of solving this problem with string concatenation. More clean would be using a url or path in the addScriptTag method.

---

Or use exposeFunction (but now functions are wrapped in Promise):

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

var functionToInject = function(){
    return 1+1;
}

var otherFunctionToInject = function(input){
    return 6
}

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

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

console.log(data);

await browser.close();