How to check if jsHandle returned from page.evaluateHandle is empty/null?

2 min read 06-10-2024

How to check if jsHandle returned from page.evaluateHandle is empty/null?

Navigating the Empty Canvas: Detecting Null or Empty jsHandles in Puppeteer

In the world of web scraping and automation with Puppeteer, page.evaluateHandle is a powerful tool for interacting with the DOM and retrieving data from web pages. However, sometimes the jsHandle returned by page.evaluateHandle might be empty or null, leading to unexpected errors in your scripts. This article will guide you through detecting and handling these empty jsHandles effectively.

The Scenario: An Empty Promise

Imagine you're scraping a website for product names using Puppeteer. You use page.evaluateHandle to locate the elements containing the product names, but sometimes these elements might not exist on the page. Here's a simplified code example:

const puppeteer = require('puppeteer');

async function scrapeProducts(url) {
  const browser = await puppeteer.launch();
  const page = await browser.newPage();
  await page.goto(url);

  // Attempt to get product names
  const productHandles = await page.evaluateHandle(() => {
    return document.querySelectorAll('.product-name');
  });

  // Here lies the problem: productHandles might be empty or null!

  // ... further code to process productHandles ...

  await browser.close();
}

scrapeProducts('https://example.com');

The problem arises when page.evaluateHandle returns an empty jsHandle because no elements with the selector .product-name exist on the page. Trying to access properties or call methods on an empty jsHandle will lead to errors, crashing your script.

The Solution: Checking for Empty Handles

Here's how you can effectively check if a jsHandle is empty or null:

**Using .properties(): ** The .properties() method on a jsHandle returns a Map containing all its properties. An empty jsHandle will have no properties.
```
if (productHandles.properties().size === 0) {
  console.log('No products found!');
} else {
  // Process the productHandles
}
```

**Using .jsonValue(): ** The .jsonValue() method attempts to convert the jsHandle to a JSON value. If the jsHandle is empty, it will return null.

const productData = await productHandles.jsonValue();

if (productData === null) {
  console.log('No products found!');
} else {
  // Process the productData
}

**Using .asElement(): ** This method attempts to convert the jsHandle into an ElementHandle. If it fails, the jsHandle is likely empty or not an element.

const productElement = await productHandles.asElement();

if (productElement === null) {
  console.log('No products found!');
} else {
  // Process the productElement
}

Optimizing for Robustness

By implementing these checks, your Puppeteer scripts will gracefully handle situations where page.evaluateHandle returns empty jsHandles. This improves your script's robustness and prevents unexpected crashes.

Additional Tips

Error Handling: Consider using try...catch blocks to catch potential errors when working with jsHandles.
Debugging: Utilize Puppeteer's debugging tools and browser DevTools to inspect the state of your jsHandles and ensure they're behaving as expected.

Conclusion

Understanding how to check for empty or null jsHandles is essential for building robust Puppeteer scripts. By incorporating these checks into your code, you'll create a more reliable and predictable scraping experience.

References: