How to wait for a javascript expression to be true

Contributors: Darío Kondratiuk

Problem

WaitForSelectorAsync is not enough, you want to wait for a more complex javascript expression to be truthly.

Solution

Use Page.WaitForExpressionAsync or Page.WaitForFunctionAsync to delay execution until the result of a javascription expression is truthly.

If it’s a simple expression you can use WaitForFunctionAsync:

using (var browser = await Puppeteer.LaunchAsync(options))
using (var page = await browser.NewPageAsync())
{
    await page.GoToAsync("https://www.somepage.com");
    await Page.WaitForExpressionAsync("document.queryselector('#status_info').innerText.match('^Showing ([1-9][0-9]*?) to ([1-9][0-9]*?)') of ([1-9][0-9]*?) entries') != null");
}

If the evaluation is more complex, you could wrap it inside a function and use WaitForFunctionAsync:

var waitTask = Page.WaitForFunctionAsync(@"() =>
{
    return document.queryselector('#status_info').innerText.match('^Showing ([1-9][0-9]*?) to ([1-9][0-9]*?)') of ([1-9][0-9]*?) entries') != null;
}");