puppeteer: how to wait until an element is visible?

Note, All the answers submitted until today are incorrect

Because it answer for an element if Exist or Located but NOT Visible or Displayed

The right answer is to check an element size or visibility using page.waitFor() or page.waitForFunction(), see explaination below.

// wait until present on the DOM
// await page.waitForSelector( css_selector );
// wait until "display"-ed
await page.waitForFunction("document.querySelector('.btnNext') && document.querySelector('.btnNext').clientHeight != 0");
// or wait until "visibility" not hidden
await page.waitForFunction("document.querySelector('.btnNext') && document.querySelector('.btnNext').style.visibility != 'hidden'");

const btnNext = await page.$('.btnNext');
await btnNext.click();

Explanation

The element that Exist on the DOM of page not always Visible if has CSS property display:none or visibility:hidden that why using page.waitForSelector(selector) is not good idea, let see the different in the snippet below.

function isExist(selector) {
  let el = document.querySelector(selector);
  let exist = el.length != 0 ? 'Exist!' : 'Not Exist!';
  console.log(selector + ' is ' + exist)
}

function isVisible(selector) {
  let el = document.querySelector(selector).clientHeight;
  let visible = el != 0 ? 'Visible, ' + el : 'Not Visible, ' + el;
  console.log(selector + ' is ' + visible + 'px')
}

isExist('#idA');
isVisible('#idA');
console.log('=============================')
isExist('#idB')
isVisible('#idB')

.bd {border: solid 2px blue;}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div class="bd">
  <div id="idA" style="display:none">#idA, hidden element</div>
</div>
<br>
<div class="bd">
  <div id="idB">#idB, visible element</div>
</div>

on the snippet above the function isExist() is simulate

page.waitForSelector('#myId');

and we can see while running isExist() for both element #idA an #idB is return exist.

But when running isVisible() the #idA is not visible or dislayed.

And here other objects to check if an element is displayed or using CSS property display.

scrollWidth
scrollHeight
offsetTop
offsetWidth
offsetHeight
offsetLeft
clientWidth
clientHeight

for style visibility check with not hidden.

note: I'm not good in Javascript or English, feel free to improve this answer.

If you want to ensure the element is actually visible, you have to use

page.waitForSelector('#myId', {visible: true})

Otherwise you are just looking for the element in the DOM and not checking for visibility.

I think you can use page.waitForSelector(selector[, options]) function for that purpose.

const puppeteer = require('puppeteer');

puppeteer.launch().then(async browser => {
  const page = await browser.newPage();
  page
    .waitForSelector('#myId')
    .then(() => console.log('got it'));
    browser.close();
});

To check the options avaible, please see the github link.