function changeBackGroundOfPs(id) {
var paragraphs = document.querySelectorAll("#" + id + " p");
// Another way to iterate on all elements in a collection
for (var i = 0; i < paragraphs.length; i++ ) {
paragraphs[i].style.backgroundColor = "lightGreen";
}
}
JS.DOM.AccessHtml.QuerySelectorAll.Ex1
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[License](https://codepen.io/Onlyforbopi/pen/vzwYLE/license).
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width">
<title>Change background color of p under an element whose id is known</title>
</head>
<body>
<button onclick="changeBackGroundOfPs('firstDiv');">Change backgrounds of p under a given element known by id</button>
<br>
<div id="firstDiv">
<p>First paragraph.</p>
<p>Second paragraph.</p>
</div>
</body>
</html>