<!DOCTYPE html>
<html>
<body>
<p>Click the button to check get the index of the first element in the array that has a value of 18 or more.</p>
<button onclick="myFunction()">Try it</button>
<p id="demo"></p>
<p><strong>Note:</strong> The findIndex() method is not supported in IE 11 (and earlier versions).</p>
<script>
var ages = [3, 10, 18, 20];
function checkAdult(age) {
return age >= 18;
}
function myFunction() {
document.getElementById("demo").innerHTML = ages.findIndex(checkAdult);
}
</script>
</body>
</html>