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<!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>