2020 Solution:

When the audio metadata has not been loaded, you will get undefined or NaN (not a number). Therefore, some people recommend first using onloadedmetadata to ensure that the audio file's metadata is obtained. Also, what most people don't mention is that you have to use [0] to locate the first index of the audio DOM element, like this:

-- Native JavaScript:

var audio = document.getElementById('audio-1');
audio.onloadedmetadata = function() {
  alert(audio.duration);
};

If this doesn't work, try the following method, but this method is less reliable and relies on the user's connection:

setTimeout(function () {
    var audio = document.getElementById('audio-1');
    console.log("audio", audio.duration);
}, 100);

-- JQuery：

$(document).ready(function() {
   var audio = $("#audio-1")[0];
   $("#audio-1").on("loadedmetadata", function() {
       alert(audio.duration);
   }); 
});