audio_file.onchange = function() {

  var file = this.files[0];

  var reader = new FileReader();

  var context = new(window.AudioContext || window.webkitAudioContext)();

  reader.onload = function() {

    context.decodeAudioData(reader.result, function(buffer) {

      prepare(buffer);

    });

  };

  reader.readAsArrayBuffer(file);

};



function prepare(buffer) {

  var offlineContext = new OfflineAudioContext(1, buffer.length, buffer.sampleRate);

  var source = offlineContext.createBufferSource();

  source.buffer = buffer;

  var filter = offlineContext.createBiquadFilter();

  filter.type = lowpass;

  source.connect(filter);

  filter.connect(offlineContext.destination);

  source.start(0);

  offlineContext.startRendering();

  offlineContext.oncomplete = function(e) {

    process(e);

  };

}



function process(e) {

  var filteredBuffer = e.renderedBuffer;

  //If you want to analyze both channels, use the other channel later

  var data = filteredBuffer.getChannelData(0);

  var max = arrayMax(data);

  var min = arrayMin(data);

  var threshold = min + (max - min) * 0.98;

  var peaks = getPeaksAtThreshold(data, threshold);

  var intervalCounts = countIntervalsBetweenNearbyPeaks(peaks);

  var tempoCounts = groupNeighborsByTempo(intervalCounts);

  tempoCounts.sort(function(a, b) {

    return b.count - a.count;

  });

  if (tempoCounts.length) {

    output.innerHTML = tempoCounts[0].tempo;

  }

}



// http://tech.beatport.com/2014/web-audio/beat-detection-using-web-audio/

function getPeaksAtThreshold(data, threshold) {

  var peaksArray = [];

  var length = data.length;

  for (var i = 0; i < length;) {

    if (data[i] > threshold) {

      peaksArray.push(i);

      // Skip forward ~ 1/4s to get past this peak.

      i += 10000;

    }

    i++;

  }

  return peaksArray;

}



function countIntervalsBetweenNearbyPeaks(peaks) {

  var intervalCounts = [];

  peaks.forEach(function(peak, index) {

    for (var i = 0; i < 10; i++) {

      var interval = peaks[index + i] - peak;

      var foundInterval = intervalCounts.some(function(intervalCount) {

        if (intervalCount.interval === interval) return intervalCount.count++;

      });

      //Additional checks to avoid infinite loops in later processing

      if (!isNaN(interval) && interval !== 0 && !foundInterval) {

        intervalCounts.push({

          interval: interval,

          count: 1

        });

      }

    }

  });

  return intervalCounts;

}



function groupNeighborsByTempo(intervalCounts) {

  var tempoCounts = [];

  intervalCounts.forEach(function(intervalCount) {

    //Convert an interval to tempo

    var theoreticalTempo = 60 / (intervalCount.interval / 44100);

    theoreticalTempo = Math.round(theoreticalTempo);

    if (theoreticalTempo === 0) {

      return;

    }

    // Adjust the tempo to fit within the 90-180 BPM range

    while (theoreticalTempo < 90) theoreticalTempo *= 2;

    while (theoreticalTempo > 180) theoreticalTempo /= 2;



    var foundTempo = tempoCounts.some(function(tempoCount) {

      if (tempoCount.tempo === theoreticalTempo) return tempoCount.count += intervalCount.count;

    });

    if (!foundTempo) {

      tempoCounts.push({

        tempo: theoreticalTempo,

        count: intervalCount.count

      });

    }

  });

  return tempoCounts;

}



// http://stackoverflow.com/questions/1669190/javascript-min-max-array-values

function arrayMin(arr) {

  var len = arr.length,

    min = Infinity;

  while (len--) {

    if (arr[len] < min) {

      min = arr[len];

    }

  }

  return min;

}



function arrayMax(arr) {

  var len = arr.length,

    max = -Infinity;

  while (len--) {

    if (arr[len] > max) {

      max = arr[len];

    }

  }

  return max;

}

<input id=audio_file type=file accept=audio/*></input>

<audio id=audio_player></audio>

<p>

  Most likely tempo: <span id=output></span>

</p>