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How to set a variable once a day in Javascript

Coniser a variable list = ["A", "B",...] as list of strings. I want to use a Javascript programm that picks three strings from this list once a day and writes it into a HTML field.
Currently I use
function getRandom(arr, n) {
var result = new Array(n),
len = arr.length,
taken = new Array(len);
if (n > len)
throw new RangeError("getRandom: more elements taken than available");
while (n--) {
var x = Math.floor(Math.random() * len);
result[n] = arr[x in taken ? taken[x] : x];
taken[x] = --len in taken ? taken[len] : len;
}
return result;
}
smallList = getRandom(list, 3);
var htmlTags = [
"tag1",
"tag2",
"tag3"
];
for (var i = 0; i < htmlTags.length; i++) {
document.getElementById(htmlTags[i]).innerHTML = smallList[i];
}
Now this list gets new entries every time I refresh the website. Is there a way that smallList is only set once a day/hour/min/ in a pedriod of time only using javascript?

So you want to:
Pick three values from your list and show them on your web page
Each day, pick three new values to show for the whole day
Everyone who visits the page should see the same values regardless of client
As others have suggested, it would be a better candidate for a server-side task than client-side.
For example, you might have a server page which checks for the existence of a value stored in cache. The cache would be set to 24 hours. If the cache is not available, then a new cache object is created and given a half-life of 24 hours. Inside the cache, you could also store the values you wish to retrieve.
Then, you could retrieve the cache and output the values. The particular implementation of the cache would depend on your server-side language.

OKAY: Via Stored Values (COOKIE, SESSION, LOCAL STORAGE, MEMORY):
per user you'd have to use a cookie, session, or write to local storage in the browser.
for all users you'd have to use a server variable somewhere like a database, file, or memory.
you'd set the value to expire in a day and regenerate if expired. this is the answer you will get from most people because it is the only way they know how to solve this, single set value polled from all locations.
BETTER: Via Deterministic Pseudo-Random Number Generator Seeded with Date:
or if you are really ambitious and don't want to rely on a value that you set somewhere, you could use or write a:
deterministic pseudo-random number generator that you seed off of the date. Since deterministic pseudo-random generators produce reproducible "randoms" from the same seed, seeding the date gives you a unique seed per day, hence a unique random per day.
function RC4(seed) {
this.s = new Array(256);
this.i = 0;
this.j = 0;
for (var i = 0; i < 256; i++) {
this.s[i] = i;
}
if (seed) {
this.mix(seed);
}
};
RC4.getStringBytes = function(string) {
var output = [];
for (var i = 0; i < string.length; i++) {
var c = string.charCodeAt(i);
var bytes = [];
do {
bytes.push(c & 0xFF);
c = c >> 8;
} while (c > 0);
output = output.concat(bytes.reverse());
}
return output;
};
RC4.prototype._swap = function(i, j) {
var tmp = this.s[i];
this.s[i] = this.s[j];
this.s[j] = tmp;
};
RC4.prototype.mix = function(seed) {
var input = RC4.getStringBytes(seed);
var j = 0;
for (var i = 0; i < this.s.length; i++) {
j += this.s[i] + input[i % input.length];
j %= 256;
this._swap(i, j);
}
};
RC4.prototype.next = function() {
this.i = (this.i + 1) % 256;
this.j = (this.j + this.s[this.i]) % 256;
this._swap(this.i, this.j);
return this.s[(this.s[this.i] + this.s[this.j]) % 256];
};
function RNG(seed) {
if (seed == null) {
seed = '' + Math.random() + Date.now();
} else if (typeof seed === "function") {
// Use it as a uniform number generator
this.uniform = seed;
this.nextByte = function() {
return ~~(this.uniform() * 256);
};
seed = null;
} else if (Object.prototype.toString.call(seed) !== "[object String]") {
seed = JSON.stringify(seed);
}
this._normal = null;
if (seed) {
this._state = new RC4(seed);
} else {
this._state = null;
}
}
RNG.prototype.nextByte = function() {
return this._state.next();
};
RNG.prototype.uniform = function() {
var BYTES = 7; // 56 bits to make a 53-bit double
var output = 0;
for (var i = 0; i < BYTES; i++) {
output *= 256;
output += this.nextByte();
}
return output / (Math.pow(2, BYTES * 8) - 1);
};
RNG.prototype.random = function(n, m) {
if (n == null) {
return this.uniform();
} else if (m == null) {
m = n;
n = 0;
}
return n + Math.floor(this.uniform() * (m - n));
};
RNG.$ = new RNG();
Date.prototype.yyyymmdd = function() {
var mm = this.getMonth() + 1; // getMonth() is zero-based
var dd = this.getDate();
return [this.getFullYear(), !mm[1] && '0', mm, !dd[1] && '0', dd].join(''); // padding
};
// Using the Date like so will give you the same random between 40 and 50 for the same day
var rng = new RNG((new Date).yyyymmdd()); rng.random(40, 50);
// Test with dates
var rng = new RNG('20180301'); rng.random(40, 50);
var rng = new RNG('20180302'); rng.random(40, 50);
var rng = new RNG('20180301'); rng.random(40, 50);

Store the list in localStorage or a Cookie. Also store the timestamp.
Use setTimeout(function(){...}, n) to examine the timestamp and update the values as needed.
If the page refreshes or is loaded anew, then perform the check on what is stored. If nothing exists, create your list and set the timestamp. If data does exist, then compare the timestamp and update the list as needed.
If you need the list to be consistent across users, then everything needs to be stored, examined and calculated on the server-side.
localStorage.savedData = {
timestamp: new Date(),
dataList: ['a','b','c']
}
To get the values from localStorage:
// you don't have to create variables, you can just use localStorage.[property] to get compare any value
let ts = localStorage.timestamp; // Date object
let dl = localStorage.dataList; // Array of values
For more information on localStorage see (or search the web) -> https://www.w3schools.com/html/html5_webstorage.asp

Calculate unique time given multiple start and end dates

If you have an array of appointments with start and end dates how do you calculate the unique time for all of the appointments?
Example:
var appointments = {
0:{"start":"2015-01-20 09:00:00","end":"2015-01-20 09:30:00"},
1:{"start":"2015-01-20 09:15:00","end":"2015-01-20 09:42:22"},
2:{"start":"2015-01-20 10:00:00","end":"2015-01-20 10:25:00"},
3:{"start":"2015-01-20 10:10:00","end":"2015-01-20 10:53:00"}
}
So in this example I would want to get a unique time (activity) value of 1H 35M 22S.
Anyone know any formulas for this?
So far I have this, seems to work but I think dates have to be sorted by start time. Is this the most efficient way to calculate this?:
var totalElapsedAppointmentSeconds = 0;
var lastActiveTimestamp;
for (i in appointments) {
if (totalElapsedAppointmentSeconds == 0) {
totalElapsedAppointmentSeconds = new Date(appointments[i].end) - new Date(appointments[i].start);
lastActiveTimestamp = new Date(appointments[i].end);
} else {
if (new Date(appointments[i].start) < lastActiveTimestamp) {
if (new Date(appointments[i].end) > lastActiveTimestamp) {
totalElapsedAppointmentSeconds += new Date(appointments[i].end) - lastActiveTimestamp;
lastActiveTimestamp = new Date(appointments[i].end);
} else {
//nothing, already completely accounted for
}
} else {
totalElapsedAppointmentSeconds += new Date(appointments[i].end) - new Date(appointments[i].start);
lastActiveTimestamp = new Date(appointments[i].end);
}
}
}
totalElapsedAppointmentSeconds = totalElapsedAppointmentSeconds/1000;
var totalElapsedTime = Math.floor(totalElapsedAppointmentSeconds / 3600) + "H " + Math.floor((totalElapsedAppointmentSeconds % 3600)/60) + "M " + (totalElapsedAppointmentSeconds % 3600) % 60 + "S";
console.log("totalElapsedTime",totalElapsedTime);

unclear what you are asking but this demonstrates calculating a time difference
EDIT whoops javascript says these are invalid dates, where did they come from?
moment.js is a good option to parse them if you must use these as inputs
var data = {
"appointments": {
0:{"start":"2015-01-20 09:00:00","end":"2015-01-20 09:30:00"},
1:{"start":"20-01-2015 09:15:00","end":"20-01-2015 09:42:22"},
2:{"start":"20-01-2015 10:00:00","end":"20-01-2015 10:25:00"},
3:{"start":"20-01-2015 10:10:00","end":"20-01-2015 10:53:00"},
}
}
function secondsDifference(ts1, ts2){
startMs = new Date(ts1).valueOf();
endMs = new Date(ts2).valueOf();
deltaMs = endMs - startMs;
deltaS = deltaMs /1000;
deltaS = Math.floor(deltaS);
return deltaS;
}
var a = data.appointments[0];
var result = secondsDifference(a.start, a.end);
console.log('first appointment length seconds:', result)

How can I use JS WebAudioAPI for beat detection?

I'm interested in using the JavaScript WebAudioAPI to detect song beats, and then render them in a canvas.
I can handle the canvas part, but I'm not a big audio guy and really don't understand how to make a beat detector in JavaScript.
I've tried following this article but cannot, for the life of me, connect the dots between each function to make a functional program.
I know I should show you some code but honestly I don't have any, all my attempts have failed miserably and the relevant code it's in the previously mentioned article.
Anyways I'd really appreciate some guidance, or even better a demo of how to actually detect song beats with the WebAudioAPI.
Thanks!

The main thing to understand about the referenced article by Joe Sullivan is that even though it gives a lot of source code, it's far from final and complete code. To reach a working solution you will still need both some coding and debugging skills.
This answer draws most of its code from the referenced article, original licensing applies where appropriate.
Below is a naïve sample implementation for using the functions described by the above article, you still need to figure out correct thresholds for a functional solution.
The code consists of preparation code written for the answer:
reading a local file over the FileReader API
decoding the file as audio data using the AudioContext API
and then, as described in the article:
filtering the audio, in this example with a low-pass filter
calculating peaks using a threshold
grouping interval counts and then tempo counts
For the threshold I used an arbitrary value of .98 of the range between maximum and minimum values; when grouping I added some additional checks and arbitrary rounding to avoid possible infinite loops and make it an easy-to-debug sample.
Note that commenting is scarce to keep the sample implementation brief because:
the logic behind processing is explained in the referenced article
the syntax can be referenced in the API docs of the related methods
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>

I wrote a tutorial here which shows how to do this with the javascript Web Audio API.
https://askmacgyver.com/blog/tutorial/how-to-implement-tempo-detection-in-your-application
Outline of Steps
Transform Audio File into an Array Buffer
Run Array Buffer Through Low Pass Filter
Trim a 10 second Clip from the Array Buffer
Down Sample the Data
Normalize the Data
Count Volume Groupings
Infer Tempo from Groupings Count
This code below does the heavy lifting.
Load Audio File Into Array Buffer and Run Through Low Pass Filter
function createBuffers(url) {
// Fetch Audio Track via AJAX with URL
request = new XMLHttpRequest();
request.open('GET', url, true);
request.responseType = 'arraybuffer';
request.onload = function(ajaxResponseBuffer) {
// Create and Save Original Buffer Audio Context in 'originalBuffer'
var audioCtx = new AudioContext();
var songLength = ajaxResponseBuffer.total;
// Arguments: Channels, Length, Sample Rate
var offlineCtx = new OfflineAudioContext(1, songLength, 44100);
source = offlineCtx.createBufferSource();
var audioData = request.response;
audioCtx.decodeAudioData(audioData, function(buffer) {
window.originalBuffer = buffer.getChannelData(0);
var source = offlineCtx.createBufferSource();
source.buffer = buffer;
// Create a Low Pass Filter to Isolate Low End Beat
var filter = offlineCtx.createBiquadFilter();
filter.type = "lowpass";
filter.frequency.value = 140;
source.connect(filter);
filter.connect(offlineCtx.destination);
// Render this low pass filter data to new Audio Context and Save in 'lowPassBuffer'
offlineCtx.startRendering().then(function(lowPassAudioBuffer) {
var audioCtx = new(window.AudioContext || window.webkitAudioContext)();
var song = audioCtx.createBufferSource();
song.buffer = lowPassAudioBuffer;
song.connect(audioCtx.destination);
// Save lowPassBuffer in Global Array
window.lowPassBuffer = song.buffer.getChannelData(0);
console.log("Low Pass Buffer Rendered!");
});
},
function(e) {});
}
request.send();
}
createBuffers('https://askmacgyver.com/test/Maroon5-Moves-Like-Jagger-128bpm.mp3');
You Now Have an Array Buffer of the Low Pass Filtered Song (And Original)
It's comprised of a number of entries, sampleRate (44100 multiplied by the number of seconds of the song).
window.lowPassBuffer // Low Pass Array Buffer
window.originalBuffer // Original Non Filtered Array Buffer
Trim a 10 Second Clip from the Song
function getClip(length, startTime, data) {
var clip_length = length * 44100;
var section = startTime * 44100;
var newArr = [];
for (var i = 0; i < clip_length; i++) {
newArr.push(data[section + i]);
}
return newArr;
}
// Overwrite our array buffer to a 10 second clip starting from 00:10s
window.lowPassFilter = getClip(10, 10, lowPassFilter);
Down Sample Your Clip
function getSampleClip(data, samples) {
var newArray = [];
var modulus_coefficient = Math.round(data.length / samples);
for (var i = 0; i < data.length; i++) {
if (i % modulus_coefficient == 0) {
newArray.push(data[i]);
}
}
return newArray;
}
// Overwrite our array to down-sampled array.
lowPassBuffer = getSampleClip(lowPassFilter, 300);
Normalize Your Data
function normalizeArray(data) {
var newArray = [];
for (var i = 0; i < data.length; i++) {
newArray.push(Math.abs(Math.round((data[i + 1] - data[i]) * 1000)));
}
return newArray;
}
// Overwrite our array to the normalized array
lowPassBuffer = normalizeArray(lowPassBuffer);
Count the Flat Line Groupings
function countFlatLineGroupings(data) {
var groupings = 0;
var newArray = normalizeArray(data);
function getMax(a) {
var m = -Infinity,
i = 0,
n = a.length;
for (; i != n; ++i) {
if (a[i] > m) {
m = a[i];
}
}
return m;
}
function getMin(a) {
var m = Infinity,
i = 0,
n = a.length;
for (; i != n; ++i) {
if (a[i] < m) {
m = a[i];
}
}
return m;
}
var max = getMax(newArray);
var min = getMin(newArray);
var count = 0;
var threshold = Math.round((max - min) * 0.2);
for (var i = 0; i < newArray.length; i++) {
if (newArray[i] > threshold && newArray[i + 1] < threshold && newArray[i + 2] < threshold && newArray[i + 3] < threshold && newArray[i + 6] < threshold) {
count++;
}
}
return count;
}
// Count the Groupings
countFlatLineGroupings(lowPassBuffer);
Scale 10 Second Grouping Count to 60 Seconds to Derive Beats Per Minute
var final_tempo = countFlatLineGroupings(lowPassBuffer);
// final_tempo will be 21
final_tempo = final_tempo * 6;
console.log("Tempo: " + final_tempo);
// final_tempo will be 126

Record and "playback" css 3d transforms

I'm trying to record a user's mouse and key inputs, then "play them back" to the user by triggering events at chronologically the same time they occurred. That sentence is a little confusing so I'll explain my code:
var $dancerContainer = $('.dancerContainer');
var count = 3;
var captured;
var countInterval;
$dancerContainer is the element I want to animate, which is a div.
count is the duration of the 'recording' phase.
captured will eventually be an object that holds the events, keyed by the elapsed time in milliseconds they occurred since the start of the recording.
function captureInput() {
var mouseCapture = [];
var keyCapture = [];
var start = new Date().getTime();
$(document).on('mousemove.capture', function(event) {
event.t = new Date().getTime() - start;
mouseCapture.push(event);
});
$(document).on('keyup.capture', function(event) {
event.t = new Date().getTime() - start;
keyCapture.push(event);
});
setTimeout(function() {
$(document).off('.capture');
captured = chronoCaptures(mouseCapture, keyCapture);
}, 3000);
}
The captureInput func tags an elapsed time on the event before pushing them to mouse and keyup arrays, mouseCapture and keyCapture, then unbinds the listeners.
function chronoCaptures(mouse, keyboard) {
var greater = (mouse.length > keyboard.length) ? mouse.length : keyboard.length;
var chrono = {};
var j = 0;
var k = 0;
for (var i = 0; i < greater; i++) {
if (keyboard[k] == undefined) {
chrono[mouse[j].t] = mouse[j];
j++;
} else if (mouse[j] == undefined) {
chrono[keyboard[k].t] = keyboard[k];
k++;
} else {
if (mouse[j].t < keyboard[k].t) {
chrono[mouse[j].t] = mouse[j];
j++;
} else {
chrono[keyboard[k].t] = keyboard[k];
k++;
}
}
}
return chrono;
}
chronoOrderCaptures then takes the two arrays of events and returns an object whose keys are the times each event occurred. Now that I look at this code again, since I'm putting the events into an object anyway, it doesn't matter what order they get put in. I might overwrite an event with another, in which case I want the key event to take precedence (going to refactor this, but this is beside the point).
function replayDance(captured, duration) {
var d = duration * 1000;
var start = new Date().getTime();
var elapsed = 0;
while (elapsed <= d) {
elapsed = new Date().getTime() - start;
var c = captured[elapsed];
if (c) {
$dancerContainer.trigger(c)
}
}
}
Finally, replayDance waits for the duration of the recording and checks how much time has elapsed. If the captured object contains an entry # that amount of elapsed time, I trigger the event on the document.
WHEW. Thank you if you've gotten this far. Now to the problem!! (what?) The problem I'm having is the mouse events all get played back 'at once'. There's no pausing occurring, I don't see them executed in sequence as if I was actually moving the mouse, although it seems like they ought to be getting triggered at roughly the same time they were recorded.
Finally here is the handler for mousemove events:
$(document).on('mousemove', followMouse);
function followMouse(event) {
var width = $(window).width();
var height = $(window).height();
var mouseX = event.pageX - (width * 0.25);
var mouseY = event.pageY - (height * 0.25);
var angleX = (mouseY / height) * 45;
var angleY = (mouseX / width) * 45;
dancer.style.webkitTransform = "rotateX(" + angleX + "deg) rotateY(" + angleY + "deg)";
}

Optimizing a functions performance using a loops wall-time

I use the following function below to update and call a drawAccel();function that builds out an animated strip chart.
function messagecb(header, message) {
if(header.type == 6) {
// processEchoReply(message);
} else if(header.type == 4) {
// accel
var accels = message.b64UnpackAccelMsg();
for(var index = 0; index < accels.length; ++index) {
var accel = accels[index];
var totalClock = accelEpochAdjust(accel.clock);
addAccelDatum(totalClock, accel.x, accel.y, accel.z);
}
if ( typeof messagecb.counter == 'undefined' ) {
messagecb.counter = 0;
}
++messagecb.counter;
if (messagecb.counter % 10 === 0) {
drawAccel();
}
} else if(header.type == 3) {
// info
var info2 = message.b64UnpackInfo2Msg();
displayCurrentPosition(info2.fixtime, info2.lat, info2.lon, info2.alt);
displayMobileStatus(info2.rssi, info2.bandClass, info2.batt);
} else if(header.type == 11) {
btReceive(header, message);
}
}
I come across some intermittent performance issues in IE8 though. So I would like to
collect the elapsed wall time running inside the update for loop, and not call the drawAccel() renderer unless I'm using less than 50% of the wall time.
Pseudo code example:
if ((lastEnteredTime - lastExitedTime)/(currentTime - lastEnteredTime) < .5){
drawAccel();
} else {
//do nothing
}
My problem is I'm not sure how I can go about getting the last entered time and the last exited time of the loop so that I can run this condition. Any ideas? Thanks!

It's not clear to me exactly what you're trying to do, but something like this should get you close. +new Date() will give you the number of milliseconds since 1/1/1970, so making that same call at various places should be able to get you what you want
var start = +new Date();
for(var index = 0; index < accels.length; ++index) {
var accel = accels[index];
var totalClock = accelEpochAdjust(accel.clock);
var current = +new Date();
var timeElapsedInMs = current - start;
//not sure the exact test you want to run here
addAccelDatum(totalClock, accel.x, accel.y, accel.z);
}
Edit based on your comment. So if you always want to have a lastEntered and lastExited values, something like this might be what you want
var lastEntered, lastExisted = +new Date();
for(var index = 0; index < accels.length; ++index) {
lastEntered = +new Date();
var accel = accels[index];
var totalClock = accelEpochAdjust(accel.clock);
var timeElapsedInMs = current - start;
//not sure the exact test you want to run here
addAccelDatum(totalClock, accel.x, accel.y, accel.z);
lastExisted = +new Date();
}
And from there you can do whatever comparisons you need.