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Resolve the promise only if xhr.onloadend has fired before

I want to assign videoLoaded to true right after myVideo.mp4 is fully loaded. I can do this at the last lines of the code (This is our promise):
preload.fetch([
clipSource
]).then(items => {
// Using a promise it'll fire when we are sure that video clip has finished loading completely
videoLoaded = true;
});
The first issue is if our URL is not valid we get a 404 response status code. the 404 itself is a valid response so we will not trigger xhr.onerror() because technically it's not an error.
we can track 404 status using:
xhr.onloadend = function() {
if(xhr.status == 404) { // do something }
}
The issue is onloadend event fired only after the promise .then(items => { .... so if there is not a valid URL we can not prevent the promise to resolve and videoLoaded will be assigned to true although there is not a valid URL...
I want to resolve the promise and assign videoLoaded to true only if xhr.status !== 404 in this situation we can be sure that we have a valid URL.
Here is the code (I have used a setInterval and it works but I think there are cleaner solutions that you can share):
let onLoadPassed = false;
let videoLoaded = false;
let clipSource = 'https://mysite/myVideo.mp4';
preload();
// Make sure the video clip is fully loaded
function preload(){
(function (global, factory) {
typeof exports === 'object' && typeof module !== 'undefined' ? module.exports = factory() :
typeof define === 'function' && define.amd ? define(factory) :
(global.Preload = factory());
}(this, (function () { 'use strict';
function preloadOne(url, done) {
const xhr = new XMLHttpRequest();
xhr.open('GET', url, true);
xhr.responseType = 'blob';
xhr.onprogress = event => {
if (!event.lengthComputable) return false
let item = this.getItemByUrl(event.target.responseURL);
item.completion = parseInt((event.loaded / event.total) * 100);
item.downloaded = event.loaded;
item.total = event.total;
this.updateProgressBar(item);
};
xhr.onload = event => {
let type = event.target.response.type;
let blob = new Blob([event.target.response], { type: type });
let url = URL.createObjectURL(blob);
let responseURL = event.target.responseURL;
let item = this.getItemByUrl(responseURL);
item.blobUrl = url;
item.fileName = responseURL.substring(responseURL.lastIndexOf('/') + 1);
item.type = type;
item.size = blob.size;
done(item);
};
xhr.onerror = event => {
console.log('Error has happend so we restart the video preloading..');
preload();
};
xhr.onloadend = function() {
if(xhr.status == 404){
console.log('404 not found');
onLoadPassed = false;
} else {
console.log('File exist');
onLoadPassed = true;
}
}
xhr.send();
}
function updateProgressBar(item) {
var sumCompletion = 0;
var maxCompletion = this.status.length * 100;
for (var itemStatus of this.status) {
if (itemStatus.completion) {
sumCompletion += itemStatus.completion;
}
}
var totalCompletion = parseInt((sumCompletion / maxCompletion) * 100);
if (!isNaN(totalCompletion)) {
this.onprogress({
progress: totalCompletion,
item: item
});
}
}
function getItemByUrl(rawUrl) {
for (var item of this.status) {
if (item.url == rawUrl) return item
}
}
function fetch(list) {
return new Promise((resolve, reject) => {
this.loaded = list.length;
for (let item of list) {
this.status.push({ url: item });
this.preloadOne(item, item => {
this.onfetched(item);
this.loaded--;
if (this.loaded == 0) {
this.oncomplete(this.status);
resolve(this.status);
}
});
}
})
}
function Preload() {
return {
status: [],
loaded: false,
onprogress: () => {},
oncomplete: () => {},
onfetched: () => {},
fetch,
updateProgressBar,
preloadOne,
getItemByUrl
}
}
return Preload;
})));
const preload = Preload();
preload.fetch([
clipSource
]).then(items => {
// Fired when we are sure that video clip has finished loading completely
let check = setInterval(passedFunc, 50);
function passedFunc() {
if(onLoadPassed === true){
videoLoaded = true;
clearInterval(check);
console.log('videoLoaded: ' + videoLoaded);
};
}
});
};

You can intercept the promise and throw an error if the status code is 404, this way the subsequent .then statements will be ignored and the result will be captured by the .catch statement.
preload.fetch([
clipSource
])
.then(response => {
if(!response.ok) //better to use response.ok as it checks a range of status codes
throw Error(response.statusText);
return response;
})
.then(items => {
// Using a promise it'll fire when we are sure that video clip has finished loading completely
videoLoaded = true;
})
.catch(error => {
//do something
console.log(error)
});

Time limit for code execution

Considering the following execution of a JS program in a JS settlement, how can I implement a time limit to prevent infinite loops in program?
try {
var x = eval(document.getElementById("program").value);
} catch(e) {
...
}
Note: the call should be able to specify a maximum execution time, just in case program enters an infinite loop.

You could use a webworker to run this in another thread:
// Worker-helper.js
self.onmessage = function(e) {
self.postMessage(eval(e.data));
};
Them use the worker as:
var worker = new Worker('Worker-helper.js');
worker.postMessage(document.getElementById("program").value);
worker.onmessage = result => {
alert(result);
};
setTimeout(() => worker.terminate(), 10 * 1000);
...which will kill the worker after 10 seconds.
Easy to use utility:
function funToWorker(fn) {
var response = "(" + fn.toString() + ")()";
var blob;
try {
blob = new Blob([response], {type: 'application/javascript'});
} catch (e) { // Backwards-compatibility
window.BlobBuilder = window.BlobBuilder || window.WebKitBlobBuilder || window.MozBlobBuilder;
blob = new BlobBuilder();
blob.append(response);
blob = blob.getBlob();
}
return new Worker(URL.createObjectURL(blob));
}
function limitedEval(string, timeout) {
return new Promise((resolve, reject) => {
const worker = funToWorker(function() {
self.onmessage = e => {
self.postMessage(eval(e.data));
}
});
worker.onmessage = e => resolve(e.data);
worker.postMessage(string);
setTimeout(() => {
worker.terminate();
reject();
}, timeout);
});
}
Usable as:
limitedEval("1 + 2", 1000)
.then(console.log)
.catch(console.error);
Try it!

Client PNG compression using javascript like pngcrush?

I'm developing HTML5 apps.
When user uploads image from their mobile, the size was too large.
I want to compress the image as PNG like the pngcrush way.
Is there any good way to choice on the frontend (like a javascript library)?
Or is it possible to port the pngcrush library to javascript?

There are a few projects out there which seem to be based around the idea of using emscripten (a LLVM-to-JavaScript compiler) to actually compile the source code from pngcrush to working JavaScript for the browser.
JavaScript-Packer/PNGCrush.html - based on pngcrush-1.7.27
richardassar/pngcrush.js - based on pngcrush-1.7.27
pngcrush-crushed - based on pngcrush-1.7.58
The version for pngcrush-1.7.27 is currently the only one that doesn't seem to produce corrupted images for me. I put together an example which uses promises here: http://plnkr.co/edit/iLpbOjlYiacR04oGdXSI?p=preview
Here's a basic usage example:
var instance = new pngcrush();
instance.exec(inputFile, function (stdoutEvent) {
console.log(stdoutEvent.data.line);
}).then(function (doneEvent) {
var outputFile = new Blob([doneEvent.data.data], { type: 'image/png' });
// do something with the outputFile
});
Here are the contents of the pngcrush-class.js file from the above plunker for reference:
(function(exports) {
var noop = function () {};
function pngcrush () {
this.callbacks = {
'error': [],
'done': [],
'start': [],
'stdout': []
};
}
pngcrush.prototype.exec = function (file, notify) {
var self = this;
if (this.execPromise) {
return this.execPromise.catch(noop).then(function () {
return self.exec(file, notify);
});
}
if (file.type !== 'image/png') {
return Promise.reject(file);
}
var promise = this.execPromise = this.readAsArrayBuffer(file).then(function (event) {
var arrayBuffer = event.target.result;
return self.initWorker().then(function (worker) {
var done = new Promise(function (resolve, reject) {
var offDone, offError, offStdout;
offDone = self.once('done', function (event) {
offError();
offStdout();
resolve(event);
});
offError = self.once('error', function (event) {
offDone();
offStdout();
reject(event);
});
offStdout = self.on('stdout', function (event) {
if (typeof notify === 'function') {
notify.call(self, event);
}
});
worker.postMessage({
'type': 'file',
'data': new Uint8Array(arrayBuffer)
});
worker.postMessage({
'type': 'command',
'command': 'go'
});
});
done.catch(noop).then(function () {
worker.terminate();
});
return done;
});
});
promise.catch(noop).then(function () {
if (promise === self.execPromise) {
delete self.execPromise;
}
});
return promise;
};
pngcrush.prototype.initWorker = function () {
var self = this;
if (this.workerPromise) {
return this.workerPromise;
}
var promise = this.workerPromise = new Promise(function (resolve, reject) {
var worker = new Worker('worker.js');
worker.onerror = function (event) {
var callbacks = [];
reject(event);
Array.prototype.push.apply(callbacks, self.callbacks.error);
while (callbacks.length) {
callbacks.shift().call(self, event);
}
};
worker.onmessage = function (event) {
if (event.data.type === 'ready') {
worker.onmessage = function (event) {
var name = event.data.type;
if (typeof self.callbacks[name] !== 'undefined') {
var callbacks = [];
Array.prototype.push.apply(callbacks, self.callbacks[name]);
while (callbacks.length) {
callbacks.shift().call(self, event);
}
}
};
resolve(worker);
}
};
});
promise.catch(noop).then(function () {
if (promise === self.workerPromise) {
delete self.workerPromise;
}
});
return promise;
};
pngcrush.prototype.on = function (name, callback) {
var self = this;
if (typeof this.callbacks[name] !== 'undefined' && typeof callback === 'function') {
this.callbacks[name].push(callback);
var off = (function () {
var ran = false;
return function () {
if (ran === true) {
return;
}
ran = true;
var idx = self.callbacks[name].lastIndexOf(callback);
if (idx !== -1) {
self.callbacks[name].splice(idx - 1, 1);
}
};
})();
return off;
}
return noop;
};
pngcrush.prototype.once = function (name, callback) {
var off = this.on(name, function () {
off();
callback.apply(this, arguments);
});
return off;
};
pngcrush.prototype.readAsArrayBuffer = function (file) {
var fileReader = new FileReader();
return new Promise(function (resolve, reject) {
fileReader.onerror = reject;
fileReader.onload = resolve;
fileReader.readAsArrayBuffer(file);
});
};
pngcrush.prototype.readAsDataURL = function (file) {
var fileReader = new FileReader();
return new Promise(function (resolve, reject) {
fileReader.onerror = reject;
fileReader.onload = resolve;
fileReader.readAsDataURL(file);
});
};
exports.pngcrush = pngcrush;
})(this);

Resolve promise when all images are loaded

I was preloading images with the following code:
function preLoad() {
var deferred = $q.defer();
var imageArray = [];
for (var i = 0; i < $scope.abbreviations.length; i++) {
imageArray[i] = new Image();
imageArray[i].src = $scope.abbreviations[i].imgPath;
}
imageArray.forEach.onload = function () {
deferred.resolve();
console.log('Resolved');
}
imageArray.forEach.onerror = function () {
deferred.reject();
console.log('Rejected')
}
return deferred.promise;
}
preLoad();
I thought images were all loading correctly because I could see the 'Resolved' log.
Later somebody pointed out that the code above doesn't guarantee that all images are loaded before resolving the promise. In fact, only the first promise is resolved.
I was advised to use $q.all applied to an array of promises instead.
This is the resulting code:
function preLoad() {
var imageArray = [];
var promises;
for (var i = 0; i < $scope.abbreviations.length; i++) {
imageArray[i] = new Image();
imageArray[i].src = $scope.abbreviations[i].imgPath;
};
function resolvePromises(n) {
return $q.when(n);
}
promises = imageArray.map(resolvePromises);
$q.all(promises).then(function (results) {
console.log('array promises resolved with', results);
});
}
preLoad();
This works, but I want to understand:
what's happening in each function;
why I need $q.all to make sure all images are loaded before resolving the promises.
The relevant docs are somewhat cryptic.

Check out this plunkr.
Your function:
function preLoad() {
var promises = [];
function loadImage(src) {
return $q(function(resolve,reject) {
var image = new Image();
image.src = src;
image.onload = function() {
console.log("loaded image: "+src);
resolve(image);
};
image.onerror = function(e) {
reject(e);
};
})
}
$scope.images.forEach(function(src) {
promises.push(loadImage(src));
})
return $q.all(promises).then(function(results) {
console.log('promises array all resolved');
$scope.results = results;
return results;
});
}
The idea is very similar to Henrique's answer, but the onload handler is used to resolve each promise, and onerror is used to reject each promise.
To answer your questions:
1) Promise factory
$q(function(resolve,reject) { ... })
constructs a Promise. Whatever is passed to the resolve function will be used in the then function. For example:
$q(function(resolve,reject) {
if (Math.floor(Math.random() * 10) > 4) {
resolve("success")
}
else {
reject("failure")
}
}.then(function wasResolved(result) {
console.log(result) // "success"
}, function wasRejected(error) {
console.log(error) // "failure"
})
2) $q.all is passed an array of promises, then takes a function which is passed an array with the resolutions of all the original promises.

I'm not used to angular promise library, but the idea is as follows:
function getImagePromise(imgData) {
var imgEl = new Image();
imgEl.src = imgData.imgPath;
return $q(function(resolve, reject){
imgEl.addEventListener('load', function(){
if ((
'naturalHeight' in this
&& this.naturalHeight + this.naturalWidth === 0
)
|| (this.width + this.height == 0)) {
reject(new Error('Image not loaded:' + this.src));
} else {
resolve(this);
}
});
imgEl.addEventListener('error', function(){
reject(new Error('Image not loaded:' + this.src));
});
})
}
function preLoad() {
return $q.all($scope.abbreviations.map(getImagePromise));
}
// using
preLoad().then(function(data){
console.log("Loaded successfully");
data.map(console.log, console);
}, function(reason){
console.error("Error loading: " + reason);
});

Wait until a condition is true?

I'm using navigator.geolocation.watchPosition in JavaScript, and I want a way to deal with the possibility that the user might submit a form relying on location before watchPosition has found its location.
Ideally the user would see a 'Waiting for location' message periodically until the location was obtained, then the form would submit.
However, I'm not sure how to implement this in JavaScript given its lack of a wait function.
Current code:
var current_latlng = null;
function gpsSuccess(pos){
//console.log('gpsSuccess');
if (pos.coords) {
lat = pos.coords.latitude;
lng = pos.coords.longitude;
}
else {
lat = pos.latitude;
lng = pos.longitude;
}
current_latlng = new google.maps.LatLng(lat, lng);
}
watchId = navigator.geolocation.watchPosition(gpsSuccess,
gpsFail, {timeout:5000, maximumAge: 300000});
$('#route-form').submit(function(event) {
// User submits form, we need their location...
while(current_location==null) {
toastMessage('Waiting for your location...');
wait(500); // What should I use instead?
}
// Continue with location found...
});

Modern solution using Promise
function waitFor(conditionFunction) {
const poll = resolve => {
if(conditionFunction()) resolve();
else setTimeout(_ => poll(resolve), 400);
}
return new Promise(poll);
}
Usage
waitFor(_ => flag === true)
.then(_ => console.log('the wait is over!'));
or
async function demo() {
await waitFor(_ => flag === true);
console.log('the wait is over!');
}
References
Promises
Arrow Functions
Async/Await

Personally, I use a waitfor() function which encapsulates a setTimeout():
//**********************************************************************
// function waitfor - Wait until a condition is met
//
// Needed parameters:
// test: function that returns a value
// expectedValue: the value of the test function we are waiting for
// msec: delay between the calls to test
// callback: function to execute when the condition is met
// Parameters for debugging:
// count: used to count the loops
// source: a string to specify an ID, a message, etc
//**********************************************************************
function waitfor(test, expectedValue, msec, count, source, callback) {
// Check if condition met. If not, re-check later (msec).
while (test() !== expectedValue) {
count++;
setTimeout(function() {
waitfor(test, expectedValue, msec, count, source, callback);
}, msec);
return;
}
// Condition finally met. callback() can be executed.
console.log(source + ': ' + test() + ', expected: ' + expectedValue + ', ' + count + ' loops.');
callback();
}
I use my waitfor() function in the following way:
var _TIMEOUT = 50; // waitfor test rate [msec]
var bBusy = true; // Busy flag (will be changed somewhere else in the code)
...
// Test a flag
function _isBusy() {
return bBusy;
}
...
// Wait until idle (busy must be false)
waitfor(_isBusy, false, _TIMEOUT, 0, 'play->busy false', function() {
alert('The show can resume !');
});

This is precisely what promises were invented and implemented (since OP asked his question) for.
See all of the various implementations, eg promisejs.org

You'll want to use setTimeout:
function checkAndSubmit(form) {
var location = getLocation();
if (!location) {
setTimeout(checkAndSubmit, 500, form); // setTimeout(func, timeMS, params...)
} else {
// Set location on form here if it isn't in getLocation()
form.submit();
}
}
... where getLocation looks up your location.

You could use a timeout to try to re-submit the form:
$('#route-form').submit(function(event) {
// User submits form, we need their location...
if(current_location==null) {
toastMessage('Waiting for your location...');
setTimeout(function(){ $('#route-form').submit(); }, 500); // Try to submit form after timeout
return false;
} else {
// Continue with location found...
}
});

export default (condition: Function, interval = 1000) =>
new Promise((resolve) => {
const runner = () => {
const timeout = setTimeout(runner, interval);
if (condition()) {
clearTimeout(timeout);
resolve(undefined);
return;
}
};
runner();
});

class App extends React.Component {
componentDidMount() {
this.processToken();
}
processToken = () => {
try {
const params = querySearch(this.props.location.search);
if('accessToken' in params){
this.setOrderContext(params);
this.props.history.push(`/myinfo`);
}
} catch(ex) {
console.log(ex);
}
}
setOrderContext (params){
//this action calls a reducer and put the token in session storage
this.props.userActions.processUserToken({data: {accessToken:params.accessToken}});
}
render() {
return (
<Switch>
//myinfo component needs accessToken to retrieve my info
<Route path="/myInfo" component={InofUI.App} />
</Switch>
);
}
And then inside InofUI.App
componentDidMount() {
this.retrieveMyInfo();
}
retrieveMyInfo = async () => {
await this.untilTokenIsSet();
const { location, history } = this.props;
this.props.processUser(location, history);
}
untilTokenIsSet= () => {
const poll = (resolve) => {
const { user } = this.props;
const { accessToken } = user;
console.log('getting accessToken', accessToken);
if (accessToken) {
resolve();
} else {
console.log('wating for token .. ');
setTimeout(() => poll(resolve), 100);
}
};
return new Promise(poll);
}

Try using setInterval and clearInterval like this...
var current_latlng = null;
function gpsSuccess(pos) {
//console.log('gpsSuccess');
if (pos.coords) {
lat = pos.coords.latitude;
lng = pos.coords.longitude;
} else {
lat = pos.latitude;
lng = pos.longitude;
}
current_latlng = new google.maps.LatLng(lat, lng);
}
watchId = navigator.geolocation.watchPosition(gpsSuccess,
gpsFail, {
timeout: 5000,
maximumAge: 300000
});
$('#route-form').submit(function (event) {
// User submits form, we need their location...
// Checks status every half-second
var watch = setInterval(task, 500)
function task() {
if (current_latlng != null) {
clearInterval(watch)
watch = false
return callback()
} else {
toastMessage('Waiting for your location...');
}
}
function callback() {
// Continue on with location found...
}
});

This accepts any function, even if it's async, and when it evaluates to a truthy value (checking every quarter-second by default), resolves to it.
function waitFor(condition, step = 250, timeout = Infinity) {
return new Promise((resolve, reject) => {
const now = Date.now();
let running = false;
const interval = setInterval(async () => {
if (running) return;
running = true;
const result = await condition();
if (result) {
clearInterval(interval);
resolve(result);
} else if (Date.now() - now >= timeout * 1000) {
clearInterval(interval);
reject(result);
}
running = false;
}, step);
});
}
Example
example();
async function example() {
let foo = 'bar';
setTimeout(() => foo = null, 2000);
console.log(`foo === "${foo}"`);
await waitFor(() => foo === null);
console.log('2 seconds have elapsed.');
console.log(`foo === ${foo}`);
}
function waitFor(condition, step = 250, timeout = Infinity) {
return new Promise((resolve, reject) => {
const now = Date.now();
let running = false;
const interval = setInterval(async () => {
if (running) return;
running = true;
const result = await condition();
if (result) {
clearInterval(interval);
resolve(result);
} else if (Date.now() - now >= timeout * 1000) {
clearInterval(interval);
reject(result);
}
running = false;
}, step);
});
}