Develop Reference

Javascript - element.addEventListener() returns "Uncaught TypeError"

For context, I am trying to code a memory game where you have to pair two of the same colored circles until the whole board is complete. I've called it Match-Two. Here is the code that I'll reference from:
class Circle {
constructor(element, circleColor){
this.elem = element;
this.color = circleColor;
}
}
var frequency = [0, 0, 0, 0, 0, 0, 0, 0];
var num;
var hue = new Array(8);
var circle = new Array(16);
hue[0] = "#0039ff";
hue[1] = "#ff0000";
hue[2] = "#43ff00";
hue[3] = "#fffa00";
hue[4] = "#7405b5";
hue[5] = "#ff9d00";
hue[6] = "#ff00c3";
hue[7] = "#00fff6";
onload = function() {
for(var i = 0; i < 16; i++){
circle[i] = new Circle(document.getElementById("circle" + i));
while(circle[i].color === undefined){
num = Math.floor(Math.random() * 8);
if(frequency[num] != 2){
frequency[num]++;
circle[i].color = hue[num];
circle[i].elem.addEventListener('click', function(){
main(circle[i])
});
}
}
}
}
function main(circle){
circle.elem.style.backgroundColor = circle.color;
}
So in this code I create a class of Circle and I create an array of Circle objects which is identified as 'circle'. When the page is loaded, I give each circle object an element reference from my html document (There are 16 circles and they each have an id of circle0, circle1, circle2.. etc. Then there's a small algorithm to ensure there are only two of each color in the matrix so they all have a matching pair. In each iteration of the for loop, I add an event listener to each circle. If the circle is clicked, I want it to change to its color which is stored in color[i].color. However, when I click the circles all it returns is:
Uncaught TypeError: Cannot read property 'elem' of undefined
at main (script.js:39)
at HTMLDivElement.<anonymous> (script.js:31)
Which is referencing:
circle.elem.style.backgroundColor = circle.color;
So I put some console.log() functions in to see what was going on:
if(frequency[num] != 2){
frequency[num]++;
circle[i].color = hue[num];
console.log(circle[i].elem);
console.log(circle[i].color);
circle[i].elem.addEventListener('click', function(){
main(circle[i])
});
}
And this spits out exactly what I expect:
script.js:31 #ff9d00
script.js:30 div data-brackets-id=​"11" class=​"circle" id=​"circle1" /div
script.js:31 #ff9d00
script.js:30 div data-brackets-id=​"12" class=​"circle" id=​"circle2" /div
script.js:31 #0039ff
script.js:30 div data-brackets-id=​"13" class=​"circle" id=​"circle3" /div
script.js:31 #0039ff
So it returns the element reference and the color of the circle. So then I try putting the "circle[i].elem.style.backgroundColor = circle[i].color" into the event listener and I get the same issue as before...
if(frequency[num] != 2){
frequency[num]++;
circle[i].color = hue[num];
console.log(circle[i].elem);
console.log(circle[i].color);
circle[i].elem.addEventListener('click', function(){
circle[i].elem.style.backgroundColor = circle[i].color
});
}
Circles without their colors. The console log statements are on the right-hand side with their specific colors as well...
So I gave up and decided to write that exact line of code outside the event listener to see if that works, and it changed all the circle's colors to their specific color...
if(frequency[num] != 2){
frequency[num]++;
circle[i].color = hue[num];
console.log(circle[i].elem);
console.log(circle[i].color);
circle[i].elem.style.backgroundColor = circle[i].color;
circle[i].elem.addEventListener('click', function(){
circle[i].elem.style.backgroundColor = circle[i].color
});
}
The circles with their specific colors...
There is some problem the event listener not being able to pass the object of a Circle or something... I don't know please help :(

Your problem boils down to the way JS treats var variables - they sort of "leak" into the global scope.
Consider the event listener that you've attached:
circle[i].elem.addEventListener('click', function(){
main(circle[i])
});
So, whenever the listener gets triggered, it calls main() function and passes circle[i] into it. But since i is the variable that's leaked outside of the supposed scope, it always has the value of 16 - the value assigned to it during the last iteration of the for loop. That's why the main() function tries to access a style property of undefined - it's the value of circle[16] that was passed into it.
Here's a couple of ways to fix it:
If you can use ES6 let variables:
Use let i instead of var i in your for loop:
for (let i = 0; i < 16; i++) {
//...
}
If not, a classic way with function closure:
function createListener(j) {
return function () {
main(circle[j])
}
}
// and use it in your 'for' loop later:
circle[i].elem.addEventListener('click', createListener(i));
Here's a useful topic that provides more techniques to avoid this: JavaScript closure inside loops – simple practical example

Save mouse location then have a second value that updates

I'm using the Phaser engine, and I want to have a line be drawn on a click and hold event from the initial mouse position and have it constantly update to draw to the mouse position as it moves. My problem is that when i try to store the initial mouse position it keeps changing. This seems like a simple problem but i'm not very good with this stuff. Here is the code:
var unitLine;
if(game.input.activePointer.isDown) {
const firstX = game.input.x;
const firstY = game.input.y;
unitLine = game.add.graphics(100, 100);
unitLine.beginFill(0xFF3300);
unitLine.lineStyle(10, 0xffd900, 1);
unitLine.moveTo(firstX, firstY);
unitLine.lineTo(game.input.x, game.input.y);
}
that firstX and firstY are changing even when i declare them as a const. Not sure what to do here.

The problem is that you're setting firstX and firstY whenever the mouse isDown, so they're basically overwritten every frame that the mouse is down.
To get around this, try using Phaser's game.input.onDown function:
var game = new Phaser.Game(500, 500, Phaser.CANVAS, 'test', {
preload: preload,
create: create,
update: update
});
function preload() {}
let firstX;
let firstY;
function create() {
game.input.onDown.add(function() {
firstX = game.input.x;
firstY = game.input.y;
}, this);
}
var unitLine;
function update() {
if (game.input.activePointer.isDown) {
unitLine = game.add.graphics(0, 0);
unitLine.beginFill(0xFF3300);
unitLine.lineStyle(10, 0xffd900, 1);
unitLine.moveTo(firstX, firstY);
unitLine.lineTo(game.input.x, game.input.y);
}
}
<script src="https://github.com/photonstorm/phaser-ce/releases/download/v2.11.1/phaser.min.js"></script>
(Also, I had to change the 100, 100 to 0, 0)

It's because you're declaring them in the statement, so the declaration is newly hit each time and the variables are created afresh.
Firsly, you need to create the variables outside of the statement.
And then, to fix your issue, I would use a bool to lock them in.
Something like this:
var unitLine;
var firstX;
var firstY;
var needToset_XY = true;
if(game.input.activePointer.isDown) {
if(needToset_XY){
firstX = game.input.x;
firstY = game.input.y;
needToset_XY = false;
}
unitLine = game.add.graphics(100, 100);
unitLine.beginFill(0xFF3300);
unitLine.lineStyle(10, 0xffd900, 1);
unitLine.moveTo(firstX, firstY);
unitLine.lineTo(game.input.x, game.input.y);
}
This means the firstX and firstY values can't be changed after the first time.
If this is all in a game loop, you'll need to declare the top four variables outside of the loop, otherwise they'll renew themselves each time.

HTML5 Canvas, Having Trouble Running Multiple Animations At Once

I have written code that takes two arrays, both of which contain co-ordinates for a four-cornered shape (effectively a start frame and an end frame), a canvas ID and a time value. The function then calculates dX and dY of each corner and uses window.performance.now() to create a timestamp. Then, on every requestAnimationFrame(), it calculates what the co-ordinates should be by using dX, dY, the old timestamp, a new timestamp and the time value from the function call. It looks like this:
function doAnim(cv, startFrame, endFrame, animTime)
{
this.canvas = document.getElementById(cv);
this.ctx = this.canvas.getContext('2d');
if(startFrame.length != endFrame.length)
{
return('Error: Keyframe arrays do not match in length');
};
this.animChange = new Array();
for(i=1;i<=startFrame.length;i++)
{
var a = startFrame[i];
var b = endFrame[i]
var c = b - a;
this.animChange[i] = c;
}
this.timerStart = window.performance.now();
function draw()
{
this.requestAnimationFrame(draw, cv);
this.ctx.clearRect(0,0,this.canvas.width,this.canvas.height);
this.currentFrame = new Array();
for(i=1;i<=startFrame.length;i++)
{
this.currentFrame[i] = startFrame[i]+(this.animChange[i]*((window.performance.now()-this.timerStart)/animTime));
}
if((window.performance.now()-this.timerStart)>=animTime)
{
this.ctx.beginPath()
this.ctx.moveTo(endFrame[1], endFrame[2]);
this.ctx.lineTo(endFrame[3], endFrame[4]);
this.ctx.lineTo(endFrame[5], endFrame[6]);
this.ctx.lineTo(endFrame[7], endFrame[8]);
this.ctx.fill();
return;
}
else
{
this.ctx.beginPath()
this.ctx.moveTo(this.currentFrame[1], this.currentFrame[2]);
this.ctx.lineTo(this.currentFrame[3], this.currentFrame[4]);
this.ctx.lineTo(this.currentFrame[5], this.currentFrame[6]);
this.ctx.lineTo(this.currentFrame[7], this.currentFrame[8]);
this.ctx.fill();
}
}
draw();
}
The goal is to have multiple animations of objects happening at once. I took the whole co-ordinate approach because I want the objects to appear as if they are coming from the horizon, creating a fake 3D perspective effect (all objects' starting frames would be a single point at the center of the canvas), and I do not want to warp the objects' textures.
Well, it works great for a single animation, but if I try to start a new animation on a completely different canvas while the first one is running, then the first animation stops dead in its tracks.
As you can see from my JS, I've tried getting around this with gratuitous use of this (I do not fully understand how this works yet, and every explanation I've read has left me even more confused), but it has not worked. I also tried a horrific approach which stored all the functions' own variables in one global array (the first time the function runs, all the variables are put in entries 1-30, the second time they're put in 31-60, etc). Unsurprisingly, that did not work either.
Here is a JSFiddle so you can see this scenario for yourself and play with my code. I am officially out of ideas. Any help will be greatly appreciated.

Like markE linked too, trying to call requestAnimationFrame multiple times won't work.
Instead you make multiple objects and then call some sort of function on them each frame.
I have created an example using your code:
https://jsfiddle.net/samcarlin/2bxn1r79/7/
var anim0frame1 = new Array();
anim0frame1[1] = 0;
anim0frame1[2] = 0;
anim0frame1[3] = 50;
anim0frame1[4] = 0;
anim0frame1[5] = 50;
anim0frame1[6] = 150;
anim0frame1[7] = 0;
anim0frame1[8] = 150;
var anim0frame2 = new Array();
anim0frame2[1] = 200;
anim0frame2[2] = 200;
anim0frame2[3] = 300;
anim0frame2[4] = 250;
anim0frame2[5] = 300;
anim0frame2[6] = 300;
anim0frame2[7] = 200;
anim0frame2[8] = 250;
//Call global
animations = [];
requestAnimationFrame( GlobalStep );
function GlobalStep(delta){
//Functions called by request animation frame have the new time as an argument
//so delta should be approximately the same as window.performance.now()
//especially in realtime applications, which this is
//Check if we have any animation objects
if(animations.length > 0){
//Iterate through and call draw on all animations
for(var i=0; i<animations.length; i++){
if(animations[i].draw(delta)){
//Basically we have it so if the draw function returns true we stop animating the object
//And remove it from the array, so have the draw function return true when animation is complete
animations[i].splice(i, 0);
//We removed an object from the array, so we decrement i
i--;
}
}
}
//And of course call requestAnimationFrame
requestAnimationFrame( GlobalStep );
}
function AnimationObject(cv, startFrame, endFrame, animTime){
//Add this object to the objects arrays
animations.push(this);
//We need to store start and end frame
this.startFrame = startFrame;
this.endFrame = endFrame;
this.animTime = animTime;
//Your code
this.canvas = document.getElementById(cv);
this.ctx = this.canvas.getContext('2d');
if (startFrame.length != endFrame.length) {
return ('Error: Keyframe arrays do not match in length');
};
this.animChange = new Array();
for (i = 1; i <= startFrame.length; i++) {
var a = startFrame[i];
var b = endFrame[i]
var c = b - a;
this.animChange[i] = c;
}
this.timerStart = window.performance.now();
}
//This adds a function to an object, but in such a way that every object shares the same function
//Imagine a kitchen, each object is a person, and this function is a spoon
//by defining this function in this manner Object.prototype.function_name = function(arguments){}
//We make it so one function definition is needed, essentially allowing all the people to share one spoon,
//the 'this' variable still refers to whichever object we call this method, and we save memory etc.
AnimationObject.prototype.draw = function(newTime){
//I added this to start frame so we get what we stored earlier
this.ctx.clearRect(0, 0, this.canvas.width, this.canvas.height);
this.currentFrame = new Array();
for (i = 1; i <= this.startFrame.length; i++) {
this.currentFrame[i] = this.startFrame[i] + (this.animChange[i] * ((newTime - this.timerStart) / this.animTime));
}
if ((newTime - this.timerStart) >= this.animTime) {
this.ctx.beginPath()
this.ctx.moveTo(this.endFrame[1], this.endFrame[2]);
this.ctx.lineTo(this.endFrame[3], this.endFrame[4]);
this.ctx.lineTo(this.endFrame[5], this.endFrame[6]);
this.ctx.lineTo(this.endFrame[7], this.endFrame[8]);
this.ctx.fill();
return;
} else {
this.ctx.beginPath()
this.ctx.moveTo(this.currentFrame[1], this.currentFrame[2]);
this.ctx.lineTo(this.currentFrame[3], this.currentFrame[4]);
this.ctx.lineTo(this.currentFrame[5], this.currentFrame[6]);
this.ctx.lineTo(this.currentFrame[7], this.currentFrame[8]);
this.ctx.fill();
}
}
Notes:
Everytime you press the button a new object is added and simply overwrites previous ones for each frame, you should implement your program so that it checks if a specific animation has already started, you could also use the builtin mechanism to stop animation when complete (read the comments in the code)
You also need to change the on button click code
<button onclick="new AnimationObject('canvas1', anim0frame1, anim0frame2, 3000);">
Lastly if you have further questions feel free to contact me

Paper js 'dot' paths showing up in firefox but not in IE or Chrome

Using the code in the paper js sketch here I am able to see the dots that are drawn by simply clicking without moving the mouse. Also you can see the element is in fact added in all 3 browsers. However, Chrome and IE do not display the element and I cannot for the life of me understand why.
// The minimum distance the mouse has to drag
// before firing the next onMouseDrag event:
tool.minDistance = 1;
tool.maxDistance = 1;
function onMouseDown(e) {
// Create a new path and give it a stroke color:
path = new Path();
path.strokeColor = 'black';
path.strokeWidth = 2;
// Add a segment to the path where
// you clicked:
path.add(e.point);
}
function onMouseDrag(e) {
var top = e.middlePoint;
var bottom = e.middlePoint;
path.add(top);
path.insert(0, bottom);
}
function onMouseUp(e) {
var pt = e.point;
path.add(pt);
path.closed = true;
console.log(path);
}

Got a reply from Jürg Lehni
It's probably a difference of the underlying rendering system, but an
open path with only one segment should simply not be rendered,
something that we should handle in the library internally.
A closed path should be rendered if it defines handles, as that can
actually be a tiny little loop.
I had to setup specific behavior to handle dot paths. In my case I used circles.

This is how i solved it:
function onMouseUp(e) {
var pt = e.point;
if (path.segments.length < 2) {
// draw a dot
pt.y ++;
}
path.add(pt);
path.closed = true;
console.log(path);
}

this getting lost again

I thought I fixed it but looks like not. Heres whats happening:
The canvas.mousemove event is handled by viewport.onMouseMove.bind(viewport) function (viewport is an instance of a class).
At the end of the onMouseMove function it calls this.Draw() (referring to viewport.Draw() function).
viewport.Draw() loops through all the items and calls Items[i].Draw(ctx) on each of them where ctx is a back buffer canvas context.
Now if If the item that is being drawn goes ahead and uses the ctx to draw something right there and then (in its Draw function), using this to refer to itself, everything works fine. For example
this.Draw = function(ctx) {
ctx.beginPath();
ctx.moveTo(this.x1, this.y1);
ctx.lineTo(this.x2, this.y2);
ctx.lineWidth = 1;
ctx.strokeStyle = "#000000";
ctx.stroke();
};
However, if the object is a container that has items in itself and tries to loop and draw them like this
this.Draw = function(ctx) {
for (j = 0; j < this.Items.length; j++) {
this.Items[j].Draw(ctx);
}
};
When it gets into the Items[j].Draw, "this" loses all meaning. alert(this) produces "object object" and I cant figure out what its referring to (it's not the viewport nor the container nor the item it needs to be). Also another weird thing - I had to change the container object loop to use j instead of i because otherwise it would create a perpetual loop (like the i's of the viewport.draw and item[i].draw were the same).

Your question is somewhat unclear. Is this.Items an array of objects with the same prototype as this? ie. nested? Also, is the j counter intended to be shared?
Regardless, function contexts' this values can be changed rather easily to whatever you need them to be with the .apply and .call functions:
this.Draw = function(ctx) {
for (var j = 0; j < this.Items.length; j++) {
// These two are the same as what you have in the question
this.Draw.call(this.Items[j], ctx);
this.Draw.apply(this.Items[j], [ctx]);
// This is what you had in the question if Draw is different for Items:
this.Items[j].Draw(ctx);
this.Items[j].Draw.call(this.Items[j], ctx);
// Will preserve the this reference within the nested call
this.Items[j].Draw.call(this, ctx);
}
};

Not sure what the problem is but as my comment suggest this is the invoking object:
//this in someFunction is window
setTimeout(myObject.someFunction, 200);
//this in someFunction is button
button.onClick=myObject.someFunction;
Not sure what you would like this to be when it's called but if it has to be Items[j] then your code is fine and maybe something else is causing you problems. I suggest console.log objects in Chrome or Firefox with firebug, use F12 to open the console and inspect the logged objects.
Here is sample code of items that can be Square or Circle;
var Shape = function Shape(args){
//args.x1 or y1 can be 0, defaults to 2
this.x1 = (args.x1 === undefined)? 2:args.x1;
this.y1 = (args.y1 === undefined)? 2:args.y1;
this.name = args.name||"unnamed";
}
//in this example Square and Cirle draw does the same
// so they can inherit it from Shape
Shape.prototype.draw=function(){
console.log("this x1:",this.x1,"this y1:",this.y1,"name",this.name);
//you can log complex values as well and click on them in the console
// to inspect the details of the complex values (objects)
// the above can be done in the following log
console.log("in draw, this is:",this);
}
var Square = function Square(args){
//re use parent constructor (parent is Shape)
Shape.call(this,args);
}
//set prototype part of inheritance and repair constructor
Square.prototype=Object.create(Shape.prototype);
Square.prototype.constructor=Square;
var Circle = function Circle(args){
//re use parent constructor (parent is Shape)
Shape.call(this,args);
}
//set prototype part of inheritance
Circle.prototype=Object.create(Shape.prototype);
Circle.prototype.constructor=Circle;
//there is only one app so will define it as object literal
var app = {
items:[],
init:function(){
var i = -1;
while(++i<10){
this.items.push(new Circle({x1:i,y1:i,name:"circle"+i}));
}
while(++i<20){
this.items.push(new Square({x1:i,y1:i,name:"square"+i}));
}
},
draw:function(){
var i = -1;len=this.items.length;
while(++i<len){
this.items[i].draw();
}
}
}
app.init();
app.draw();//causes console.logs