Cannot read property 'getContext' of null, using canvas
JavascriptHtmlCanvasJavascript Problem Overview
I get the error Uncaught TypeError: Cannot read property 'getContext' of null and the important parts in files are... I am wondering since game.js is in a directory below, it cannot find canvas? What should I do?
./index.html:
<canvas id="canvas" width="640" height="480"></canvas>
./javascript/game.js:
var Grid = function(width, height) {
...
this.draw = function() {
var canvas = document.getElementById("canvas");
if(canvas.getContext) {
var context = canvas.getContext("2d");
for(var i = 0; i < width; i++) {
for(var j = 0; j < height; j++) {
if(isLive(i, j)) {
context.fillStyle = "lightblue";
}
else {
context.fillStyle = "yellowgreen";
}
context.fillRect(i*15, j*15, 14, 14);
}
}
}
}
}
Javascript Solutions
Solution 1 - Javascript
I guess the problem is your js runs before the html is loaded.
If you are using jquery, you can use the document ready function to wrap your code:
$(function() {
var Grid = function(width, height) {
// codes...
}
});
Or simply put your js after the <canvas>.
Solution 2 - Javascript
Put your JavaScript code after your tag <canvas></canvas>
Solution 3 - Javascript
You don't have to include JQuery.
In the index.html:
<canvas id="canvas" width="640" height="480"></canvas><script src="javascript/game.js">
This should work without JQuery...
Edit: You should put the script tag IN the body tag...
Solution 4 - Javascript
You should put javascript tag in your html file.
because browser load your webpage according to html flow, you should put your javascript file<script src="javascript/game.js"> after the <canvas>element tag. otherwise,if you put your javascript in the header of html.Browser load script first but it doesn't find the canvas. So your canvas doesn't work.
Solution 5 - Javascript
Write code in this manner ...
<canvas id="canvas" width="640" height="480"></canvas>
<script>
var Grid = function(width, height) {
...
this.draw = function() {
var canvas = document.getElementById("canvas");
if(canvas.getContext) {
var context = canvas.getContext("2d");
for(var i = 0; i < width; i++) {
for(var j = 0; j < height; j++) {
if(isLive(i, j)) {
context.fillStyle = "lightblue";
}
else {
context.fillStyle = "yellowgreen";
}
context.fillRect(i*15, j*15, 14, 14);
}
}
}
}
}
First write canvas tag and then write script tag. And write script tag in body.
Solution 6 - Javascript
You just need to put<script src='./javascript/game.js'></script> after your <canvas>.
Because the browser don't find your javascript file before the canvas
Solution 7 - Javascript
I assume you have your JS file declared inside the <head> tag so it keeps it consistent, like standard, then in your JS make sure the canvas initialization is after the page is loaded:
window.onload = function () {
var myCanvas = document.getElementById('canvas');
var ctx = myCanvas.getContext('2d');
}
There is no need to use jQuery just to initialize a canvas, it's very evident most of the programmers all around the world use it unnecessarily and the accepted answer is a probe of that.
Solution 8 - Javascript
This might seem like overkill, but if in another case you were trying to load a canvas from js (like I am doing), you could use a setInterval function and an if statement to constantly check if the canvas has loaded.
//set up the interval
var thisInterval = setInterval(function{
//this if statment checks if the id "thisCanvas" is linked to something
if(document.getElementById("thisCanvas") != null){
//do what you want
//clearInterval() will remove the interval if you have given your interval a name.
clearInterval(thisInterval)
}
//the 500 means that you will loop through this every 500 milliseconds (1/2 a second)
},500)
(In this example the canvas I am trying to load has an id of "thisCanvas")