Cannot Read Property 'getContext' Of Null

September 15, 2022 Post a Comment

I have a button which on click executes this function.This code is to draw a line on canvas element on which PDF file gets rendered on a webpage by using PDF.JS. But i get an erro

Solution 1:

Firstly, you should check for null:

var c = document.getElementById("canvas1");
if (c != null) {
  // proceed
} else {
  // a problem: what can you do about it?
}

Secondly, make sure you have an element canvas1 - if it exists then c should not be null; if it doesn't exist then there's a discrepancy between the code and content, and you need to decide what should happen in circumstances when this occurs, if it should never occur then it's exceptional and maybe you want the error the be raised, or a message of your own specified, or something.

Solution 2:

I was using var ctx= c.getContext('2D'); with the capital 'D' in 2D and that is not correct. It should be a lowercase 'd' in 2d. I realize this is a very simple error, but it brought me here. Hopefully I no one else starts to pull out hair on something so simple.

Solution 3:

Check the position of your script. I have included it in the <head> section so obviously, it tried to find canvas element even before letting it appended to the DOM. Thus giving undefined or null error.

Solution 4:

Your code works as far as I can tell (just added the following HTML):

<button onclick="abc()">button</button>
<canvas id="canvas1" height="300" width="300"/>

http://jsfiddle.net/qBHRg/

Solution 5:

Assuming your alerts report that you have valid references to your canvas and context:

Alert#1: HTMLCanvasElement and

Alert#2: CanvasRenderingContext2D (or your browsers version of these)

Then the code you have presented will work in html5 canvas (no errors).

Since you are working with pdf.js, your error might be in code you haven't shown us.

Make sure you are feeding pdf.js a valid context

This alert should be CanvasRenderingContext2D

// test if the context you're feeding pdf.js is valid

alert(ctx);

// feed pdf.js the context

var myPDFContext = {
  canvasContext: ctx,
  viewport: viewport
};
page.render(myPDFContext);

JavaScript Gen