Add canvas to a page with javascript

问题:

I am trying to use Javascript in order to add a canvas to one page which originally does not have one. I am trying to do the following:

var canv=document.createElement("canvas");
canv.setAttribute("id", "canvasID");
alert(canv.id);
var c=document.getElementById("canvasID");
alert(c.id);

The problem is the the first alert(canv.id) results in canvasID, while the second alert is undefined because c is null.

Can anybody tell me what am I doing wrong?

PS: the code is designed to run under Greasemonkey so adding the canvas and its ID in the HTML itself is not a viable option.

回答1:

Use something like Node.appendChild( child ) for adding it to the DOM:

var canv = document.createElement('canvas');
canv.id = 'someId';

document.body.appendChild(canv); // adds the canvas to the body element
document.getElementById('someBox').appendChild(canv); // adds the canvas to #someBox

---

Or you can use element.innerHTML:

document.body.innerHTML += '<canvas id="someId"></canvas>'; // the += means we add this to the inner HTML of body
document.getElementById('someBox').innerHTML = '<canvas id="someId"></canvas>'; // replaces the inner HTML of #someBox to a canvas

回答2:

    var canvas = document.getElementById('canvas'); //finds Original Canvas
    img = document.createElement('img'); 
    img.src = 'images/a.jpg'; //stores image src

    var canv = document.createElement('canvas'); // creates new canvas element
    canv.id = 'canvasdummy'; // gives canvas id
    canv.height = canvas.height; //get original canvas height
    canv.width = canvas.width; // get original canvas width
    document.body.appendChild(canv); // adds the canvas to the body element

    var canvas1 = document.getElementById('canvasdummy'); //find new canvas we created
    var context = canvas1.getContext('2d');

    context.drawImage(img, 0, 0, canvas.width, canvas.height); //draws background image
    context.drawImage(canvas, 0, 0); //draws original canvas on top of background
    cscreen = canvas1.toDataURL(); //generates PNG of newly created canvas
    document.body.removeChild(canv); // removes new canvas

I use this all the time and works great...

回答3:

var canv=document.createElement("canvas");
canv.setAttribute("id", "canvasID");
document.body.appendChild(canv);

Without something like that third line, your new canvas is never actually inserted into the page.