I have multiple canvases which are used as layers.
canvas c1 = background image. canvas c2 = charactor image. canvas c3 = item image.
My source code is as follows.
Set three canvases at the same location.
<div canvas="canvas-wrap">
<canvas class="canvas" id="c1">
<canvas class="canvas" id="c2">
<canvas class="canvas" id="c3">
</div>
.canvas-wrap{
width: 600px;
height:500px;
max-width: 100%;
position: relative;
padding: 0;
box-sizing: content-box;
}
.canvas{
position: absolute;
left:0;
top:0;
border: 0;
max-width:100%;
box-sizing: content-box;
padding: 0;
margin: 0;
}
Now, I want to export these canvases as a jpg.
I have source code that can export one canvas, but I want to export one jpg from three canvases.
function saveCanvas(canvas_id)
{
var canvas = document.getElementById("c1");
var a = document.createElement('a');
a.href = canvas.toDataURL('image/jpeg', 0.85);
a.download = 'download.jpg';
a.click();
} 
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It is possible, but a better way is to use 1 canvas and draw everything there.
Here we create a virtual canvas, loop through all canvases to draw data onto it, and then save the image generated by that canvas.
//store all canvasses in array so we can loop through them later const canvasses = []; ['c1','c2','c3'].forEach(canvasId => { canvasses.push(document.getElementById(canvasId)); }); function save(){ //create one canvas to draw everything to const canvas = document.createElement("canvas"); canvas.width = canvasses[0].width; canvas.height = canvasses[0].height; //draw other canvases here const ctx = canvas.getContext('2d'); canvasses.forEach(data => { ctx.drawImage(data, 0, 0); }); //original saving code, using the fake canvas we created var a = document.createElement('a'); a.href = canvas.toDataURL('image/jpeg', 0.85); a.download = 'download.jpg'; a.click(); }