在画布html5上创建/绘制多个图像

Question

I was wondering what I was doing wrong because I'm making a memory game but the images wouldn't draw on my canvas.

    var ctx; 
    var cardBacks = "Resources/monster.png"; //shows back of card when turned over
    var faces = []; 
    var tiles = [];
    var Tile;
    var columns = 5;
    var rows = 4;

    function createTile(x,y){
        this.x = x;
        this.y = y;
        this.width = 70;
        ctx.drawImage(cardBacks, this.x, this.y, this.width, this.width);
    }

    function initialize(){
        ctx = document.getElementById("myCanvas").getContext("2d");
        for (var i = 0; i < columns; i++) {
            for (var j = 0; j < rows; j++) {
                tiles.push(createTile(i * 78 + 10, j * 78 + 40));
            }
        }
    }

Answer 1

You need to draw an HTML image element onto the canvas, not a url. I think you could do

var cardBacks = new Image();
cardBacks.src = "Resources/monster.png";

Here's an example: https://jsfiddle.net/yarhqskx/
Here's with multiple (added onload): https://jsfiddle.net/yarhqskx/7/

Answer 2

Try this code with onload callback function.

var ctx;
var cardBacks = new Image();
cardBacks.src = "Resources/monster.png"; //shows back of card when turned over
cardBacks.onload = function() {
    ctx.drawImage(cardBacks, this.x, this.y, this.width, this.width);

}
var faces = [];
var tiles = [];
var Tile;
var columns = 5;
var rows = 4;

function createTile(x, y) {
    this.x = x;
    this.y = y;
    this.width = 70;
}

function initialize() {
    ctx = document.getElementById("myCanvas").getContext("2d");
    for (var i = 0; i < columns; i++) {
        for (var j = 0; j < rows; j++) {
            tiles.push(createTile(i * 78 + 10, j * 78 + 40));
        }
    }
}