How to move image on canvas using arrow keys in javascript

Question

I've tried a few different ways that I have seen on here, but I can't quite get my image to move. Whenever I try adapting code for arrow key presses, it just seems to make my canvas shrink and my player model (spaceperson) disappear.

here is the "drawing board" I keep returning to, and what I have so far.

// Get the canvas and context
var canvas = document.getElementById("space"); 
var ctx = canvas.getContext("2d");
canvas.width = 1920;
canvas.height = 700;


// Create the image object
var spaceperson = new Image();

// Add onload event handler
spaceperson.onload = function () {
   // Done loading, now we can use the image
   ctx.drawImage(spaceperson, 280, 300);
};


// artwork by Harrison Marley (using make8bitart.com)
spaceperson.src = "http://i.imgur.com/Eh9Dpq2.png";`

I am quite new to javascript, and I am just trying to work out how I can move the specperson image using arrow keys. I was trying to make a class for space person to access their x,y values, but I can't seem to draw the image without using .onload

Answer 1

here a more complete example:

//just a utility
function image(url, callback){
    var img = new Image();
    if(typeof callback === "function"){
        img.onload = function(){
            //just to ensure that the callback is executed async
            setTimeout(function(){ callback(img, url) }, 0)
        }
    }
    img.src = url;
    return img;
}

//a utility to keep a value constrained between a min and a max
function clamp(v, min, max){
    return v > min? v < max? v: max: min;
}

//returns a function that can be called with a keyCode or one of the known aliases 
//and returns true||false wether the button is down
var isKeyDown = (function(aliases){
    for(var i=256, keyDown=Array(i); i--; )keyDown[i]=false;
    var handler = function(e){ 
        keyDown[e.keyCode] = e.type === "keydown";
        e.preventDefault();  //scrolling; if you have to suppress it
    };

    addEventListener("keydown", handler, false);
    addEventListener("keyup", handler, false);

    return function(key){
        return(true === keyDown[ key in aliases? aliases[ key ]: key ])
    }
})({
    //some aliases, to be extended
    up: 38,
    down: 40,
    left: 37,
    right: 39
});



// Get the canvas and context
var canvas = document.getElementById("space"); 
canvas.width = 1920;
canvas.height = 700;
var ctx = canvas.getContext("2d");

//the acutal image is just a little-part of what defines your figue
var spaceperson = {
    image: image("//i.imgur.com/Eh9Dpq2.png", function(img){ 
        spaceperson.width = img.naturalWidth;
        spaceperson.height = img.naturalHeight;

        //start the rendering by calling update
        update();
    }),

    //position
    x: 60, y: 310,
    width: 0, height: 0,

    speed: 200  // 200px/s
};

var lastCall = 0;  //to calculate the (real) time between two update-calls
//the render-fucntion
function update(){
    //taking account for (sometimes changing) framerates
    var now = Date.now(), time = lastCall|0 && (now-lastCall)/1000;
    lastCall = now;
    requestAnimationFrame(update);

    var sp = spaceperson,
        speed = sp.speed;

    //checking the pressed buttons and calculates the direction
    //two opposite buttons cancel out each other, like left and right
    var dx = (isKeyDown('right') - isKeyDown('left')) * time, 
        dy = (isKeyDown('down') - isKeyDown('up')) * time;

    //fix the speed for diagonals
    if(dx && dy) speed *= 0.7071067811865475;   // * 1 / Math.sqrt(2)

    if(dx) { //there is some movement on the x-axes
        sp.x = clamp(
            //calculate the new x-Position
            //currentPos + direction * speed
            sp.x + dx * sp.speed, 

            //restraining the result to the bounds of the map
            0, canvas.width - sp.width
        );
    }

    //same for y
    if(dy) sp.y = clamp(sp.y + dy * sp.speed, 0, canvas.height - sp.height);

    ctx.clearRect(0, 0, canvas.width, canvas.height);
    ctx.drawImage(sp.image, sp.x, sp.y);
}

Edit:

A quick question (I hope); if I was to later add other objects, would I check for collisions in update()?

This is still just a very basic example. The main purpose of the update()-function should be to work as the main event-loop. To trigger all Events that have to happen each frame in the order they have to happen.

var lastCall = 0;
function update(){
    //I always want a next frame
    requestAnimationFrame(update);

    //handle timing
    var now = Date.now(), 
        //time since the last call in seconds
        //cause usually it's easier for us to think in 
        //tems like 50px/s than 0.05px/ms or 0.8333px/frame
        time = lastCall|0 && (now-lastCall) / 1000;

    lastCall = now;

    movePlayer(time);
    moveEnemies(time);
    moveBullets(time);

    collisionDetection();

    render();
}

function render(){
    ctx.clear(0, 0, canvas.width, canvas.height);
    drawBackground(ctx);
    for(var i=0; i<enemies.length; ++i)
        enemies[i].render(ctx);
    player.render(ctx);
}

Not saying that you have to implement all these functions now, but to give you an idea of a possible structure. Don't be scared to break big tasks (functions) up into subtasks.
And it might make sense to give each enemy a move()-function so you can implement different movement-patterns per enemy, or you say that the pattern is (and will be) all the same for each enemy, parameterized at the best, then you can handle that in a loop.
Same thing for rendering, as I'm showing in the last part of code.