WebGL 1.0. Square translation using WASD and arrow keys

HTML

<p>Click here to active the window</p>
<p>Use WASD or arrow keys</p>
<canvas id="renderCanvas" width="250" height="250"></canvas>

<script src="https://cdnjs.cloudflare.com/ajax/libs/gl-matrix/2.4.0/gl-matrix-min.js"></script>

CSS

#renderCanvas {
    border: 5px solid #aaaaaa;
}

JavaScript

var vertexShaderSource =
    `attribute vec2 a_Position;
    uniform mat4 u_ModelMatrix;

    void main()
    {
        gl_Position = u_ModelMatrix * vec4(a_Position, 0.0, 1.0);
    }`;

var fragmentShaderSource =
    `precision mediump float;
    uniform vec3 u_Color;

    void main()
    {
        gl_FragColor = vec4(u_Color, 1.0);
    }`;

var gl = document.getElementById("renderCanvas").getContext("webgl");

var vShader = gl.createShader(gl.VERTEX_SHADER);
gl.shaderSource(vShader, vertexShaderSource);
gl.compileShader(vShader);

var fShader = gl.createShader(gl.FRAGMENT_SHADER);
gl.shaderSource(fShader, fragmentShaderSource);
gl.compileShader(fShader);

var program = gl.createProgram();
gl.attachShader(program, vShader);
gl.attachShader(program, fShader);
gl.linkProgram(program);
gl.useProgram(program);

var vertices = new Float32Array([
    -0.5, 0.5, -0.5, -0.5, 0.5, 0.5, 0.5, -0.5
]);

var vbo = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, vbo);
gl.bufferData(gl.ARRAY_BUFFER, vertices, gl.STATIC_DRAW);

var a_Position = gl.getAttribLocation(program, "a_Position");
gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, 0, 0);
gl.enableVertexAttribArray(a_Position);

var u_Color = gl.getUniformLocation(program, "u_Color");
gl.uniform3f(u_Color, 0.635, 0.450, 0.125);

var modelMatrix = mat4.create();
var pos = { x: 0.1, y: 0.1 };
var step = 0.2;

window.addEventListener('keydown', function (event)
{
    if (event.keyCode === 87 || event.keyCode === 38) // W, Up
        pos.y += step;
    else if (event.keyCode === 65 || event.keyCode === 37) // A, Left
        pos.x -= step;
    else if (event.keyCode === 83 || event.keyCode === 40) // S, Down
        pos.y -= step;
    else if (event.keyCode === 68 || event.keyCode === 39) // D, Right
        pos.x += step;
    draw(pos);
});

draw(pos);

function draw(pos)
{
    mat4.identity(modelMatrix);
    mat4.translate(modelMatrix, modelMatrix, vec3.fromValues(pos.x, pos.y, 0.0));
    mat4.scale(modelMatrix,...