[WebGL] shaderとprogramのエラーを得る方法

gistfile1.txt

var canvas;
var gl;
var program;

function createGlContext()
{
    canvas = document.getElementById("container");
    gl = canvas.getContext("webgl");
}

function createShader(gl, source, type) {
    var shader = gl.createShader(type);
    source = document.getElementById(source).text;
    gl.shaderSource(shader, source);
    gl.compileShader(shader);

    //! getShaderParameterでshaderの状態取得
    if (!gl.getShaderParameter(shader, gl.COMPILE_STATUS)) {
        //! 失敗していたらgetShaderInfoLogでエラー文字列取得
        alert(gl.getShaderInfoLog(shader));
        return null;
    }
    return shader;
}

function linkProgram()
{
    var vertexShader = createShader(gl, 'vert-shader', gl.VERTEX_SHADER);
    var fragShader = createShader(gl, 'frag-shader', gl.FRAGMENT_SHADER);

    program = gl.createProgram();
    gl.attachShader(program, vertexShader);
    gl.attachShader(program, fragShader);

    gl.linkProgram(program);
    // シェーダのリンクが正しく行なわれたかチェック
    if(gl.getProgramParameter(program, gl.LINK_STATUS)){
        gl.useProgram(program);
    } else {
        //! 失敗していたらgetProgramInfoLogでエラー文字列取得
        alert(gl.getProgramInfoLog(program));
    }
}

function drawArrays()
{
    var buffer = gl.createBuffer();
    gl.bindBuffer(gl.ARRAY_BUFFER, buffer);
    gl.bufferData(
            gl.ARRAY_BUFFER,
            new Float32Array([
                -1.0, -1.0,
                 1.0, -1.0,
                -1.0,  1.0,
                -1.0,  1.0,
                 1.0, -1.0,
                 1.0,  1.0]),
            gl.STATIC_DRAW);

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

    gl.drawArrays(gl.TRIANGLES, 0, 6);
}

function run()
{
    createGlContext();
    linkProgram();
    drawArrays();
}