Drawing simple texture in WebGL

// Vertex shader program
var VSHADER_SOURCE =
    'attribute vec3 a_Position;\n' +
    'attribute vec2 a_TexCoord;\n' +
    'varying vec2 v_TexCoord;\n' +
    'void main() {\n' +
    ' gl_Position = vec4(a_Position, 1.0);\n' +
    ' v_TexCoord = a_TexCoord;\n' +
    '}\n';

// Fragment shader program
var FSHADER_SOURCE =
    'precision highp float;\n' +
    'uniform sampler2D u_Sampler;\n' +
    'varying vec2 v_TexCoord;\n' +
    'void main() {\n' +
    ' gl_FragColor = texture2D(u_Sampler, vec2(v_TexCoord.s, v_TexCoord.t));\n' +
    '}\n';

// Global gl variable
var gl;

// Global shader variables
var a_Position;
var a_TexCoord;

// Global buffers
var verticesBuffer;
var triangleFacesBuffer;
var textureCoordBuffer;

// Global texture
var texture;


function main() {
    // Get the rendering context for WebGL

    initializeWebGL();
    initializeShaders();
    initializeBuffers();

    //   Initialize html elements and set all events

    // Image selector
    var imageSel = document.getElementById("textureSelector");
    imageSel.onchange = function(){loadImage(imageSel)};
}

function initializeWebGL() {
    // Get the rendering context for WebGL
    var canvas = document.getElementById("webglCanvas");
    gl = canvas.getContext("webgl");
    if (!gl) {
        console.log('Failed to get the rendering context for WebGL');
        return;
    }

    // Clear and init canvas
    gl.clearColor(0.0, 0.0, 0.0, 1.0);
    gl.enable(gl.DEPTH_TEST);
    gl.depthFunc(gl.LEQUAL);
    gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
}

function initializeShaders() {
    // Initialize shaders through cuon-utils.js
    if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {
        console.log("Failed to initialize shaders.");
        return null;
    }

    // Fetch and enable shader attributes
    a_Position = gl.getAttribLocation(gl.program, "a_Position");
    gl.enableVertexAttribArray(a_Position);

    a_TexCoord = gl.getAttribLocation(gl.program, "a_TexCoord");
    gl.enableVertexAttribArray(a_TexCoord);
}

function initializeBuffers() {
    // Vertex positional data
    var vertices = [
        1.0,  1.0,  //Top right corner
        -1.0, 1.0,  //Top left corner
        1.0,  -1.0, //Bottom right corner
        -1.0, -1.0  //Bottom left corner
    ];

    verticesBuffer = gl.createBuffer();
    gl.bindBuffer(gl.ARRAY_BUFFER, verticesBuffer);
    gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(vertices), gl.STATIC_DRAW);


    // Texture positional data
    var textureCoords = [
        1.0,  1.0,  //Top right corner
        0.0,  1.0   //Top left corner
        1.0,  0.0,  //Bottom right corner
        0.0,  0.0,  //Bottom left corner
    ];

    textureCoordBuffer  = gl.createBuffer();
    gl.bindBuffer(gl.ARRAY_BUFFER, textureCoordBuffer );
    gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(textureCoords), gl.STATIC_DRAW);

    var triangleFaces = [
                0, 1, 2
                1, 2, 3
            ];

    triangleFacesBuffer  = gl.createBuffer();
    gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, triangleFacesBuffer );
    gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, new Uint16Array(triangleFaces), gl.STATIC_DRAW);
}

function render() {
    // Clear canvas
    gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);

    // Activate and bind texture
    gl.activeTexture(gl.TEXTURE0);
    gl.bindTexture(gl.TEXTURE_2D, texture);
    gl.uniform1i(gl.getUniformLocation(gl.program, "u_Sampler"), 0);

    // Bind and point vertex data buffer to a_Position
    gl.bindBuffer(gl.ARRAY_BUFFER, verticesBuffer);
    gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, 0, 0);

    // UPDATED
    gl.bindBuffer(gl.ARRAY_BUFFER, textureCoordBuffer );
    gl.vertexAttribPointer(a_TexCoord, 2, gl.FLOAT, false, 0, 0);

    // Bind triangle faces data
    gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, triangleFacesBuffer);

    // Draw elements
    gl.drawElements(gl.TRIANGLES, 6, gl.UNSIGNED_SHORT, 0);
}


// Texture functions

function loadImage(imageSelector) {
    texture = gl.createTexture();
    var image = new Image();
    image.onload = loadTexture(gl, image);
    image.src = imageSelector.value;
}

function loadTexture(gl, image) {
    gl.pixelStorei(gl.UNPACK_FLIP_Y_WEBGL, true);
    gl.activeTexture(gl.TEXTURE0);
    gl.bindTexture(gl.TEXTURE_2D, texture);
    gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGB, gl.RGB, gl.UNSIGNED_BYTE, image);
    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
    gl.bindTexture(gl.TEXTURE_2D, null);

    render();
}