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// WebGL1
var gl = canvas.getContext('webgl');

// WebGL2 @ script.js:121
var gl = canvas.getContext('webgl2');
function getWebGLContext(canvas) {

    ...

    if (isWebGL2) {
        gl.getExtension('EXT_color_buffer_float');
        supportLinearFiltering = gl.getExtension('OES_texture_float_linear');
    } else {
        halfFloat = gl.getExtension('OES_texture_half_float');
        supportLinearFiltering = gl.getExtension('OES_texture_half_float_linear');
    }

    ...

}
// WebGL1
var ext = gl.getExtension("OES_vertex_array_object");
if (!ext) {
  // tell user they don't have the required extension or work around it
} else {
  var someVAO = ext.createVertexArrayOES();
}

// WebGL2
var someVAO = gl.createVertexArray();
// BAD                    +---There's a new line here!
// BAD                    V
var vertexShaderSource = `
#version 300 es
...
`;

// GOOD
var vertexShaderSource = `#version 300 es
...
`;
// WebGL1
attribute vec4 a_position;
attribute vec2 a_texcoord;
attribute vec3 a_normal;

// WebGL2
in vec4 a_position;
in vec2 a_texcoord;
in vec3 a_normal;
// WebGL1
varying vec2 v_texcoord;
varying vec3 v_normal;

// WebGL2
// Vertex Shader
out vec2 v_texcoord;
out vec3 v_normal;

// Framgent Shader
in vec2 v_texcoord;
in vec3 v_normal;
// WebGL1
gl_FragColor = vec4(1, 0, 0, 1);  // red

// WebGL2
out vec4 myOutputColor;

void main() {
   myOutputColor = vec4(1, 0, 0, 1);  // red
}
// WebGL1
vec4 color1 = texture2D(u_some2DTexture, ...);
vec4 color2 = textureCube(u_someCubeTexture, ...);

// WebGL2
vec4 color1 = texture(u_some2DTexture, ...);
vec4 color2 = texture(u_someCubeTexture, ...);