How to read webgl GL.bufferData in javascript

It is not directly possible to read the data back in WebGL1. (see below for WebGL2). This is limitation of OpenGL ES 2.0 on which WebGL is based.

There are some workarounds:

1. You could try to render that data to a texture then use readPixels to read the data.

   You'd have to encode the data into bytes in your shader because readPixels in WebGL can only read bytes

2. You can wrap your WebGL to store the data yourself something like

   var buffers = {};
   var nextId = 1;
   var targets = {};
   
   function copyBuffer(buffer) {
      // make a Uint8 view of buffer in case it's not already
      var view = new Uint8Buffer(buffer.buffer); 
      // now copy it
      return new UintBuffer(view);
   }
   
   gl.bindBuffer = function(oldBindBufferFn) {
     return function(target, buffer) {
       targets[target] = new Uint8Buffer(buffer.buffer);
       oldBindBufferFn(target, buffer);
     };
   }(gl.bindBuffer.bind(gl));
   
   gl.bufferData = function(oldBufferDataFn) {
      return function(target, data, hint) {
        var buffer = targets[target];
        if (!buffer.id) {
          buffer.id = nextId++;
        }
        buffers[buffer.id] = copyBuffer(data);
        oldBufferDataFn(target, data, hint);
      };
   }(gl.bufferData.bind(gl)));
   

   Now you can get the data with

   data = buffers[someBuffer.id];
   

   This is probably what the WebGL Inspector does

   Note that there are a few issues with the code above. One it doesn't check for errors. Checking for errors would make it way slower but not checking for error will give you incorrect results if your code generates errors. A simple example

   gl.bufferData(someBuffer, someData, 123456);
   

   This would generate an INVALID_ENUM error and not update the data in someBuffer but our code isn't checking for errors so it would have put someData into buffers and if you read that data it wouldn't match what's in WebGL.

   Note the code above is pseudo code. For example I didn't supply a wrapper for gl.bufferSubData.

WebGL2

In WebGL2 there is a function gl.getBufferSubData that will allow you to read the contents of a buffer. Note that WebGL2, even though it's based on OpenGL ES 3.0 does not support gl.mapBuffer because there is no performant and safe way to expose that function.