constrain trouble

// the way I see this done in one example is like:

vbuffer = _gl.createBuffer();
_gl.bindBuffer(ARRAY_BUFFER, vbuffer);
_gl.bufferData(ARRAY_BUFFER, vertices, STATIC_DRAW);
numItems = vertices.length ~/ 2;

tbuffer = _gl.createBuffer();
_gl.bindBuffer(ARRAY_BUFFER, tbuffer);                                       
_gl.bufferData(ARRAY_BUFFER, textureCoords, STATIC_DRAW);

aVertexPosition = _gl.getAttribLocation(program, "aVertexPosition"); // but things like this
_gl.enableVertexAttribArray(aVertexPosition); // which this part relies on

aTextureCoord = _gl.getAttribLocation(program, "aTextureCoord"); // and this
_gl.enableVertexAttribArray(aTextureCoord); // and this

uSampler = _gl.getUniformLocation(program, "uSampler"); // are values that were set in the shader code directly,
// and I'm not sure how to set them otherwise

// and some rendering code:

_gl.bindBuffer(ARRAY_BUFFER, vbuffer);
_gl.vertexAttribPointer(aVertexPosition, 2, FLOAT, false, 0, 0);

_gl.bindBuffer(ARRAY_BUFFER, tbuffer);
_gl.vertexAttribPointer(aTextureCoord, 2, FLOAT, false, 0, 0);

_gl.bindTexture(TEXTURE_2D, playerTexture); // plus, binding the texture is done in a way I donnt see an example for in 
// our kernel code and he defines the texture manually, which it might be trivial to take the texture being worked with 
// already somewhere in the code instead, but I'm not sure how I would do it from looking at the existing code.
_gl.uniform1i(uSampler, 0);

_gl.drawArrays(TRIANGLES, 0, numItems);


// and the way that texture is defined in the example Im citing is like:

playerTexture = _gl.createTexture();
_gl.bindTexture(TEXTURE_2D, playerTexture);
_gl.texImage2DUntyped(TEXTURE_2D, 0, RGBA, RGBA, UNSIGNED_BYTE, document.querySelector('#img-player'));
_gl.texParameteri(TEXTURE_2D, TEXTURE_MAG_FILTER, NEAREST);
_gl.texParameteri(TEXTURE_2D, TEXTURE_MIN_FILTER, LINEAR_MIPMAP_NEAREST);
_gl.generateMipmap(TEXTURE_2D);
_gl.bindTexture(TEXTURE_2D, null);

and his shaders are like:

<script id="vertex" type="x-shader">
  attribute vec2 aVertexPosition;

  void main() {
    gl_Position = vec4(aVertexPosition, 0.0, 1.0);
  }
</script>
<script id="fragment" type="x-shader">
  #ifdef GL_ES
  precision highp float;
  #endif

  uniform vec4 uColor;

  void main() {
    gl_FragColor = uColor;
  }
</script>