ANGLE_instanced_arrays

// enable the extension
const ext = gl.getExtension("ANGLE_instanced_arrays");

// binding the geometry buffer as usual
gl.bindBuffer(gl.ARRAY_BUFFER, geometryVertexBuffer);
gl.enableVertexAttribArray(vertexPositionAttributeLocation);
gl.vertexAttribPointer(
  vertexPositionAttributeLocation,
  3,
  gl.FLOAT,
  false,
  0,
  0,
);

// build position buffer
const instancePositions = [];
for (const instance of instances) {
  instancePositions.push(
    instance.position.x,
    instance.position.y,
    instance.position.z,
  );
}
const instancePositionBuffer = createWebGLBufferFromData(instancePositions);

// binding the instance position buffer as you would with any attribute
gl.bindBuffer(gl.ARRAY_BUFFER, instancePositionBuffer);
gl.enableVertexAttribArray(instancePositionAttributeLocation);
gl.vertexAttribPointer(
  instancePositionAttributeLocation,
  3,
  gl.FLOAT,
  false,
  0,
  0,
);

// mark the attribute as instanced and advance it every single(1) instance rather than every vertex
ext.vertexAttribDivisorANGLE(instancePositionAttributeLocation, 1);

// draw geometry for each instance
ext.drawArraysInstancedANGLE(
  gl.TRIANGLES,
  0,
  numGeometryVertices,
  instances.length,
);