mat4 type in attribute shader

Question

how to send to a shader attribute with the MAT4 type?

attribute mat4 attr;
...

JS:

var attr=gl.getAttribLocation(_program,"attr");

Answer 1

From the spec section 2.10.4

When an attribute variable is declared as a mat4, its matrix columns are taken from the (x, y, z, w) components of generic attributes i through i + 3.

so

JS:

var row0Location = gl.getAttribLocation(_program, "attr");
var row1Location = row0Location + 1;
var row2Location = row0Location + 2;
var row3Location = row0Location + 3; 

As for getting the data the most common way would be to put all the matrices in one buffer so

var matrices = new Float32Array(numMatrices * 16);

... // fill out your matrices

gl.bufferData(gl.ARRAY_BUFFER, matrices, gl.STATIC_DRAW);

then to set up the attributes

var floatsPerRow = 4
var bytesPerRow = floatsPerRow * 4;
var bytesPerMatrix = bytesPerRow * 4;
var row0Offset = bytesPerRow * 0;
var row1Offset = bytesPerRow * 1;
var row2Offset = bytesPerRow * 2;
var row3Offset = bytesPerRow * 3;
gl.enableVertexAttribArray(row0Location);
gl.vertexAttribPointer(row0Location, floatsPerRow, gl.FLOAT, 
                       false, bytesPerMatrix, row0Offset);
gl.enableVertexAttribArray(row1Location);
gl.vertexAttribPointer(row1Location, floatsPerRow, gl.FLOAT, 
                       false, bytesPerMatrix, row1Offset);
gl.enableVertexAttribArray(row2Location);
gl.vertexAttribPointer(row2Location, floatsPerRow, gl.FLOAT, 
                       false, bytesPerMatrix, row2Offset);
gl.enableVertexAttribArray(row3Location);
gl.vertexAttribPointer(row3Location, floatsPerRow, gl.FLOAT, 
                       false, bytesPerMatrix, row3Offset);

Something to be aware of. If you're debugging and you comment out attr in your shader then row0Location will be -1 and calling all the gl.vertexAttrib functions with a -1 location is a no-op which is good. But, because you compute the other locations row1Location, row2Location and row3Location will be valid attribute locations as far as WebGL is concern but invalid as far as your program is concerned. Not a big deal just something to keep in mind.