webgl matrix set

I would understand how i can assign a value to an index in a webgl matrix.
I tryed with
var mx = mat4.create();
mx[0][0] = 0.31312312 or
mx[1][0] = 0.2222222

but don’t work.

It looks like you are using glMatrix. If that is the case, the matrices are stored as flat arrays (both for speed and compatibility with shader uniforms).

As such, mx[0][0] is just mx[0], mx[1][0] is mx[4], etc.