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.
the matrices in webgl are row major or column major?
I think that i must do with only one index from 0 to 15 , but how i can calculate the indexes?