JS substitua o espaço pelo sublinhado
var string = "my name";
string = string.replace(/ /g,"_"); //returns my_name
If-dev
var string = "my name";
string = string.replace(/ /g,"_"); //returns my_name// replaces space with '_'
str = str.replace(/ /g, "_");
// or
str = str.split(' ').join('_');