getting “function toUpperCase() { [native code] }” error in my browser console [closed]

I don’t know why this error “function toUpperCase() { [native code] }” pops up when, I try JavaScript’s toUpperClass() , toLowerClass() comamand.

var name="hello stack overflow";
alert(name.toUpperCase);

I want to solve this issue and want to know what is “native code”.