What does if(x){some code} mean?

1 
let x=5;
if(x){
//some code
}

What does x mean inside of if condition expression?
Does it equivalent to if(x==true), or if(x==!undefined) ?

2

An if statement always evaluates the expression inside of the parens to either true or false.

3

Thanks @bbsmooth, read the article and now I get it.