The problem of variable advance in JS scope chain

The JS access variable is from the inside out, and each activity variable above the scope chain is also from the inside out, such as a function, first the variable declared by arguments and inside the function, and then the variable that can be accessed by the outer layer, until the last window global object, when the function is out, Well done. Internally declared activity objects are destroyed, so the outer drops simply cannot access the objects declared inside the function. The reason that JS will be all the variables in advance also for different scopes, on the outermost side, is to put all global variables and global function declaration in advance, inside the function is to first declare the function inside the variables and functions in advance

The problem of variable advance in JS scope chain