Use eval () to "spoof" JavaScript lexical scopes

We know that the so-called "lexical scope" is the way the code is written when the internal function can access variables outside the function, if there is a function outside the same name variable, then the function inside to get the same name variable value will be masked out of the function of the same name variable (originally two different variables, just the same name.) Also note that the same scope is the same variable, so do not repeat the declaration, otherwise the second declaration will be ignored).

Use eval () to "spoof" the lexical scope:

function foo (str) {    eval (str);    Console.log (a);} var a=100; foo ("var a=12");   //  A

According to the logic should output 100, can actually output is 12, because Eval can cause Var a=12; This statement is as if it were in the same position as eval (). This code actually creates a variable B in the Foo function, so the variable b outside of the function is naturally masked.

eval () can insert code dynamically, but it actually has an impact on performance, so it's best not to use it.

Use eval () to "spoof" JavaScript lexical scopes