Specify scope for eval() in JavaScript?

Question

is there any way I can execute eval() on a specific scope (but NOT global)?

for example, the following code doesn't work (a is undefined on the second statement) because they are on different scope:

eval(var a = 1); 
eval(alert(a));

If possible, I would like to create a scope on the fly. for example (the syntax is definitely wrong, but just to illustrate the idea)

var scope1;
var scope2;
with scope1{
    eval(var a = 1); eval(alert(a));  // this will alert 1
}
with scope2{
    eval(var a = 1); eval(a++); eval(alert(a));  // this will alert 2
}
with scope1{
    eval(a += 2); eval(alert(a)); // this will alert 3 because a is already defined in scope1
}

Any idea on how to achieve something like this? Thanks!

Answer 1

you can use the "use strict" to contain the eval'ed code within the eval itself.

Second, eval of strict mode code does not introduce new variables into the surrounding scope. In normal code eval("var x;") introduces a variable x into the surrounding function or the global scope. This means that, in general, in a function containing a call to eval every name not referring to an argument or local variable must be mapped to a particular definition at runtime (because that eval might have introduced a new variable that would hide the outer variable). In strict mode eval creates variables only for the code being evaluated, so eval can't affect whether a name refers to an outer variable or some local variable

var x = 17;                                       //a local variable
var evalX = eval("'use strict'; var x = 42; x");  //eval an x internally
assert(x === 17);                                 //x is still 17 here
assert(evalX === 42);                             //evalX takes 42 from eval'ed x

If a function is declared with "use strict", everything in it will be executed in strict mode. the following will do the same as above:

function foo(){
    "use strict";

     var x = 17;
     var evalX = eval("var x = 42; x");
     assert(x === 17);
     assert(evalX === 42);
}