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 codeeval("var x;")
introduces a variablex
into the surrounding function or the global scope. This means that, in general, in a function containing a call toeval
every name not referring to an argument or local variable must be mapped to a particular definition at runtime (because thateval
might have introduced a new variable that would hide the outer variable). In strict modeeval
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);
}