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4.12.2 Global eval()

4.12.2 Global eval()


It is the ability of eval() to change local variables that is so problematic to JavaScript optimizers. As a workaround, however, interpreters simply do less optimization on any function that calls eval(). But what should a JavaScript interpreter do, however, if a script defines an alias for eval() and then calls that function by another name? In order to simplify the job of JavaScript implementors, the ECMAScript 3 standard declared that interpreters did not have to allow this. If the eval() function was invoked by any name other than “eval”, it was allowed to throw an EvalError.

In practice, most implementors did something else. When invoked by any other name, eval() would evaluate the string as if it were top-level global code. The evaluated code might define new global variables or global functions, and it might set global variables, but it could not use or modify any variables local to the calling function, and would not, therefore, interfere with local optimizations.

ECMAScript 5 deprecates EvalError and standardizes the de facto behavior of eval(). A “direct eval” is a call to the eval() function with an expression that uses the exact, unqualified name “eval” (which is beginning to feel like a reserved word). Direct calls to eval() use the variable environment of the calling context. Any other call—an indirect call—uses the global object as its variable environment and cannot read, write, or define local variables or functions. The following code demonstrates:

var geval = eval; // Using another name does a global eval

var x = "global", y = "global"; // Two global variables

function f() { // This function does a local eval

var x = "local"; // Define a local variable

eval("x += 'changed';"); // Direct eval sets local variable

return x; // Return changed local variable

}

function g() { // This function does a global eval

var y = "local"; // A local variable

geval("y += 'changed';"); // Indirect eval sets global variable

return y; // Return unchanged local variable

}

console.log(f(), x); // Local variable changed: prints "localchanged global":

console.log(g(), y); // Global variable changed: prints "local globalchanged":

Notice that the ability to do a global eval is not just an accommodation to the needs of the optimizer, it is actually a tremendously useful feature: it allows you to execute strings of code as if they were independent, top-level scripts. As noted at the beginning of this section, it is rare to truly need to evaluate a string of code. But if you do find it necessary, you are more likely to want to do a global eval than a local eval.

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