tl;dr

Ideally you shouldn’t use eval, because it inhibits many optimizations and makes code run slower. new Function(argname1, argname2, ..., code) doesn’t inhibit optimizations, so it’s a better way to dynamically generate code. (This may require passing in arguments instead of using names for local variables. That’s the price you pay to play nice with compilers that could optimize but for eval.)

Nevertheless, it’s still possible to use eval in ES5. eval of normal code behaves as it always has. But eval of strict mode code behaves differently: any variables created by the code being evaluated affect only that code, not the enclosing scope. (The enclosing scope’s variables are still visible in the eval code if they aren’t shadowed.) Firefox now correctly binds variables created by strict mode eval code, completing the last major component of strict mode (though bugs remain). These programs demonstrate the idea:

var x = 2, y = 3;
print(eval("var x = 9; x"));               // prints 9
print(x);                                  // prints 9
print(eval("'use strict'; var x = 5; x")); // prints 5
print(eval("'use strict'; var x = 7; x")); // prints 7
print(eval("'use strict'; y"));            // prints 3
print(x);                                  // prints 9

"use strict";
var x = 2, y = 3;
// NB: Strictness propagates into eval code evaluated by a
//     direct call to eval — a call occurring through an
//     expression of the form eval(...).
print(eval("var x = 5; x")); // prints 5
print(eval("var x = 7; x")); // prints 7
print(eval("y"));            // prints 3
print(x);                    // prints 2

This partially defangs eval. But even strict mode eval inhibits optimizations, so you are still better off avoiding it.

eval is a double-edged sword

eval is one of the most powerful parts of JavaScript: it enables runtime code generation. You can compile code to perform specific operations, avoiding unnecessary general-purpose overhead — a powerful concept. (But you’d be better off using new Function(argname1, argname2, ..., code), which doesn’t inhibit optimizations and still enables code generation, at loss of the ability to capture the local scope. Code using eval may see considerable speedups: for example, roc’s CPU emulator sped up ~14% switching from eval to Function. Less beefy code won’t see that magnitude of win, yet why give up performance when you have a ready alternative?)

Yet at the same time, eval is too powerful. As inline assembly is to C or C++ (at least without the information gcc‘s asm syntax requires), so is eval to JavaScript. In both instances a powerful construct inhibits many optimizations. Even if you don’t care about optimizations or performance, eval‘s ability to introduce and delete bindings makes code that uses it much harder to reason about.

eval arbitrarily mutates variables

At its simplest, eval can change the value of any variable:

function test(code)
{
  var v = 1;
  eval(code);
  return v;
}
assert(test("v = 2") === 2);

Thus you can’t reorder or constant-fold assignments past eval: eval forces everything to be “written to disk” so that the eval code can observe it, and likewise it forces everything to be read back “from disk” when needed next. Without costly analysis you can’t store v in a register across the call.

eval can insert bindings after compilation

eval‘s ability to add bindings is worse. This can make it impossible to say what a name refers to until runtime:

var v;
function test(code)
{
  eval(code);
  return v;
}

Does the v in the return statement mean the global variable? You can’t know without knowing the code eval will compile and run. If that code is "var v = 17;" it refers to a new variable. If that code is "/* psych! */" it refers to the global variable. eval in a function will deoptimize any name in that function which refers to a variable in an enclosing scope. (And don’t forget that the name test itself is in an enclosing scope: if the function returned test instead of v, you couldn’t say whether that test referred to the enclosing function or to a new variable without knowing code.)

eval can remove bindings added after compilation

You can also delete bindings introduced by eval (but not any other variables):

var v = 42;
function test(code)
{
  eval(code);
  function f(code2)
  {
    eval(code2);
    return function g(code3) { eval(code3); return v; };
  }
  return f;
}
var f = test("var v = 17;");
var g = f("var v = 8675309;");
assert(g("/* nada */") === 8675309);
assert(g("var v = 5;") === 5);
assert(g("delete v") === 17);
assert(g("delete v") === 42);
assert(g("delete v") === 42); // can't delete non-eval var (thankfully)

So not only can you not be sure what binding a name refers to given eval, you can’t even be sure what binding it refers to over time! (Throw generators into the game and you also have to account for a scope without a binding containing that binding even after a function has “returned”.)

eval can affect enclosing scopes

Worst, none of these complications (and I’ve listed only a few) are limited to purely local variables. eval can affect any variable it can see at runtime, whether in its immediate function or in any enclosing function or globally. eval is the fruit of the poisonous tree: it taints not just the scope containing it, but all scopes containing it.

Save us, ES5!

ES5 brings some relief from this madness: strict mode eval can no longer introduce or delete bindings. (Normal eval remains unchanged.) Deleting a binding is impossible in strict mode because delete name is a syntax error. And instead of introducing bindings in the calling scope, eval of strict mode code introduces bindings for that code only:

var x = 2, y = 3;
print(eval("var x = 9; x"));               // prints 9
print(x);                                  // prints 9
print(eval("'use strict'; var x = 5; x")); // prints 5
print(eval("'use strict'; var x = 7; x")); // prints 7
print(eval("'use strict'; y"));            // prints 3
print(x);                                  // prints 9

This works best if you have strict mode all the way down, so that eval can never affect the bindings of any scope (and so you don’t need "use strict" at the start of every eval code):

"use strict";
var x = 2, y = 3;
// NB: Strictness propagates into eval code evaluated by a
//     direct call to eval — a call occurring through an
//     expression of the form eval(...).
print(eval("var x = 5; x")); // prints 5
print(eval("var x = 7; x")); // prints 7
print(eval("y"));            // prints 3
print(x);                    // prints 2

Names in strict mode code can thus be associated without having to worry about eval in strict mode code altering bindings, preserving additional optimization opportunities.

Firefox now correctly implements strict mode eval code binding semantics (modulo bugs, of course).

So if I write strict mode code, should I use eval?

eval‘s worst aspects are gone in strict mode, but using it still isn’t a good idea. It can still change variables in ways the JavaScript compiler can’t detect, so strict mode eval still generally forces every variable to be saved before it occurs and to be reloaded when needed. This deoptimization is unavoidable if runtime code generation can affect dynamically-determined local variables. It’s still better to use Function than to use eval.

Also, as a temporary SpiderMonkey-specific concern, we don’t perform many of the binding optimizations strict mode eval enables. Binding semantics might (I haven’t tested, and it’s entirely possible the extra work is unnoticeable in practice) slow down strict eval compared to normal eval. Strict mode eval performance in SpiderMonkey won’t be much better than that of regular eval, and it might be slightly worse. We’ll fix this over time, but for now don’t expect strict mode eval to improve performance. (If you really need performance, don’t use eval.)

Conclusion

eval is powerful — arguably too powerful. ES5’s strict mode blunts eval‘s sharpest corners to simplify it and permit typical optimizations in code using it. But while strict mode eval is better than regular eval, Function is still the best way to generate code at runtime. If you must use eval, consider using strict mode eval for a simpler binding model and eventual performance benefits.

You can experiment with a version of Firefox with these changes by downloading a nightly build. (Don’t forget to use the profile manager if you want to keep the settings you use with your primary Firefox installation pristine.)

tl;dr

Don’t try to use the arguments or caller properties of functions created in strict mode code. Don’t try to use the callee or caller properties of arguments objects corresponding to invocations of functions in strict mode code. Don’t try to use the caller property of a function if it might be called from strict mode code. You are in for an unpleasant surprise (a thrown TypeError) if you do.

function strict() { "use strict"; return arguments; }
function outer() { "use strict"; return inner(); }
function inner() { return inner.caller; }

strict.caller;    // !!! BAD IDEA
strict.arguments; // !!! BAD IDEA
strict().caller;  // !!! BAD IDEA
strict().callee;  // !!! BAD IDEA
outer();          // !!! BAD IDEA

Really, it’s best not to access the caller of a function, the current function (except by naming it), or the arguments for a given function (except via arguments or by use of the named parameter) at all.

ES5 strict mode: self-limitation, not wish fulfillment

ES5 introduces the curious concept of strict mode. Strict mode, whose name and concept derive from the similar feature in Perl, is a new feature in ES5 whose purpose is to deliberately reduce the things you can do in JavaScript. Instead of a feature, it’s really more the absence of several features, within the scope of the strict-annotated code: with, particularly intrusive forms of eval, silent failure of writes to non-writable properties, silent failure of deletions of non-configurable properties, implicit creation of global-object properties, and so on. The goal of these removals is to simplify both the reasoning necessary to understand such code and the implementation of it in JavaScript engines: to sand away some rough edges in the language.

Magical stack-introspective properties of functions and arguments

Consider this code, and note the expected behavior (expected per the web, but not as part of ES3):

function getSelf() { return arguments.callee; }
assertEq(getSelf(), getSelf); // arguments.callee is the enclosing function

function outer()
{
  function inner() { return arguments.callee.caller; } // inner.caller === outer
  return inner();
}
assertEq(outer(), outer); // fun.caller === nearest function in stack that called fun, or null

function args2()
{
  return args2.arguments;
}
assertEq(args2(17)[0], 17); // fun.arguments === arguments object for nearest call to fun in stack, or null

Real-world JavaScript implementations take many shortcuts for top performance. These shortcuts are not (supposed to be) observable, except by timing the relevant functionality. Two common optimizations are function inlining and avoiding creating an arguments object. The above “features” play havoc with both of these optimizations (as well as others, one of which will be the subject of a forthcoming post).

Inlining a function should conceptually be equivalent to splatting the function’s contents in that location in the calling function and doing some α-renaming to ensure no names in the splatted body conflict with the surrounding code. The ability to access the calling function defeats this: there’s no function being invoked any more, so what does it even mean to ask for the function’s caller? (Don’t simply say you’d hard-code the surrounding function: how do you know which property lookups in the inlined code will occur upon precisely the function being called, looking for precisely the caller property?) It is also possible to access a function’s arguments through fun.arguments. While the “proper” behavior here is more obvious, implementing it would be a large hassle: either the arguments would have to be created when the function was inlined (in the general case where you can’t be sure the function will never be used this way), or you’d have to inline code in such a way as to be able to “work backward” to the argument values.

Speaking of arguments, in offering access to the corresponding function via arguments.callee it has the same problems as fun.caller. It also presents one further problem: in (some, mostly old) engines, arguments.caller provides access to the variables declared within that function when it was most recently called. (If you’re thinking security/integrity/optimization hazard, well, you now know why engines no longer support it.)

In sum these features are bad for optimization. Further, since they’re a form of dynamic scoping, they’re basically bad style in many other languages already.

Per ES5, SpiderMonkey no longer supports this stack-inspecting magic when it interacts with strict mode

As of the most recent Firefox nightly, SpiderMonkey now rejects code like that given above when it occurs in strict mode (more or less). (The properties in question are now generally implemented through a so-called “poison pill” mechanism, an immutable accessor property which throws a TypeError when retrieved or set.) The specific scenarios which we reject are as follows.

First, attempts to access the caller or arguments (except by directly naming the object) of a strict mode function throw a TypeError, because these properties are poison pills:

function strict()
{
  "use strict";
  strict.caller;    // !!! TypeError
  strict.arguments; // !!! TypeError
  return arguments; // direct name: perfectly cromulent
}
strict();

Second, attempts to access the enclosing function or caller variables via the arguments of a strict mode function throw a TypeError. These properties too are poison pills:

function strict()
{
  "use strict";
  arguments.callee; // !!! TypeError
  arguments.caller; // !!! TypeError
}
strict();

Third (and most trickily, because non-strict code is affected), attempts to access a function’s caller when that caller is in strict mode will throw a TypeError. This isn’t a poison pill, because if the "use strict" directive weren’t there it would still “work”:

function outer()
{
  "use strict";
  return inner();
}
function inner()
{
  return inner.caller; // !!! TypeError
}
outer();

But if there’s no strict mode in sight, nothing will throw exceptions, and what worked before will still work:

function fun()
{
  assertEq(fun.caller, null); // global scope
  assertEq(fun.arguments, arguments);
  assertEq(arguments.callee, fun);
  arguments.caller; // won't throw, won't do anything special
  return arguments;
}
fun();

Conclusion

With these changes, which are required by ES5, stack inspection is slowly going the way of the dodo. Don’t use it or rely on it! Even if you never use strict mode, beware the third change, for it still might affect you if you provide methods for other code to call. (But don’t expect to be able to avoid strict mode forever: I expect all JavaScript libraries will adopt strict mode in short order, given its benefits.)

(For those curious about new Error().stack, we’re still considering what to do about it. Regrettably, we may need to kill it for information-privacy reasons too, or at least censor it to something less useful. Nothing’s certain yet; stay tuned for further announcements should we make changes.)

You can experiment with a version of Firefox with these changes by downloading a TraceMonkey branch nightly; these changes should also make their way into mozilla-central nightlies shortly, if you’d rather stick to trunk builds. (Don’t forget to use the profile manager if you want to keep the settings you use with your primary Firefox installation pristine.)