ECMAScript 5 strict mode in Firefox 4

Editor’s note: This article is posted by Chris Heilmann but authored by Jeff Walden – credit where credit is due.

Developers in the Mozilla community have made major improvements to the JavaScript engine in Firefox 4. We have devoted much effort to improving performance, but we’ve also worked on new features. We have particularly focused on ECMAScript 5, the latest update to the standard underlying JavaScript.

Strict mode is arguably the most interesting new feature in ECMAScript 5. It’s a way to opt in to a restricted variant of JavaScript. Strict mode isn’t just a subset: it intentionally has different semantics from normal code. Browsers not supporting strict mode will run strict mode code with different behavior from browsers that do, so don’t rely on strict mode without feature-testing for support for the relevant aspects of strict mode.

Strict mode code and non-strict mode code can coexist, so scripts can opt into strict mode incrementally. Strict mode blazes a path to future ECMAScript editions where new code with a particular <script type="..."> will likely automatically be executed in strict mode.

What does strict mode do? First, it eliminates some JavaScript pitfalls that didn’t cause errors by changing them to produce errors. Second, it fixes mistakes that make it difficult for JavaScript engines to perform optimizations: strict mode code can sometimes be made to run faster than identical code that’s not strict mode. Firefox 4 generally hasn’t optimized strict mode yet, but subsequent versions will. Third, it prohibits some syntax likely to be defined in future versions of ECMAScript.

Invoking strict mode

Strict mode applies to entire scripts or to individual functions. It doesn’t apply to block statements enclosed in {} braces; attempting to apply it to such contexts does nothing. eval code, event handler attributes, strings passed to setTimeout, and the like are entire scripts, and invoking strict mode in them works as expected.

Strict mode for scripts

To invoke strict mode for an entire script, put the exact statement "use strict"; (or 'use strict';) before any other statements.

// Whole-script strict mode syntax
"use strict";
var v = "Hi!  I'm a strict mode script!";

This syntax has a trap that has already bitten a major site: it isn’t possible to blindly concatenate non-conflicting scripts. Consider concatenating a strict mode script with a non-strict mode script: the entire concatenation looks strict! The inverse is also true: non-strict plus strict looks non-strict. Concatenation of strict mode scripts with each other is fine, and concatenation of non-strict mode scripts is fine. Only crossing the streams by concatenating strict and non-strict scripts is problematic.

Strict mode for functions

Likewise, to invoke strict mode for a function, put the exact statement "use strict"; (or 'use strict';) in the function’s body before any other statements.

function strict()
  // Function-level strict mode syntax
  'use strict';
  function nested() { return "And so am I!"; }
  return "Hi!  I'm a strict mode function!  " + nested();
function notStrict() { return "I'm not strict."; }

Changes in strict mode

Strict mode changes both syntax and runtime behavior. Changes generally fall into these categories:

  • Converting mistakes into errors (as syntax errors or at runtime)
  • Simplifying how the particular variable for a given use of a name is computed
  • Simplifying eval and arguments
  • Making it easier to write “secure” JavaScript
  • Anticipating future ECMAScript evolution

Converting mistakes into errors

Strict mode changes some previously-accepted mistakes into errors. JavaScript was designed to be easy for novice developers, and sometimes it gives operations which should be errors non-error semantics. Sometimes this fixes the immediate problem, but sometimes this creates worse problems in the future. Strict mode treats these mistakes as errors so that they’re discovered and promptly fixed.

First, strict mode makes it impossible to accidentally create global variables. In normal JavaScript, mistyping a variable in an assignment creates a new property on the global object and continues to “work” (although future failure is possible: likely, in modern JavaScript). Assignments which would accidentally create global variables instead throw errors in strict mode:

"use strict";
mistypedVaraible = 17; // throws a ReferenceError

Second, strict mode makes assignments which would otherwise silently fail throw an exception. For example, NaN is a non-writable global variable. In normal code assigning to NaN does nothing; the developer receives no failure feedback. In strict mode assigning to NaN throws an exception. Any assignment that silently fails in normal code will throw errors in strict mode:

"use strict";
NaN = 42; // throws a TypeError
var obj = { get x() { return 17; } };
obj.x = 5; // throws a TypeError
var fixed = {};
fixed.newProp = "ohai"; // throws a TypeError

Third, if you attempt to delete undeletable properties, strict mode throws errors (where before the attempt would simply have no effect):

"use strict";
delete Object.prototype; // throws a TypeError

Fourth, strict mode requires that all properties named in an object literal be unique. Normal code may duplicate property names, with the last one determining the property’s value. But since only the last one does anything, the duplication is simply a vector for bugs, if the code is modified to change the property value other than by changing the last instance. Duplicate property names are a syntax error in strict mode:

"use strict";
var o = { p: 1, p: 2 }; // !!! syntax error

Fifth, strict mode requires that function argument names be unique. In normal code the last duplicated argument hides previous identically-named arguments. Those previous arguments remain available through arguments[i], so they’re not completely inaccessible. Still, this hiding makes little sense and is probably undesirable (it might hide a typo, for example), so in strict mode duplicate argument names are a syntax error:

function sum(a, a, c) // !!! syntax error
  "use strict";
  return a + b + c; // wrong if this code ran

Sixth, strict mode forbids octal syntax. Octal syntax isn’t part of ECMAScript, but it’s supported in all browsers by prefixing the octal number with a zero: 0644 === 420 and "\045" === "%". Novice developers sometimes believe a leading zero prefix has no semantic meaning, so they use it as an alignment device — but this changes the number’s meaning! Octal syntax is rarely useful and can be mistakenly used, so strict mode makes octal a syntax error:

"use strict";
var sum = 015 + // !!! syntax error
          197 +

Simplifying variable uses

Strict mode simplifies how variable uses map to particular variable definitions in the code. Many compiler optimizations rely on the ability to say that this variable is stored in this location: this is critical to fully optimizing JavaScript code. JavaScript sometimes makes this basic mapping of name to variable definition in the code impossible to perform except at runtime. Strict mode removes most cases where this happens, so the compiler can better optimize strict mode code.

First, strict mode prohibits with. The problem with with is that any name in it might map either to a property of the object passed to it, or to a variable in surrounding code, at runtime: it’s impossible to know which beforehand. Strict mode makes with a syntax error, so there’s no chance for a name in a with to refer to an unknown location at runtime:

"use strict";
var x = 17;
with (obj) // !!! syntax error
  // If this weren't strict mode, would this be var x, or
  // would it instead be obj.x?  It's impossible in general
  // to say without running the code, so the name can't be
  // optimized.

The simple alternative of assigning the object to a variable, then accessing the corresponding property on that variable, stands ready to replace with.

Second, eval of strict mode code does not introduce new variables into the surrounding code. 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;
var evalX = eval("'use strict'; var x = 42; x");
assert(x === 17);
assert(evalX === 42);

Relatedly, if the function eval is invoked by an expression of the form eval(...) in strict mode code, the code will be evaluated as strict mode code. The code may explicitly invoke strict mode, but it’s unnecessary to do so.

function strict1(str)
  "use strict";
  return eval(str); // str will be treated as strict mode code
function strict2(f, str)
  "use strict";
  return f(str); // not eval(...): str is strict iff it invokes strict mode
function nonstrict(str)
  return eval(str); // str is strict iff it invokes strict mode
strict1("'Strict mode code!'");
strict1("'use strict'; 'Strict mode code!'");
strict2(eval, "'Non-strict code.'");
strict2(eval, "'use strict'; 'Strict mode code!'");
nonstrict("'Non-strict code.'");
nonstrict("'use strict'; 'Strict mode code!'");

Third, strict mode forbids deleting plain names. Thus names in strict mode eval code behave identically to names in strict mode code not being evaluated as the result of eval. Using delete name in strict mode is a syntax error:

"use strict";
eval("var x; delete x;"); // !!! syntax error

Making eval and arguments simpler

Strict mode makes arguments and eval less bizarrely magical. Both involve a considerable amount of magical behavior in normal code: eval to add or remove bindings and to change binding values, and arguments by its indexed properties aliasing named arguments. Strict mode makes great strides toward treating eval and arguments as keywords, although full fixes will not come until a future edition of ECMAScript.

First, the names eval and arguments can’t be bound or assigned in language syntax. All these attempts to do so are syntax errors:

"use strict";
eval = 17;
var obj = { set p(arguments) { } };
var eval;
try { } catch (arguments) { }
function x(eval) { }
function arguments() { }
var y = function eval() { };
var f = new Function("arguments", "'use strict'; return 17;");

Second, strict mode code doesn’t alias properties of arguments objects created within it. In normal code within a function whose first argument is arg, setting arg also sets arguments[0], and vice versa (unless no arguments were provided or arguments[0] is deleted). For strict mode functions, arguments objects store the original arguments when the function was invoked. The value of arguments[i] does not track the value of the corresponding named argument, nor does a named argument track the value in the corresponding arguments[i].

function f(a)
  "use strict";
  a = 42;
  return [a, arguments[0]];
var pair = f(17);
assert(pair[0] === 42);
assert(pair[1] === 17);

Third, arguments.callee is no longer supported. In normal code arguments.callee refers to the enclosing function. This use case is weak: simply name the enclosing function! Moreover, arguments.callee substantially hinders optimizations like inlining functions, because it must be made possible to provide a reference to the un-inlined function if arguments.callee is accessed. For strict mode functions, arguments.callee is a non-deletable property which throws an error when set or retrieved:

"use strict";
var f = function() { return arguments.callee; };
f(); // throws a TypeError

“Securing” JavaScript

Strict mode makes it easier to write “secure” JavaScript. Some websites now provide ways for users to write JavaScript which will be run by the website on behalf of other users. JavaScript in browsers can access the user’s private information, so such JavaScript must be partially transformed before it is run, to censor access to forbidden functionality. JavaScript’s flexibility makes it effectively impossible to do this without many runtime checks. Certain language functions are so pervasive that performing runtime checks has considerable performance cost. A few strict mode tweaks, plus requiring that user-submitted JavaScript be strict mode code and that it be invoked in a certain manner, substantially reduce the need for those runtime checks.

First, the value passed as this to a function in strict mode isn’t boxed into an object. For a normal function, this is always an object: the provided object if called with an object-valued this; the value, boxed, if called with a Boolean, string, or number this; or the global object if called with an undefined or null this. (Use call, apply, or bind to specify a particular this.) Automatic boxing is a performance cost, but exposing the global object in browsers is a security hazard, because the global object provides access to functionality “secure” JavaScript environments must invariably. Thus for a strict mode function, the specified this is used unchanged:

"use strict";
function fun() { return this; }
assert(fun() === undefined);
assert( === 2);
assert(fun.apply(null) === null);
assert( === undefined);
assert(fun.bind(true)() === true);

(Tangentially, built-in methods also now won’t box this if it is null or undefined. [This change is independent of strict mode but is motivated by the same concern about exposing the global object.] Historically, passing null or undefined to a built-in method like Array.prototype.sort() would act as if the global object had been specified. Now passing either value as this to most built-in methods throws a TypeError. Booleans, numbers, and strings are still boxed by these methods: it’s only when these methods would otherwise act on the global object that they’ve been changed.)

Second, in strict mode it’s no longer possible to “walk” the JavaScript stack via commonly-implemented extensions to ECMAScript. In normal code with these extensions, when a function fun is in the middle of being called, fun.caller is the function that most recently called fun, and fun.arguments is the arguments for that invocation of fun. Both extensions are problematic for “secure” JavaScript, because they allow “secured” code to access “privileged” functions and their (potentially unsecured) arguments. If fun is in strict mode, both fun.caller and fun.arguments are non-deletable properties which throw an error when set or retrieved:

function restricted()
  "use strict";
  restricted.caller;    // throws a TypeError
  restricted.arguments; // throws a TypeError
function privilegedInvoker()
  return restricted();

Third, arguments for strict mode functions no longer provide access to the corresponding function call’s variables. In some old ECMAScript implementations arguments.caller was an object whose properties aliased variables in that function. This is a security hazard because it breaks the ability to hide privileged values via function abstraction; it also precludes most optimizations. For these reasons no recent browsers implement it. Yet because of its historical functionality, arguments.caller for a strict mode function is also a non-deletable property which throws an error when set or retrieved:

"use strict";
function fun(a, b)
  "use strict";
  var v = 12;
  return arguments.caller; // throws a TypeError
fun(1, 2); // doesn't expose v (or a or b)

Paving the way for future ECMAScript versions

Future ECMAScript versions will likely introduce new syntax, and strict mode in ECMAScript 5 applies some restrictions to ease the transition. It will be easier to make some changes if the foundations of those changes are prohibited in strict mode.

First, in strict mode a short list of identifiers become reserved keywords. These words are implements, interface, let, package, private, protected, public, static, and yield. In strict mode, then, you can’t name or use variables or arguments with these names. A Mozilla-specific caveat: if your code is JavaScript 1.7 or greater (you’re chrome code, or you’ve used the right <script type="">) and is strict mode code, let and yield have the functionality they’ve had since those keywords were first introduced. But strict mode code on the web, loaded with <script src=""> or <script>...</script>, won’t be able to use let/yield as identifiers.

Second, strict mode prohibits function statements not at the top level of a script or function. In normal code in browsers, function statements are permitted “everywhere”. This is not part of ES5! It’s an extension with incompatible semantics in different browsers. Future ECMAScript editions hope to specify new semantics for function statements not at the top level of a script or function. Prohibiting such function statements in strict mode “clears the deck” for specification in a future ECMAScript release:

"use strict";
if (true)
  function f() { } // !!! syntax error
for (var i = 0; i < 5; i++)
  function f2() { } // !!! syntax error
function baz() // kosher
  function eit() { } // also kosher

This prohibition isn’t strict mode proper, because such function statements are an extension. But it is the recommendation of the ECMAScript committee, and browsers will implement it.

Strict mode in browsers

Firefox 4 is the first browser to fully implement strict mode. The Nitro engine found in many WebKit browsers isn’t far behind with nearly-complete strict mode support. Chrome has also started to implement strict mode. Internet Explorer and Opera haven’t started to implement strict mode; feel free to send those browser makers feedback requesting strict mode support.

Browsers don’t reliably implement strict mode, so don’t blindly depend on it. Strict mode changes semantics. Relying on those changes will cause mistakes and errors in browsers which don’t implement strict mode. Exercise caution in using strict mode, and back up reliance on strict mode with feature tests that check whether relevant features of strict mode are implemented.

To test out strict mode, download a Firefox nightly and start playing. Also consider its restrictions when writing new code and when updating existing code. (To be absolutely safe, however, it’s probably best to wait to use it in production until it’s shipped in browsers.)

About Chris Heilmann

Evangelist for HTML5 and open web. Let's fix this!

More articles by Chris Heilmann…


  1. JulienW

    Thanks for this Mozilla, that’s just great. Let’s just use “use strict” during development for now and remove it for production until it’s shipped and we’re sure it’s bugfree. Or not, let’s use it now.

    Will this get into next beta ? I don’t understand if betas comes directly from the trunk or if Firefox 4 is already on its own branch (although it doesn’t seem so).

    January 25th, 2011 at 08:55

    1. Jeff Walden

      It will be in the next beta release. Numerous portions of strict mode are in previous betas as well; the eval variable binding change was the last big bit we were working on implementing before we were willing to say strict mode was “done”. (“done” because bugs will remain, and in some sense probably always will remain.)

      January 25th, 2011 at 10:08

  2. Jeff Walden

    “Tangentially, built-in methods also now won’t box this if it is null or undefined.”

    It belatedly occurs to me that we haven’t implemented this yet: bug 619283. We’ll fix this fairly shortly, but if you happen to be particularly interested in “secure” variant functionality, hold off on relying on it until we fix this. (Not that you were likely to start relying on it this early anyway! You’d have to be insanely forward-looking to be doing that now. :-) )

    January 25th, 2011 at 09:25

  3. oliver

    What is missing from JSC’s strict mode?

    January 25th, 2011 at 09:49

    1. Jeff Walden

      The last time I checked there were a few niggling bits that kangax’s strict mode page was still flagging. Also, I filed bug 53014 and CC’d you yesterday over the one last bit I very nearly forgot before writing this post (we literally timed this post to be immediately after today’s nightly build were done to have that change in them). Aside from those bits I think you’re set, at least as far as features go (bugs like the eval tests I mentioned to you in IRC a couple weeks back aside). But I admit to not following Nitro development all that closely, so I’m mostly relying on others’ evaluations plus a very small bit of my own testing.

      January 25th, 2011 at 10:06

  4. Weaner

    Isn’t “function f() { }” always the same as “var f = function () {}”? I remember Douglas Crockford saying that in one of his videos. Does this mean that latter also is disallowed in such cases?

    January 25th, 2011 at 10:32

    1. Jeff Walden

      It’s not the same. In the two cases where ECMAScript specifies behavior, function statements are processed before any code in that scope (be it program or function) executes. Thus this example works without throwing an exception:

      var v = g();
      function g() { return "processed before other statements execute"; }

      Likewise for this one:

      function f()
        return g();
        function g() { return "processed before other statements"; }

      If instead you assigned into a var in those examples, the assignment would occur in source order, so the examples would fail — the assignments would happen after the calls to the function, but the call attempt would throw because the variables would be undefined.

      January 25th, 2011 at 12:38

  5. Philip Jägenstedt

    Are there any optimizations made possible by strict mode that aren’t possible without it as well? To someone who hasn’t implemented an ECMAScript engine (me) it looks like all the optimizations mentioned could be done for non-strict code with some analysis during parsing. In the simplest approach, check during parsing if there are any instances of eval, with, arguments.callee, arguments.caller, etc and if there wasn’t, perform the clever optimizations you say strict mode enables. Are there loopholes that would make the bookkeeping more expensive than the gains from the optimizations made possible? On the face of things that seems unlikely, as the bookkeeping would be more or less identical to what you need to do to throw the new syntax errors. Maybe the magic sauce is in some of the new runtime behavior? Am I missing something, or is strict-mode actually not really that useful for performance?

    January 25th, 2011 at 13:28

    1. Jeff Walden

      That’s basically what SpiderMonkey does. It’s what other high-quality engines do. But there are wrinkles.

      “some analysis” turns out to be too much in many cases. You can’t spend too much time on analysis, else the time spent will offset the win. Making it special enough to adapt to register allocation, say, is nigh-impossible. You could perhaps only optimize on subsequent re-parses, but then you’re getting a bit more intelligence than a seemingly simple fix. We (and other browsers, blame SunSpider’s dumb JS code) do that sometimes — if you recompile the exact same eval code at the exact same location, we cache the “script” for the code — but it’s not as easy as you make it sound to implement. It would also probably be pretty fragile.

      Really, the new runtime behavior is much of the possible win. Without that you can’t perform that name optimization which underlies most optimizations JS engines can perform. You might be able to fast-path somewhat — only sometimes — but it is a good bit of work. And really, most JS developers agree the stuff strict mode kills is not good form (arguments.callee is probably the only one that might not fit that, but that’s mostly because of old IE bugs that make named function expressions not work properly, somewhat).

      January 26th, 2011 at 18:22

    2. Jeff Walden

      Answering somewhat more precisely: strict mode lets you optimize name bindings even in the presence of eval. So if I name an outer variable in strict mode code within a function that uses eval, I know that that’ll always refer to the outer variable, and never to a binding introduced by that eval. This is a place where current JS engines just give up — when they see eval they throw all optimizations (like that one) out the window. They must: without strict mode semantics the eval might introduce a local variable that hides the outer name. If you make eval respect strict mode, then, you can optimize this name reference where you couldn’t before.

      To be sure, you can imagine super-clever cheating that optimizes even this case and somehow avoids getting caught, in the words of the Smalltalk maxim. SpiderMonkey might implement some rocket surgery like that at some point. It still seems better to me to avoid the need for it by “fixing” the language. :-)

      I should also note that the end game for these strict mode fixes is for ES6 to not use a shared global object when running scripts in the even-more-opt-in mode enabled by a type="" on the script element. With strict mode restrictions applied to the entire script, and with the shared global object taken out of the picture, all name use to binding mappings can be determined at compilation time, before execution. And that gives you compile-time detection and rejection of mistyped and undeclared variable names. No more pussyfooting around with making assignment to undeclared global variables a runtime error, that only happens if you run that code! If you screw up anywhere, the code just won’t work at all. You can’t do better than reporting the error at the earliest possible point like this.

      January 28th, 2011 at 10:44

  6. oxdef

    Hmm, syntax for statement is not clear but statement irself is good idea.
    Like in Perl.

    January 25th, 2011 at 14:06

  7. marcoos

    Awesome post by Chris. I translated it into Polish. :)

    Polish translation of this MozHacks post / polskie tłumaczenie tej notki:

    January 25th, 2011 at 14:08

  8. Daniel Kirsch

    Is there a way to enable strict mode in general for all scripts? At least within a chrome environment for addons or XULRunner based apps.

    January 25th, 2011 at 17:18

    1. Jeff Walden

      There isn’t at the moment. I’m not sure whether or not it would be worth adding one; feel free to file a bug.

      January 25th, 2011 at 17:30

  9. Steffen

    How does this relate to the javascript.options.strict pref?
    Is that a different strict mode?

    January 26th, 2011 at 01:14

    1. Jeff Walden

      It’s a different strict mode. How awesome is that? Perhaps we should rename our mode to Firebird mode. Thoughts?

      (…I kid!)

      Seriously, the collision is unfortunate, but strict mode is the right name for the language feature, and it’s just our loss that we implemented something somewhat similar with the same name but which is invoked far differently and has very different behavior in some cases.

      January 26th, 2011 at 18:27

  10. Neil Rashbrook

    Are all of the strict mode errors exposed in non-strict mode as warnings (with javascript.options.strict turned on)?

    January 26th, 2011 at 04:12

    1. Jeff Walden

      Most are, I believe, but the comprehensive answer is no, because with triggers no warnings. (It used to, but warnings just weren’t a big enough stick to get developers to change their code, so we don’t spam about with.)

      January 26th, 2011 at 09:02

  11. Jethro Larson

    I don’t see strict mode getting much use until it actually enables something. Web devs will just see it as new constraints that just make it harder to get stuff done. If strict mode is actually more performant or includes features that let you write more, faster, then the adoption may raise, but it might not happen otherwise.

    January 26th, 2011 at 11:45

  12. JulienW

    Jethro, what you don’t understand is that strict mode enables you to make less errors : with it you can detect unwanted errors before they actually happen.

    All Javascript “features” removed by “strict mode” are just very bad practices.

    January 26th, 2011 at 15:31

  13. uploading

    Firefox is very distinctive.

    January 29th, 2011 at 14:12

  14. OldDog

    I’m all for zapping developers into writing proper code with strict rules but “use strict”;? seriously, couldn’t think of a better syntax?

    How about?


    or just plain

    use strict; ?

    What’s wrong with that?

    February 27th, 2011 at 06:00

    1. Jeff Walden

      I don’t disagree. And, seeing how many sites have wrongly, early, incorrectly started using strict mode (tagging stuff as strict that really isn’t, e.g. with assignments to undeclared globals, use of arguments.callee, and the like), I think were we to do it over again we might have chosen a non-backwards-compatible syntax. But at this point it’s water under the bridge. Strict mode syntax is what it is, no use crying over spilled milk.

      February 28th, 2011 at 11:14

  15. JulienW

    Because it must be backward compatible with old engines.

    February 28th, 2011 at 08:27

  16. PabloCubico

    Wouldn’t be dangerous to use strict mode in development but NOT in production?

    As the docs say: “Strict mode changes semantics”. So having it removed in production would be the same as using a browser that doesn’t support it.

    June 4th, 2011 at 05:06

    1. JulienW

      As I understand (but I may be wrong), “strict mode” is a subset of “legacy javascript”. So it should be safe.

      (right ?)

      June 6th, 2011 at 04:32

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