JavaScript Strict Mode

The fifth edition of the ECMAScript specification introduced Strict Mode. Strict Mode imposes a layer of constraint on JavaScript – intended to protect you from the more perilous aspects of the language.

While researching this article I wrote 38 tests covering all the Strict Mode rules as defined in the ES5 specification. You can see how your favorite browser shapes up by clicking here.

The code for each test is reproduced at the end of the article as an aid to understanding the specification. You can also run the tests manually by copying and pasting the source into the console. The full source code is on my github repo.

Firefox 4 and IE10 (preview 1) already fully support Strict Mode, and Chrome 12 is nearly there. Strict Mode is here to stay – let’s dive in…

How do I invoke Strict Mode?

Adding “use strict” as the first statement¹ in your JavaScript code will enforce Strict Mode over the entire source…

"use strict";
012; //Octal literal throws a SyntaxError in Strict Mode

 
Alternatively you can restrict Strict Mode enforcement to a given function, by adding a “use strict” statement to the first line¹ of the function body…

012; //No Error (Strict Mode not enforced globally)
function foo() {
    "use strict";
    x=3; //Strict Mode will throw an error for implicit globals
}
foo(); //ReferenceError (Strict Mode is enforced for function foo)

 
¹ @kangax pointed out that ‘use strict’ can appear anywhere within a leading set of string literal statements – presumably to allow future directives to act in parallel.
 
Do functions reflect the Strict Mode directives of their outer functions?

Inner functions defined within an outer function that carries the “use strict” directive, will also be subject to Strict Mode…

var wrapper = function(fn) {
  'use strict';
  var deleteNonConfigurable = function () {
    var obj = {};
    Object.defineProperty(obj, "name", {
      configurable: false
    });
    delete obj.name; //Will throw TypeError in Strict Mode
  }
  return deleteNonConfigurable;
}

wrapper()(); //TypeError (Strict Mode enforced)

 
However it is important to note, Strict Mode is not enforced on non-strict functions that are invoked inside the body of a strict function (either because they were passed as arguments or invoked using call or apply)…

var test = function(fn) {
  'use strict';
  fn();
}

var deleteNonConfigurable = function () {
  var obj = {};
  Object.defineProperty(obj, "name", {
    configurable: false
  });
  delete obj.name; //will throw TypeError in Strict Mode
}

test(deleteNonConfigurable); //no error (Strict Mode not enforced)

 
Why can’t I run global Strict Mode in my browser console?

When running code in firebug and other browser consoles, an initial “use strict” (outside a function) has no effect. This is because most console runners wrap all code in an eval call – so your “use strict” is no longer the first statement. A partial workaround is to wrap your code in a self invoking function starting with “use strict” (but even then, I found enforcement of Strict Mode within the console to be pretty flakey – particularly when using webkit developer tools – better to test your code in a web page):

(function() {
    "use strict";
    var a;
    var b;
    function bar() {
        x = 5; //Strict Mode will throw an error for implicit globals
    }
    bar(); //ReferenceError (Strict Mode is enforced)
})();

 
What happens if my browser doesn’t support Strict Mode?

Nothing. The “use strict” directive is simply a string statement that will be ignored by JavaScript engines that don’t support Strict Mode. This allows safe cross-browser use of Strict Mode syntax while ensuring built-in forward compatibility with browsers that might support Strict Mode in the future.

What are the rules of Strict Mode?

The restrictions defined by the Strict Mode specification encompass both load-time and runtime behaviors. Here’s a brief overview (each rule is covered in detail, with an example, in the next section):

Syntax Errors

In many cases, Strict Mode will prevent ambiguous or allegedly misleading code from even loading. Octal literals, duplicate property names, incorrect usage of delete and attempts to do anything dodgy with the eval and arguments keywords will throw a SyntaxError as will any use of the with statement (thanks to @brendaneich for clarifying why with was omitted)

The ‘this’ value

In Strict Mode the value of this will not be auto-coerced to an object. This is probably the most interesting part of Strict Mode and the one that is likely to have the most significant impact on developers. Most notably, if the first argument to call or apply is null or undefined, the this value of the invoked function will not be converted to the global object.

Implicit globals

Not many folks will argue against this one. Creation of implicit globals is almost always a mistake. in Strict Mode you’ll get a ReferenceError. That’ll teach you 😉

arguments.caller and arguments.callee

These useful properties are frowned upon in Strict Mode. Definitely controversial. The lesser used function.arguments and function.caller properties are also banned.

Object property definition violations

Attempting to perform an update on a property when its property definition defines otherwise will throw a TypeError in Strict Mode.

The Tests

Here is the full source code from my Strict Mode tests. Each set of tests is preempted by a comment lifted directly from the ECMAScript specification being tested. This version of the source is set to run in “console mode” – meaning it can be copied and pasted into a developer console and run as is. The same source run in “HTML mode” is used to generate the visual test page that I introduced at the top of the article. That source, with supporting objects is on my github repo. I’m sure to have made a few mistakes – feel free to let me know!

(function() {

  ////////////////////////////////
  //TEST UTILS...
  ////////////////////////////////

  var HTML_MODE = 0;
  var CONSOLE_MODE = 1;

  var mode = CONSOLE_MODE;

  var banner = document.getElementById('banner');
  var currentTestWidget;

  var testsPassed = 0;
  var totalTests = 0;

  window.console = window.console || {log:alert};

  function testException(testName, code, expectedError) {
      'use strict';
      startTest(testName);
      try {
          expectedError == SyntaxError ? eval(code) : code();
          finishTest(false);
      } catch (e) {
          (e instanceof expectedError) ? finishTest(true) : finishTest(false);
      }
  }

  function testValue(testName, fn, expectedValue, options) {
      'use strict';
      options = options || {};
      startTest(testName);
      var result = (fn.apply(options.ctx, options.args || []) === expectedValue);
      finishTest(result);
  }

  function startTest(testName) {
    if (mode == CONSOLE_MODE) {
      console.log("testing..." + testName);
    } else {
      this.currentWidget = document.createElement('DIV');
      this.currentWidget.innerHTML = testName;
      document.body.appendChild(this.currentWidget);
    }
  }

  function finishTest(passed) {
    totalTests++;
    passed && testsPassed++;
    var result = passed ? "passed" : "failed";
    if (mode == CONSOLE_MODE) {
      console.log(result);
    } else {
      this.currentWidget.className = result;
    }
  }

  function startAll() {
    if (mode == HTML_MODE) {
      banner.innerHTML += [":", browser.browserName, browser.fullVersion].join(' ');
    }
  }

  function finishAll() {
    var result = ["","(", testsPassed, "out of", totalTests, "tests passed", ")"].join(' ');
    if (mode == HTML_MODE) {
      banner.innerHTML += result;
    } else if (mode == CONSOLE_MODE) {
      console.log(result);
    }
  }

  ////////////////////////////////
  //THE TESTS...
  ////////////////////////////////

  startAll();

  // A conforming implementation, when processing strict mode code, may not extend the
  //syntax of NumericLiteral (7.8.3) to include OctalIntegerLiteral as described in B.1.1.
  testException("no octal literals", '012', SyntaxError);

  // A conforming implementation, when processing strict mode code (see 10.1.1), may not
  //extend the syntax of EscapeSequence to include OctalEscapeSequence as described in B.1.2.
  testException("no octal escape sequence", '"\\012"', SyntaxError);

  // Assignment to an undeclared identifier or otherwise unresolvable reference does not
  //create a property in the global object. When a simple assignment occurs within strict
  //mode code, its LeftHandSide must not evaluate to an unresolvable Reference. If it does
  //a ReferenceError exception is thrown (8.7.2).
  testException(
    "no implied globals",
    function () {'use strict'; x = 3;},
    ReferenceError
  );

  //The LeftHandSide also may not be a reference to a data property with the attribute
  //value {[[Writable]]:false}, to an accessor property with the attribute value
  //{[[Set]]:undefined}, nor to a non-existent property of an object whose [[Extensible]]
  //internal property has the value false. In these cases a TypeError exception is thrown
  //(11.13.1).
  var assignToNonWritable = function () {
      'use strict';
      var obj = {};
      Object.defineProperty(obj, "name", {
          writable: false
      });
      obj.name = "octopus";
  }

  testException("can't assign to non-writable properties", assignToNonWritable, TypeError);

  var assignWhenSetterUndefined = function () {
      'use strict';
      var obj = {};
      Object.defineProperty(obj, "name", {
          set: undefined
      });
      obj.name = "cuttlefish";
  }

  testException("can't assign when setter undefined", assignWhenSetterUndefined, TypeError);

  var assignToNonExtensible = function () {
      'use strict';
      var obj = {};
      Object.preventExtensions(obj);
      obj.name = "jellyfish";
  }

  testException("can't assign to non extensible", assignToNonExtensible, TypeError);

  //The identifier eval or arguments may not appear as the LeftHandSideExpression of an
  //Assignment operator (11.13) or of a PostfixExpression (11.13) or as the UnaryExpression
  //operated upon by a Prefix Increment (11.4.4) or a Prefix Decrement (11.4.5) operator.
  testException("can't assign to eval", "eval=3", SyntaxError);
  testException("can't assign to arguments", "arguments=3", SyntaxError);
  testException("can't postfix eval", "eval++", SyntaxError);
  testException("can't postfix arguments", "arguments++", SyntaxError);
  testException("can't prefix eval", "++eval", SyntaxError);
  testException("can't prefix arguments", "++arguments", SyntaxError);

  //Arguments objects for strict mode functions define non-configurable accessor properties
  //named "caller" and "callee" which throw a TypeError exception on access (10.6).
  testException(
    "can't use arguments.caller",
    function () {'use strict'; arguments.caller;},
    TypeError
  );

  testException(
    "can't use arguments.callee",
    function () {'use strict'; arguments.callee},
    TypeError
  );

  //Arguments objects for strict mode functions do not dynamically share their array indexed
  //property values with the corresponding formal parameter bindings of their functions. (10.6).
  var assignToArguments = function (x) {
    'use strict';
    arguments[0] = 3;
    return x;
  }

  testValue(
    "arguments not bound to formal params",
    assignToArguments,
    5,
    {args: [5]}
  );

  //For strict mode functions, if an arguments object is created the binding of the local
  //identifier arguments to the arguments object is immutable and hence may not be the
  //target of an assignment expression. (10.5).
  var assignToFormalParams = function (x) {
      'use strict';
      x = 3;
      return arguments[0];
  }

  testValue(
    "arguments object is immutable",
    assignToFormalParams,
    5,
    {args: [5]}
  );

  //It is a SyntaxError if strict mode code contains an ObjectLiteral with more than one
  //definition of any data property (11.1.5).
  testException("no duplicate properties", "({a:1, a:2})", SyntaxError);

  //It is a SyntaxError if the Identifier "eval" or the Identifier "arguments occurs as the
  //Identifier in a PropertySetParameterList of a PropertyAssignment that is contained in
  //strict code or if its FunctionBody is strict code (11.1.5).
  testException(
    "eval not allowed in propertySetParameterList",
    "({set a(eval){ }})",
    SyntaxError
  );

  testException(
    "arguments not allowed in propertySetParameterList",
    "({set a(arguments){ }})",
    SyntaxError
  );

  //Strict mode eval code cannot instantiate variables or functions in the variable environment
  //of the caller to eval. Instead, a new variable environment is created and that environment
  //is used for declaration binding instantiation for the eval code (10.4.2).
  testException(
    "eval cannot create var in calling context",
    function () {'use strict'; eval('var a = 99'); a},
    ReferenceError
  );

  //If this is evaluated within strict mode code, then the this value is not coerced to an object.
  //A this value of null or undefined is not converted to the global object and primitive values
  //are not converted to wrapper objects. The this value passed via a function call (including
  //calls made using Function.prototype.apply and Function.prototype.call) do not coerce the
  //passed this value to an object (10.4.3, 11.1.1, 15.3.4.3, 15.3.4.4).
  var getThis = function () {
      'use strict';
      return this;
  }

  testValue(
    "this is not coerced",
    getThis,
    4,
    {ctx: 4}
  );

  testValue(
    "no global coercion for null",
    getThis,
    null,
    {ctx: null}
  );

  //When a delete operator occurs within strict mode code, a SyntaxError is thrown if its
  //UnaryExpression is a direct reference to a variable, function argument, or function name
  //(11.4.1).
  testException("can't delete variable directly", "var a = 3; delete a", SyntaxError);
  testException("can't delete argument", "function(a) {delete a}", SyntaxError);
  testException("can't delete function by name", "function fn() {}; delete fn", SyntaxError);

  //When a delete operator occurs within strict mode code, a TypeError is thrown if the
  //property to be deleted has the attribute { [[Configurable]]:false } (11.4.1).
  var deleteNonConfigurable = function () {
      'use strict';
      var obj = {};
      Object.defineProperty(obj, "name", {
          configurable: false
      });
      delete obj.name;
  }

  testException("error when deleting non configurable", deleteNonConfigurable, TypeError);

  //It is a SyntaxError if a VariableDeclaration or VariableDeclarationNoIn occurs within
  //strict code and its Identifier is eval or arguments (12.2.1).
  testException("can't use eval as var name", "var eval;", SyntaxError);
  testException("can't use arguments as var name", "var arguments;", SyntaxError);

  //Strict mode code may not include a WithStatement. The occurrence of a WithStatement
  //in such a context is an SyntaxError (12.10).
  testException("can't use with", "with (Math) {round(sqrt(56.67))}", SyntaxError);

  //It is a SyntaxError if a TryStatement with a Catch occurs within strict code and the
  //Identifier of the Catch production is eval or arguments (12.14.1)
  testException("can't use eval as catch id", "try {'cake'} catch(eval) {}", SyntaxError);
  testException("can't use arguments as catch id", "try {'cake'} catch(arguments) {}", SyntaxError);

  //It is a SyntaxError if the identifier eval or arguments appears within a
  //FormalParameterList of a strict mode FunctionDeclaration or FunctionExpression (13.1)
  testException("can't use eval as formal param", "function(eval) {}", SyntaxError);
  testException("can't use arguments as formal param", "function(arguments) {}", SyntaxError);

  //A strict mode function may not have two or more formal parameters that have the same
  //name. An attempt to create such a function using a FunctionDeclaration, FunctionExpression,
  //or Function constructor is a SyntaxError (13.1, 15.3.2).
  testException("can't duplicate formal params", "function(me, me, me) {}", SyntaxError);

  //An implementation may not associate special meanings within strict mode functions to
  //properties named caller or arguments of function instances. ECMAScript code may not
  //create or modify properties with these names on function objects that correspond to
  //strict mode functions (13.2).
  testException(
    "can't use caller obj of function",
    function () {'use strict'; (function () {}).caller},
    TypeError
  );

  //It is a SyntaxError to use within strict mode code the identifiers eval or arguments as
  //the Identifier of a FunctionDeclaration or FunctionExpression or as a formal parameter
  //name (13.1). Attempting to dynamically define such a strict mode function using the
  //Function constructor (15.3.2) will throw a SyntaxError exception.
  testException("can't use eval as function name", "function eval() {}", SyntaxError);
  testException("can't use arguments as function name", "function arguments() {}", SyntaxError);

  var functionConstructorStr = "new Function('eval', 'use strict')";
  testException("can't use eval as param name via constructor", functionConstructorStr, SyntaxError);

  finishAll();

})();

 
Wrap up

Whether restricting access to the language makes for better developers is questionable – but lets save that debate for another time. In its defense, Strict Mode is a nice compromise between a complete language change (which would have broken backwards-compatibility) and doing nothing (which would have raised the hackles of those who insist that the more egregious parts of the language must be phased out). If nothing else, Strict Mode is meat and drink for those of us obsessed with the nuances of this fascinating language. Enjoy!

Further Reading

ECMA-262 5th Edition: The Strict Mode of ECMAScript

Asen Bozhilov: Strict tester
A thoughtful, thorough set of Strict Mode tests.

Juriy Zaytsev (aka “kangax”): ECMAScript 5 compatibility table — Strict mode support
Another good source, part of a major compatibility reference by kangax (thanks also to kangax for a last minute IM chat about these tests)

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