What is "strict mode" and how is it used?

Its main purpose is to do more checking.

Just add "use strict"; at the top of your code, before anything else.

For example, blah = 33; is valid JavaScript. It means you create a completely global variable blah.

But in strict mode it's an error because you did not use the keyword "var" to declare the variable.

Most of the time you don't mean to create global variables in the middle of some arbitrary scope, so most of the time that blah = 33 is written it is an error and the programmer didn't actually want it to be a global variable, they meant to write var blah = 33.

It similarly disallows a lot of things that are technically valid to do. NaN = "lol" does not produce an error. It also doesn't change the value of NaN. Using strict this (and similar weird statements) produce errors. Most people appreciate this because there is no reason to ever write NaN = "lol", so there was most likely a typo.

Read more at the MDN page on strict mode.


One aspect of strict mode not already mentioned in Simon's answer is that strict mode sets this to undefined in functions invoked through function invocation.

So things like this

function Obj() {
   this.a = 12;
   this.b = "a";
   this.privilegedMethod = function () {
      this.a++;
      privateMethod();
   };

   function privateMethod() {
     this.b = "foo";
   }
}

will cause an error when privateMethod is called (since you can't add a property to undefined), rather than uselessly adding a b property to the global object.


Strict mode was added so that there would be an easily statically-analyzable subset of ECMAScript which would be a good target for future versions of the language. Strict mode was also designed in the hope that developers who limit themselves to strict mode would make fewer mistakes and that the bugs they do make would manifest in more obvious ways.

Harmony, which will hopefully become the next major version of ECMAScript is going to be built on top of ES5 strict.

Harmony builds on ES5 strict mode to avoid too many modes.

Some other language experiments also depend on strict mode. SES depends on ES5 strict mode's analyzability.

SES (Secure ECMAScript) Design Experiment

Design an Object Capability Programming Language by removing or repairing features in ES5/Strict.

There should be a straight-forward translation from SES to ES5/Strict.

Annex C of the standard explains the differences between strict mode and normal mode.

The strict mode restriction and exceptions

  • The identifiers "implements", "interface", "let", "package", "private", "protected", "public", "static", and "yield" are classified as FutureReservedWord tokens within strict mode code. (7.6.12 [?]).
  • 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.
  • 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.
  • 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). 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).
  • The identifier eval or arguments may not appear as the LeftHandSideExpression of an Assignment operator (11.13) or of a PostfixExpression (11.3) or as the UnaryExpression operated upon by a Prefix Increment (11.4.4) or a Prefix Decrement (11.4.5) operator. Arguments objects for strict mode functions define non-configurable accessor properties named "caller" and "callee" which throw a TypeError exception on access (10.6).
  • 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). 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).
  • It is a SyntaxError if strict mode code contains an ObjectLiteral with more than one definition of any data property (11.1.5). 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).
  • 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).
  • 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).
  • 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).
  • 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). It is a SyntaxError if a VariableDeclaration or VariableDeclarationNoIn occurs within strict code and its Identifier is eval or arguments (12.2.1).
  • Strict mode code may not include a WithStatement. The occurrence of a WithStatement in such a context is an SyntaxError (12.10).
  • 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)
  • It is a SyntaxError if the identifier eval or arguments appears within a FormalParameterList of a strict mode FunctionDeclaration or FunctionExpression (13.1)
  • 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).
  • An implementation may not extend, beyond that defined in this specification, meanings within strict mode functions of 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 (10.6, 13.2, 15.3.4.5.3).
  • 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.

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Javascript

Strict Mode

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