To instruct the browser to use more restrictive javascript mode, it is possible to use a “use strict” directive in javascript context. This feature is possible since EcmaScript Version 5.

Example:

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(function () {
    "use strict";
    // Strict code comes in here
 
}());

Another example in HTML body context:

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<script type="text/javascript">
    "use strict";
    // Strict code comes in here
    foo = 123; // this is not possible anymore..
    var bar = 123; // but this is
</script>

And again in HTML body context, but with local scoping:

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<script type="text/javascript">
(function () {
    "use strict";
    // Strict code comes in here
 
}());
</script>

The Strict mode has the following restrictions 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.

More about strict mode: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions_and_function_scope/Strict_mode