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5.7.3 “use strict”

debugger was formally added to the language by ECMAScript 5, but it has been implemented by major browser vendors for quite some time. Note that it is not enough to have a debugger available: the debugger statement won’t start the debugger for you. If a debugger is already running, however, this statement will cause a breakpoint. If you use the Firebug debugging extension for Firefox, for example, you must have Firebug enabled for the web page you want to debug in order for the debuggerstatement to work.


5.7.3 “use strict”

"use strict" is a directive introduced in ECMAScript 5. Directives are not statements (but are close enough that "use strict" is documented here). There are two important differences between the "use strict" directive and regular statements:

The purpose of a "use strict"directive is to indicate that the code that follows (in the script or function) is strict code. The top-level (nonfunction) code of a script is strict code if the script has a "use strict" directive. A function body is strict code if it is defined within strict code or if it has a "use strict" directive. Code passed to the eval() method is strict code if eval() is called from strict code or if the string of code includes a "use strict" directive.

Strict code is executed in strict mode. The strict mode of ECMAScript 5 is a restricted subset of the language that fixes a few important language deficiencies and provides stronger error checking and increased security. The differences between strict mode and non-strict mode are the following (the first three are particularly important):

var hasStrictMode = (function() { "use strict"; return this===undefined}());

Also, in strict mode, when a function is invoked with call() or apply(), the this value is exactly the value passed as the first argument to call() or apply(). (In nonstrict mode, null and undefined values are replaced with the global object and non-object values are converted to objects.)

5.7 Miscellaneous Statements | 111

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