Note: Sometimes you'll see the default, non-strict mode referred to as sloppy mode. This isn't an official term, but be aware of it, just in case. JavaScript's strict mode is a way to opt in to a restricted variant of JavaScript, thereby implicitly opting-out of
"sloppy mode". 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 makes several changes to normal JavaScript semantics: Invoking strict modeStrict mode applies to entire scripts or to individual functions. It doesn't apply to block statements enclosed in Strict mode for scriptsTo invoke
strict mode for an entire script, put the exact statement
Strict mode for functionsLikewise, to invoke strict mode for a function, put the exact statement
The
Strict mode for modulesThe entire contents of JavaScript modules are automatically in strict mode, with no statement needed to initiate it.
Strict mode for classesAll parts of a class's body are strict mode code, including both class declarations and class expressions.
Changes in strict modeStrict mode changes both syntax and runtime behavior. Changes generally fall into these categories:
Converting mistakes into errorsStrict 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. Assigning to undeclared variablesStrict mode makes it impossible to accidentally create global variables. In sloppy mode, mistyping a variable in an assignment creates a new property on the global object and continues to "work". Assignments which would accidentally create global variables throw an error in strict mode:
Failing to assign to object propertiesStrict mode makes assignments which would otherwise silently fail to throw an exception. There are three ways to fail a property assignment:
For example,
Failing to delete object propertiesAttempts to delete a non-configurable or otherwise undeletable (e.g. it's intercepted by a proxy's
Strict mode also forbids deleting plain names.
If the name is a configurable global property, prefix it with
Duplicate parameter namesStrict mode requires that function parameter names be unique. In sloppy mode, the last duplicated argument hides previous identically-named arguments. Those previous arguments remain available through
Legacy octal literalsStrict mode
forbids a
The standardized way to denote octal literals is via the
Octal escape sequences, such as Setting properties on primitive valuesStrict mode forbids setting properties on primitive values. Accessing a property on a primitive implicitly creates a wrapper object that's unobservable, so in sloppy mode,
setting properties is ignored (no-op). In strict mode, a
Duplicate property namesDuplicate property names used to be considered a
Note: Making code that used to error become non-errors is always considered backwards-compatible. This is a good part of the language being strict about throwing errors: it leaves room for future semantic changes. Simplifying scope managementStrict mode simplifies how variable names map to particular variable definitions in the code. Many compiler optimizations rely on the ability to say that variable X is stored in that 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 until runtime. Strict mode removes most cases where this happens, so the compiler can better optimize strict mode code. Removal of the with statementStrict mode prohibits
The simple alternative of assigning the object to a short name variable, then accessing the corresponding property on that variable, stands ready to replace Non-leaking evalIn strict mode,
Whether the string passed to Block-scoped function declarationsThe JavaScript language specification, since its start, had not allowed function declarations nested in block statements. However, it was so intuitive that most browsers implemented it as an extension grammar. Unfortunately, the implementations' semantics diverged, and it became impossible for the language specification to reconcile all implementations. Therefore, block-scoped function declarations are only explicitly specified in strict mode (whereas they were once disallowed in strict mode), while sloppy mode behavior remains divergent among browsers. Making eval and arguments simplerStrict mode
makes Preventing binding or assigning eval and argumentsThe names
No syncing between parameters and arguments indicesStrict mode code doesn't sync indices of
the
"Securing" JavaScriptStrict 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 a 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. No this substitutionThe value passed as
Removal of stack-walking propertiesIn strict mode it's no longer possible to "walk" the JavaScript stack. Many implementations used to implement some extension features that make it possible to detect the upstream caller of a function. When a function
Similarly,
Future-proofing JavaScriptReserved words are identifiers that can't be used as variable names. Strict mode reserves some more names than sloppy mode, some of which are already used in the language, and some of which are reserved for the future to make future syntax extensions easier to implement.
Transitioning to strict modeStrict mode has been designed so that the transition to it can be made gradually. It is possible to change each file individually and even to transition code to strict mode down to the function granularity. You can migrate a
codebase to strict mode by first adding Syntax errorsWhen adding
These errors are good, because they reveal plain errors or bad practices. They occur before the code is running, so they are easily discoverable as long as the code gets parsed by the runtime. New runtime errorsJavaScript used to silently fail in contexts where what was done should be an error. Strict mode throws in such cases. If your code base contains such cases, testing will be necessary to be sure nothing is broken. You can screen for such errors at the function granularity level.
Semantic differencesThese differences are very subtle differences. It's possible that a test suite doesn't catch this kind of subtle difference. Careful review of your code base will probably be necessary to be sure these differences don't affect the semantics of your code. Fortunately, this careful review can be done gradually down the function granularity. this In sloppy mode, function calls like arguments In sloppy mode, modifying a value in the eval In strict mode code, In sloppy mode, a function declaration inside a block may be visible outside the block and even callable. In strict mode, a function declaration inside a block is only visible inside the block. See alsoWhich of the following refers to the malicious insertion of scripting code to extract data or modify a website's code application or content?Cross-Site Scripting (XSS) attacks are a type of injection, in which malicious scripts are injected into otherwise benign and trusted websites. XSS attacks occur when an attacker uses a web application to send malicious code, generally in the form of a browser side script, to a different end user.
Which of the following refers to the free encryption software based on the OpenPGP standard?GnuPG is a complete and free implementation of the OpenPGP standard as defined by RFC4880 (also known as PGP). GnuPG allows you to encrypt and sign your data and communications; it features a versatile key management system, along with access modules for all kinds of public key directories.
Which of the following refers to the free encryption software based on the OpenPGP standard that uses both symmetric and asymmetric encryption?GnuPG is a hybrid-encryption software program because it uses a combination of conventional symmetric-key cryptography for speed, and public-key cryptography for ease of secure key exchange, typically by using the recipient's public key to encrypt a session key which is used only once.
Which of the following is a DVWA security level that mimics a vulnerable web application?Damn VulnerableWeb Application (DVWA)7. Which of the following is a Damn Vulnerable Web Application (DVWA) security levelthat mimics a vulnerable Web application? Low8.
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