What does use strict do in JavaScript and what is the reasoning behind it

Question

Recently  I ran some of my JavaScript code through Crockford s JSLint  and it gave the following error      Problem at line 1 character 1  Missing  use strict  statement    Doing some searching  I realized that some people add  use strict   into their JavaScript code  Once I added the statement  the error stopped appearing  Unfortunately  Google did not reveal much of the history behind this string statement  Certainly it must have something to do with how the JavaScript is interpreted by the browser  but I have no idea what the effect would be   So what is  use strict   all about  what does it imply  and is it still relevant   Do any of the current browsers respond to the  use strict   string or is it for future use

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A word of caution  all you hard-charging programmers   applying  use strict  to existing code can be hazardous   This thing is not some feel-good  happy-face sticker that you can slap on the code to make it  better    With the  use strict  pragma  the browser will suddenly THROW exceptions in random places that it never threw before just because at that spot you are doing something that default loose JavaScript happily allows but strict JavaScript abhors   You may have strictness violations hiding in seldom used calls in your code that will only throw an exception when they do eventually get run - say  in the production environment that your paying customers use   If you are going to take the plunge  it is a good idea to apply  use strict  alongside comprehensive unit tests and a strictly configured JSHint build task that will give you some confidence that there is no dark corner of your module that will blow up horribly just because you ve turned on Strict Mode   Or  hey  here s another option   just don t add  use strict  to any of your legacy code  it s probably safer that way  honestly   DEFINITELY DO NOT add  use strict  to any modules you do not own or maintain  like third party modules   I think even though it is a deadly caged animal   use strict  can be good stuff  but you have to do it right   The best time to go strict is when your project is greenfield and you are starting from scratch  Configure JSHint JSLint with all the warnings and options cranked up as tight as your team can stomach  get a good build test assert system du jour rigged like Grunt Karma Chai  and only THEN start marking all your new modules as  use strict    Be prepared to cure lots of niggly errors and warnings   Make sure everyone understands the gravity by configuring the build to FAIL if JSHint JSLint produces any violations   My project was not a greenfield project when I adopted  use strict    As a result  my IDE is full of red marks because I don t have  use strict  on half my modules  and JSHint complains about that   It s a reminder to me about what refactoring I should do in the future   My goal is to be red mark free due to all of my missing  use strict  statements  but that is years away now

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I would like to offer a somewhat more founded answer complementing the other answers  I was hoping to edit the most popular answer  but failed  I tried to make it as comprehensive and complete as I could   You can refer to the MDN documentation for more information    use strict  a directive introduced in ECMAScript 5   Directives are similar to statements  yet different    use strict does not contain key words  The directive is a simple expression statement  which consists of a special string literal  in single or double quotes   JavaScript engines  that do not implement ECMAScript 5  merely see an expression statement without side effects  It is expected that future versions of ECMAScript standards introduce use as a real key word  the quotes would thereby become obsolete  use strict can be used only at the beginning of a script or of a function  i e  it must precede every other  real  statement  It does not have to be the first instruction in a script of function  it can be preceded by other statement expressions that consist of string literals   and JavaScript implementations can treat them as implementation specific directives   String literals statements  which follow a first real statement  in a script or function  are simple expression statements  Interpreters must not interpret them as directives and they have no effect    The use strict directive indicates that the following code  in a script or a function  is strict code  The code in the highest level of a script  code that is not in a function  is considered strict code when the script contains a use strict directive  The content of a function is considered strict code when the function itself is defined in a strict code or when the function contains a use strict directive  Code that is passed to an eval   method is considered strict code when eval   was called from a strict code or contains the use strict directive itself   The strict mode of ECMAScript 5 is a restricted subset of the JavaScript language  which eliminates relevant deficits of the language and features more stringent error checking and higher security  The following lists the differences between strict mode and normal mode  of which the first three are particularly important     You cannot use the with-statement in strict mode  In strict mode all variables have to be declared  if you assign a value to an identifier that has not been declared as variable  function  function parameter  catch-clause parameter or property of the global Object  then you will get a ReferenceError  In normal mode the identifier is implicitly declared as a global variable  as a property of the global Object  In strict mode the keyword this has the value undefined in functions that were invoked as functions  not as methods    In normal mode this always points to the global Object   This difference can be used to test if an implementation supports the strict mode     var hasStrictMode    function      use strict   return this   undefined           Also when a function is invoked with call   or apply in strict mode  then this is exactly the value of the first argument of the call  or apply   invocation   In normal mode null and undefined are replaced by the global Object and values  which are not objects  are cast into objects   In strict mode you will get a TypeError  when you try to assign to readonly properties or to define new properties for a non extensible object   In normal mode both simply fail without error message   In strict mode  when passing code to eval    you cannot declare or define variables or functions in the scope of the caller  as you can do it in normal mode   Instead  a new scope is created for eval   and the variables and functions are within that scope  That scope is destroyed after eval   finishes execution  In strict mode the arguments-object of a function contains a static copy of the values  which are passed to that function  In normal mode the arguments-object has a somewhat  magical  behaviour  The elements of the array and the named function parameters reference both the same value  In strict mode you will get a SyntaxError when the delete operator is followed by a non qualified identifier  a variable  function or function parameter   In normal mode the delete expression would do nothing and is evaluated to false  In strict mode you will get a TypeError when you try to delete a non configurable property   In normal mode the attempt simply fails and the delete expression is evaluated to false   In strict mode it is considered a syntactical error when you try to define several properties with the same name for an object literal   In normal mode there is no error   In strict mode it is considered a syntactical error when a function declaration has multiple parameters with the same name   In normal mode there is no error   In strict mode octal literals are not allowed  these are literals that start with 0x   In normal mode some implementations do allow octal literals   In strict mode the identifiers eval and arguments are treated like keywords  You cannot change their value  cannot assign a value to them  and you cannot use them as names for variables  functions  function parameters or identifiers of a catch block  In strict mode are more restrictions on the possibilities to examine the call stack  arguments caller and arguments callee cause a TypeError in a function in strict mode  Furthermore  some caller- and arguments properties of functions in strict mode cause a TypeError when you try to read them

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When adding  use strict    the following cases will throw a SyntaxError before the script is executing    Paving the way for future ECMAScript versions  using one of the newly reserved keywords  in prevision for ECMAScript 6   implements  interface  let  package  private  protected  public  static  and yield  Declaring function in blocks   if a lt b   function f        Octal syntax   var n   023   this point to the global object    function f            use strict         this a   1       f      Declaring twice the same name for a property name in an object literal     a  1  b  3  a  7     This is no longer the case in ECMAScript 6  bug 1041128   Declaring two function arguments with the same name function   f a  b  b     Setting a value to an undeclared variable  function f x       use strict      var a   12     b   a   x 35     error    f     Using delete on a variable name delete myVariable  Using eval or arguments as variable or function argument name   use strict   arguments    var obj     set p arguments         try     catch  arguments      function arguments           Sources    Transitioning to strict mode on MDN Strict mode on MDN JavaScript   s Strict Mode and Why You Should Use It on Colin J  Ihrig s blog  archived version

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Strict mode can prevent memory leaks   Please check the function below written in non-strict mode   function getname        name    Stack Overflow      Not using var keyword     return name    getname    console log name      Stack Overflow   In this function  we are using a variable called name inside the function  Internally  the compiler will first check if there is any variable declared with that particular name in that particular function scope  Since the compiler understood that there is no such variable  it will check in the outer scope  In our case  it is the global scope  Again  the compiler understood that there is also no variable declared in the global space with that name  so it creates such a variable for us in the global space  Conceptually  this variable will be created in the global scope and will be available in the entire application   Another scenario is that  say  the variable is declared in a child function  In that case  the compiler checks the validity of that variable in the outer scope  i e   the parent function  Only then it will check in the global space and create a variable for us there  That means additional checks need to be done  This will affect the performance of the application     Now let s write the same function in strict mode    use strict  function getname        name    Stack Overflow      Not using var keyword     return name    getname    console log name      We will get the following error   Uncaught ReferenceError  name is not defined at getname   lt anonymous gt  3 15  at  lt anonymous gt  6 5   Here  the compiler throws the reference error  In strict mode  the compiler does not allow us to use the variable without declaring it  So memory leaks can be prevented  In addition  we can write more optimized code

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Note that use strict was introduced in EcmaScript 5 and was kept since then   Below are the conditions to trigger strict mode in ES6 and ES7         Global code is strict mode code if it begins with a Directive    Prologue that contains a Use Strict Directive  see 14 1 1     Module code is always strict mode code    All parts of a ClassDeclaration or a ClassExpression are strict mode    code    Eval code is strict mode code if it begins with a Directive Prologue    that contains a Use Strict Directive or if the call to eval is a direct eval  see 12 3 4 1  that is contained in strict mode code    Function code is strict mode code if the associated FunctionDeclaration  FunctionExpression  GeneratorDeclaration  GeneratorExpression  MethodDefinition  or ArrowFunction is contained in strict mode code or if the code that produces the value of the function   s   ECMAScriptCode   internal slot begins with a Directive Prologue that contains a Use Strict Directive    Function code that is supplied as the arguments to the built-in    Function and Generator constructors is strict mode code if the last argument is a String that when processed is a FunctionBody that begins with a Directive Prologue that contains a Use Strict Directive

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use strict is a way to make your code safer  because you can t use dangerous features that can work not as you expect  And  as was written before  it makes code more strict

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JavaScript    strict    mode was introduced in ECMAScript 5    function        use strict     your code            Writing  use strict   at the very top of your JS file turns on strict syntax checking  It does the following tasks for us    shows an error if you try to assign to an undeclared variable stops you from overwriting key JS system libraries forbids some unsafe or error-prone language features   use strict also works inside of individual functions  It is always a better practice to include use strict in your code   Browser compatibility issue   The  use  directives are meant to be backwards-compatible  Browsers that do not support them will just see a string literal that isn t referenced further  So  they will pass over it and move on

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Using  use strict   does not suddenly make your code better   The JavaScript strict mode is a feature in ECMAScript 5  You can enable the strict mode by declaring this in the top of your script function    use strict     When a JavaScript engine sees this directive  it will start to interpret the code in a special mode  In this mode  errors are thrown up when certain coding practices that could end up being potential bugs are detected  which is the reasoning behind the strict mode    Consider this example   var a   365  var b   030    In their obsession to line up the numeric literals  the developer has inadvertently initialized variable b with an octal literal  Non-strict mode will interpret this as a numeric literal with value 24  in base 10   However  strict mode will throw an error   For a non-exhaustive list of specialties in strict mode  see this answer     Where should I use  use strict      In my new JavaScript application  Absolutely  Strict mode can be used as a whistleblower when you are doing something stupid with your code  In my existing JavaScript code  Probably not  If your existing JavaScript code has statements that are prohibited in strict-mode  the application will simply break  If you want strict mode  you should be prepared to debug and correct your existing code  This is why using  use strict   does not suddenly make your code better      How do I use strict mode    Insert a  use strict   statement on top of your script      File  myscript js   use strict   var a   2         Note that everything in the file myscript js will be interpreted in strict mode  Or  insert a  use strict   statement on top of your function body   function doSomething          use strict               Everything in the lexical scope of function doSomething will be interpreted in strict mode  The word lexical scope is important here  For example  if your strict code calls a function of a library that is not strict  only your code is executed in strict mode  and not the called function  See this answer for a better explanation      What things are prohibited in strict mode   I found a nice article describing several things that are prohibited in strict mode  note that this is not an exclusive list       Scope      Historically  JavaScript has been confused about how functions   are scoped  Sometimes they seem to be statically scoped  but some   features make them behave like they are dynamically scoped  This is   confusing  making programs difficult to read and understand    Misunderstanding causes bugs  It also is a problem for performance    Static scoping would permit variable binding to happen at compile   time  but the requirement for dynamic scope means the binding must be   deferred to runtime  which comes with a significant performance   penalty       Strict mode requires that all variable binding be done statically    That means that the features that previously required dynamic binding   must be eliminated or modified  Specifically  the with statement is   eliminated  and the eval function   s ability to tamper with the   environment of its caller is severely restricted       One of the benefits of strict code is that tools like YUI Compressor   can do a better job when processing it       Implied Global Variables      JavaScript has implied global variables  If   you do not explicitly declare a variable  a global variable is   implicitly declared for you  This makes programming easier for   beginners because they can neglect some of their basic housekeeping   chores  But it makes the management of larger programs much more   difficult and it significantly degrades reliability  So in strict   mode  implied global variables are no longer created  You should   explicitly declare all of your variables       Global Leakage      There are a number of situations that could cause this   to be bound to the global object  For example  if you forget to   provide the new prefix when calling a constructor function  the   constructor s this will be bound unexpectedly to the global object  so   instead of initializing a new object  it will instead be silently   tampering with global variables  In these situations  strict mode will   instead bind this to undefined  which will cause the constructor to   throw an exception instead  allowing the error to be detected much   sooner       Noisy Failure      JavaScript has always had read-only properties  but you   could not create them yourself until ES5   s Object createProperty   function exposed that capability  If you attempted to assign a value   to a read-only property  it would fail silently  The assignment would   not change the property   s value  but your program would proceed as   though it had  This is an integrity hazard that can cause programs to   go into an inconsistent state  In strict mode  attempting to change a   read-only property will throw an exception       Octal      The octal  or base 8  representation of numbers was extremely   useful when doing machine-level programming on machines whose word   sizes were a multiple of 3  You needed octal when working with the CDC   6600 mainframe  which had a word size of 60 bits  If you could read   octal  you could look at a word as 20 digits  Two digits represented   the op code  and one digit identified one of 8 registers  During the   slow transition from machine codes to high level languages  it was   thought to be useful to provide octal forms in programming languages       In C  an extremely unfortunate representation of octalness was   selected  Leading zero  So in C  0100 means 64  not 100  and 08 is an   error  not 8  Even more unfortunately  this anachronism has been   copied into nearly all modern languages  including JavaScript  where   it is only used to create errors  It has no other purpose  So in   strict mode  octal forms are no longer allowed       Et cetera      The arguments pseudo array becomes a little bit more   array-like in ES5  In strict mode  it loses its callee and caller   properties  This makes it possible to pass your arguments to untrusted   code without giving up a lot of confidential context  Also  the   arguments property of functions is eliminated       In strict mode  duplicate keys in a function literal will produce a   syntax error  A function can   t have two parameters with the same name    A function can   t have a variable with the same name as one of its   parameters  A function can   t delete its own variables  An attempt to   delete a non-configurable property now throws an exception  Primitive   values are not implicitly wrapped      Reserved words for future JavaScript versions  ECMAScript 5 adds a list of reserved words  If you use them as variables or arguments  strict mode will throw an error  The reserved words are      implements  interface  let  package  private  protected  public  static  and yield     Further Reading   Strict Mode - JavaScript   MDN Browser support for strict mode Transitioning to strict mode

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If people are worried about using use strict it might be worth checking out this article    ECMAScript 5  Strict mode  support in browsers  What does this mean    NovoGeek com - Krishna s weblog  It talks about browser support  but more importantly how to deal with it safely   function isStrictMode        return  this           returns false  since  this  refers to global object and       this  becomes false     function isStrictMode            use strict       return  this            returns true  since in strict mode the keyword  this     does not refer to global object  unlike traditional JS      So here   this  is  undefined  and   this  becomes true

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There s a good talk by some people who were on the ECMAScript committee  Changes to JavaScript  Part 1  ECMAScript 5  about how incremental use of the  use strict  switch allows JavaScript implementers to clean up a lot of the dangerous features of JavaScript without suddenly breaking every website in the world   Of course it also talks about just what a lot of those misfeatures are  were  and how ECMAScript 5 fixes them

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use strict   Defines that JavaScript code should be executed in     strict mode     The  use strict  directive was new in ECMAScript version 5  It is not a statement  but a literal expression  ignored by earlier versions of JavaScript  The purpose of  use strict  is to indicate that the code should be executed in  strict mode   With strict mode  you can not  for example  use undeclared variables    All modern browsers support  use strict  except Internet Explorer 9 and lower   Disadvantage  If a developer used a library that was in strict mode  but the developer was used to working in normal mode  they might call some actions on the library that wouldn   t work as expected   Worse  since the developer is in normal mode  they don   t have the advantages of extra errors being thrown  so the error might fail silently   Also  as listed above  strict mode stops you from doing certain things   People generally think that you shouldn   t use those things in the first place  but some developers don   t like the constraint and want to use all the features of the language    For basic example and for reference go through    https   www tutorialsteacher com javascript javascript-strict

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use strict   is the ECMA effort to make JavaScript a little bit more robust  It brings in JS an attempt to make it at least a little  strict   other languages implement strict rules since the 90s   It actually  forces  JavaScript developers to follow some sort of coding best practices  Still  JavaScript is very fragile  There is no such thing as typed variables  typed methods  etc  I strongly recommend JavaScript developers to learn a more robust language such as Java or ActionScript3  and implement the same best practices in your JavaScript code  it will work better and be easier to debug

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This article about Javascript Strict Mode might interest you  John Resig - ECMAScript 5 Strict Mode  JSON  and More To quote some interesting parts   Strict Mode is a new feature in ECMAScript 5 that allows you to place a program  or a function  in a  quot strict quot  operating context  This strict context prevents certain actions from being taken and throws more exceptions   And   Strict mode helps out in a couple ways   It catches some common coding bloopers  throwing exceptions  It prevents  or throws errors  when relatively  quot unsafe quot  actions are taken  such as gaining access to the global object   It disables features that are confusing or poorly thought out    Also note you can apply  quot strict mode quot  to the whole file    Or you can use it only for a specific function  still quoting from John Resig s article      Non-strict code      function       quot use strict quot         Define your library strictly              Non-strict code     Which might be helpful if you have to mix old and new code  -  So  I suppose it s a bit like the  quot use strict quot  you can use in Perl  hence the name    it helps you make fewer errors  by detecting more things that could lead to breakages  Strict mode is now supported by all major browsers  Inside native ECMAScript modules  with import and export statements  and ES6 classes  strict mode is always enabled and cannot be disabled

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It s a new feature of ECMAScript 5  John Resig wrote up a nice summary of it   It s just a string you put in your JavaScript files  either at the top of your file or inside of a function  that looks like this    use strict     Putting it in your code now shouldn t cause any problems with current browsers as it s just a string  It may cause problems with your code in the future if your code violates the pragma   For instance  if you currently have foo    bar  without defining foo first  your code will start failing   which is a good thing in my opinion

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Use Strict is used to show common and repeated errors so that it is handled differently   and changes the way java script runs   such changes are     Prevents accidental globals No duplicates Eliminates with Eliminates this coercion Safer eval   Errors for immutables   you can also read this article for the details

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Normally  JavaScript does not follow strict rules  hence increasing chances of errors  After using  use strict   the JavaScript code should follow strict set of rules as in other programming languages such as use of terminators  declaration before initialization  etc    If  use strict  is used  the code should be written by following a strict set of rules  hence decreasing the chances of errors and ambiguities

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Quoting from w3schools      The  use strict  Directive      The  use strict  directive is new in JavaScript 1 8 5  ECMAScript   version 5        It is not a statement  but a literal expression  ignored by earlier   versions of JavaScript       The purpose of  use strict  is to indicate that the code should be   executed in  strict mode        With strict mode  you can not  for example  use undeclared variables       Why Strict Mode       Strict mode makes it easier to write  secure  JavaScript       Strict mode changes previously accepted  bad syntax  into real errors       As an example  in normal JavaScript  mistyping a variable name creates   a new global variable  In strict mode  this will throw an error    making it impossible to accidentally create a global variable       In normal JavaScript  a developer will not receive any error feedback   assigning values to non-writable properties       In strict mode  any assignment to a non-writable property  a   getter-only property  a non-existing property  a non-existing   variable  or a non-existing object  will throw an error    Please refer to http   www w3schools com js js strict asp to know more

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Small examples to compare   Non-strict mode    x000D   x000D  for  i of  1 2 3   console log i  x000D       x000D     output  x000D     1 x000D     2 x000D     3 x000D   x000D   x000D    Strict mode    x000D   x000D   use strict   x000D  for  i of  1 2 3   console log i  x000D   x000D     output  x000D     Uncaught ReferenceError  i is not defined x000D   x000D   x000D    Non-strict mode    x000D   x000D  String prototype test   function      x000D    console log typeof this      string    x000D     x000D   x000D   a  test    x000D   x000D     output x000D     false x000D   x000D   x000D     x000D   x000D  String prototype test   function      x000D     use strict   x000D     x000D    console log typeof this      string    x000D     x000D   x000D   a  test    x000D   x000D     output x000D     true x000D   x000D   x000D

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The statement  use strict   instructs the browser to use the Strict mode  which is a reduced and safer feature set of JavaScript    List of features  non-exhaustive    Disallows global variables   Catches missing var declarations and typos in variable names   Silent failing assignments will throw error in strict mode  assigning NaN   5    Attempts to delete undeletable properties will throw  delete Object prototype   Requires all property names in an object literal to be unique  var x    x1   1   x1   2     Function parameter names must be unique  function sum  x  x          Forbids octal syntax  var x   023  some devs assume wrongly that a preceding zero does nothing to change the number    Forbids the with keyword  eval in strict mode does not introduce new variables   Forbids deleting plain names  delete x   Forbids binding or assignment of the names eval and arguments in any form  Strict mode does not alias properties of the arguments object with the formal parameters   i e  in function sum  a b    return arguments 0    b   This works because arguments 0  is bound to a and so on    arguments callee is not supported    Ref  Strict mode  Mozilla Developer Network

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use strict  makes JavaScript code to run in strict mode  which basically means everything needs to be defined before use  The main reason for using strict mode is to avoid accidental global uses of undefined methods   Also in strict mode  things run faster  some warnings or silent warnings throw fatal errors  it s better to always use it to make a neater code    use strict  is widely needed to be used in ECMA5  in ECMA6 it s part of JavaScript by default  so it doesn t need to be added if you re using ES6   Look at these statements and examples from MDN      The  use strict  Directive The  use strict  directive is new in   JavaScript 1 8 5  ECMAScript version 5   It is not a statement  but a   literal expression  ignored by earlier versions of JavaScript  The   purpose of  use strict  is to indicate that the code should be   executed in  strict mode   With strict mode  you can not  for example    use undeclared variables       Examples of using  use strict     Strict mode for functions  Likewise  to invoke strict mode for a   function  put the exact statement  use strict    or  use strict    in   the function s body before any other statements    1  strict mode in functions    function strict             Function-level strict mode syntax       use strict        function nested     return  And so am I           return  Hi   I m a strict mode function       nested        function notStrict     return  I m not strict        console log strict    notStrict       2  whole-script strict mode    use strict   var v    Hi  I m a strict mode script    console log v     3  Assignment to a non-writable global   use strict       Assignment to a non-writable global var undefined   5     throws a TypeError var Infinity   5     throws a TypeError     Assignment to a non-writable property var obj1       Object defineProperty obj1   x     value  42  writable  false     obj1 x   9     throws a TypeError     Assignment to a getter-only property var obj2     get x     return 17       obj2 x   5     throws a TypeError     Assignment to a new property on a non-extensible object  var fixed       Object preventExtensions fixed   fixed newProp    ohai      throws a TypeError   You can read more on MDN

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Including use strict in the beginning of your all sensitive JavaScript files from this point is a small way to be a better JavaScript programmer and avoid random variables becoming global and things change silently

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The main reasons why developers should use  use strict  are    Prevents accidental declaration of global variables Using  use strict    will make sure that variables are declared with var before use   Eg   function useStrictDemo      use strict      works fine  var a    No Problem       does not work fine and throws error  k    problem      even this will throw error  someObject     problem    lot of problem       N B  The  use strict  directive is only recognized at the beginning of a script or a function  The string  arguments  cannot be used as a variable    use strict   var arguments   3 14        This will cause an error  Will restrict uses of keywords as variables  Trying to use them will throw errors    In short will make your code less error prone and in turn will make you write good code   To read more about it you can refer here

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If you use a browser released in the last year or so then it most likely supports JavaScript Strict mode  Only older browsers around before ECMAScript 5 became the current standard don t support it    The quotes around the command make sure that the code will still work in older browsers as well  although the things that generate a syntax error in strict mode will generally just cause the script to malfunction in some hard to detect way in those older browsers

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I strongly recommend every developer to start using strict mode now  There are enough browsers supporting it that strict mode will legitimately help save us from errors we didn   t even know were in your code   Apparently  at the initial stage there will be errors we have never encountered before  To get the full benefit  we need to do proper testing after switching to strict mode to make sure we have caught everything  Definitely we don   t just throw use strict in our code and assume there are no errors  So the churn is that it   s time to start using this incredibly useful language feature to write better code   For example   var person         name    xyz       position    abc       fullname   function        use strict   return this name         JSLint is a debugger written by Douglas Crockford  Simply paste in your script  and it   ll quickly scan for any noticeable issues and errors in your code

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Strict mode makes several changes to normal JavaScript semantics    eliminates some JavaScript silent errors by changing them to throw errors  fixes mistakes that make it difficult for JavaScript engines to perform optimizations  prohibits some syntax likely to be defined in future versions of ECMAScript    for more information vistit Strict Mode- Javascript

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Use Strict   is an insurance that programmer will not use the loose or the bad properties of JavaScript  It is a guide  just like a ruler will help you make straight lines   Use Strict  will help you do  Straight coding    Those that prefer not to use rulers to do their lines straight usually end up in those pages asking for others to debug their code   Believe me  The overhead is negligible compared to poorly designed code  Doug Crockford  who has been a senior JavaScript developer for several years  has a very interesting post here  Personally  I like to return to his site all the time to make sure I don t forget my good practice   Modern JavaScript practice should always evoke the  Use Strict   pragma  The only reason that the ECMA Group has made the  Strict  mode optional is to permit less experienced coders access to JavaScript and give then time to adapt to the new and safer coding practices

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Strict mode eliminates errors that would be ignored in non-strict mode  thus making javascript    more secured      Is it considered among best practices   Yes  It s considered part of the best practices while working with javascript to include Strict mode  This is done by adding the below line of code in your JS file   use strict    in your code   What does it mean to user agents   Indicating that code should be interpreted in strict mode specifies to user agents like browsers that they should treat code literally as written  and throw an error if the code doesn t make sense  For example  Consider in your  js file you have the following code  Scenario 1   NO STRICT MODE  var city    quot Chicago quot  console log city     Prints the city name  i e  Chicago  Scenario 2   NO STRICT MODE  city    quot Chicago quot  console log city     Prints the city name  i e  Chicago   So why does the variable name is being printed in both cases   Without strict mode turned on  user agents often go through a series of modifications to problematic code in an attempt to get it to make sense  On the surface  this can seem like a fine thing  and indeed  working outside of strict mode makes it possible for people to get their feet wet with JavaScript code without having all the details quite nailed down  However  as a developer  I don t want to leave a bug in my code  because I know it could come back and bite me later on  and I also just want to write good code  And that s where strict mode helps out  Scenario 3   STRICT MODE   use strict    city    quot Chicago quot  console log city     Reference Error  asignment is undeclared variable city   Additional tip  To maintain code quality using strict mode  you don t need to write this over and again especially if you have multiple  js file  You can enforce this rule globally in eslint rules as follows  Filename   eslintrc js module exports         env            es6  true            rules             strict    error    global                             Okay  so what is prevented in strict mode    Using a variable without declaring it will throw an error in strict mode  This is to prevent unintentionally creating global variables throughout your application  The example with printing Chicago covers this in particular   Deleting a variable or a function or an argument is a no-no in strict mode   quot use strict quot    function x p1  p2        delete x     This will cause an error   Duplicating a parameter name is not allowed in strict mode    quot use strict quot    function x p1  p1           This will cause an error   Reserved words in the Javascript language are not allowed in strict mode  The words are implements interface  let  packages  private  protected  public  static  and yield   For a more comprehensive list check out the MDN documentation here  https   developer mozilla org en-US docs Web JavaScript Reference Strict mode

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My two cents   One of the goals of strict mode is to allow for faster debugging of issues  It helps the developers by throwing exception when certain wrong things occur that can cause silent  amp  strange behaviour of your webpage  The moment we use use strict  the code will throw out errors which helps developer to fix it in advance   Few important things which I have learned after using use strict    Prevents Global Variable Declaration   var tree1Data     name   Banana Tree  age  100 leafCount  100000    function Tree typeOfTree        var age      var leafCount       age   typeOfTree age      leafCount   typeOfTree leafCount      nameoftree   typeOfTree name      var tree1   new Tree tree1Data   console log window     Now this code creates nameoftree in global scope which could be accessed using window nameoftree  When we implement use strict the code would throw error      Uncaught ReferenceError  nameoftree is not defined   Sample  Eliminates with statement    with statements can t be minified using tools like uglify-js  They re also deprecated and removed from future JavaScript versions   Sample  Prevents Duplicates    When we have duplicate property  it throws an exception      Uncaught SyntaxError  Duplicate data property in object literal not   allowed in strict mode    use strict   var tree1Data         name   Banana Tree       age  100      leafCount  100000      name  Banana Tree       There are few more but I need to gain more knowledge on that

[javascript] What does "use strict" do in JavaScript, and what is the reasoning behind it?

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