Const in JavaScript when to use it and is it necessary

Question

I ve recently come across the const keyword in JavaScript  From what I can tell  it is used to create immutable variables  and I ve tested to ensure that it cannot be redefined  in Node js    const x    const   const x    not-const       Will give an error   constant  x  has already been defined    I realise that it is not yet standardized across all browsers - but I m only interested in the context of Node js V8  and I ve noticed that certain developers   projects seem to favor it heavily when the var keyword could be used to the same effect   So my questions are    When is it appropriate to use const in place of var  Should it be used every time a variable which is not going to be re-assigned is declared  Does it actually make any difference if var is used in place of const or vice-versa

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First  three useful things about const  other than the scope improvements it shares with let     It documents for people reading the code later that the value must not change  It prevents you  or anyone coming after you  from changing the value unless they go back and change the declaration intentionally  It might save the JavaScript engine some analysis in terms of optimization  E g   you ve declared that the value cannot change  so the engine doesn t have to do work to figure out whether the value changes so it can decide whether to optimize based on the value not changing    Your questions      When is it appropriate to use const in place of var    You can do it any time you re declaring a variable whose value never changes  Whether you consider that appropriate is entirely down to your preference   your team s preference      Should it be used every time a variable which is not going to be re-assigned is declared    That s up to you   your team      Does it actually make any difference if var is used in place ofconst  or vice-versa    Yes    var and const have different scope rules   You might have wanted to compare with let rather than var   Specifically  const and let are block-scoped and  when used at global scope  don t create properties on the global object  even though they do create globals   var has either global scope  when used at global scope  or function scope  even if used in a block   and when used at global scope  creates a property on the global object  See my  three useful things  above  they all apply to this question

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To integrate the previous answers  there s an obvious advantage in declaring constant variables  apart from the performance reason  if you accidentally try to change or re-declare them in the code  the program will respectively not change the value or throw an error   For example  compare      Will output  SECRET   const x    SECRET  if  x    ANOTHER SECRET         Warning  assigning a value variable in an if condition     console log  x      with      Will output  ANOTHER SECRET   var y    SECRET  if  y    ANOTHER SECRET          console log  y      or     Will throw TypeError  const  x  has already been declared  const x    SECRET       complex code     var x   0   with      Will reassign y and cause trouble  var y    SECRET       complex code     var y   0

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My opinions  Q  When is it appropriate to use const in place of var  A  Never  Q  Should it be used every time a variable which is not going to be re-assigned is declared  A  Never   Like this is going to make a dent in resource consumption    Q  Does it actually make any difference if var is used in place of const or vice-versa  A  Yes   Using var is the way to go   Much easier in dev tools and save from creating a new file s  for testing    var in not in place of const - const is trying to take var s place     Extra A  Same goes for let   JavaScript is a loose language - why constrict it

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2017 Update  This answer still receives a lot of attention  It s worth noting that this answer was posted back at the beginning of 2014 and a lot has changed since then  ecmascript-6 support is now the norm  All modern browsers now support const so it should be pretty safe to use without any problems     Original Answer from 2014  Despite having fairly decent browser support  I d avoid using it for now  From MDN s article on const      The current implementation of const is a Mozilla-specific extension and is not part of ECMAScript 5  It is supported in Firefox  amp  Chrome  V8   As of Safari 5 1 7 and Opera 12 00  if you define a variable with const in these browsers  you can still change its value later  It is not supported in Internet Explorer 6-10  but is included in Internet Explorer 11  The const keyword currently declares the constant in the function scope  like variables declared with var     It then goes on to say      const is going to be defined by ECMAScript 6  but with different semantics  Similar to variables declared with the let statement  constants declared with const will be block-scoped    If you do use const you re going to have to add in a workaround to support slightly older browsers

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When it comes to the decision between let and const  both block scoped   always prefer const so that the usage is clear in the code  That way  if you try to redeclare the variable  you ll get an error  If there s no other choice but redeclare it  just switch for let  Note that  as Anthony says  the const values aren t immutable  for instances  a const object can have properties mutated   When it comes to var  since ES6 is out  I never used it in production code and can t think of a use case for it  One point that might consider one to use it is JavaScript hosting - while let and const are not hoisted  var declaration is  Yet  beware that variables declared with var  have a function scope  not a block scope    if declared outside any function  they will be globally available throughout the program  if declared within a function  they are only available within the function itself    in HackerRank - Variable Declaration Keywords   You can think of let as the block scoped version of var

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Personal preference really  You could use const when  as you say  it will not be re-assigned and is constant  For example if you wanted to assign your birthday  Your birthday never changes so you could use it as a constant  But your age does change so that could be a variable

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There are two aspects to your questions  what are the technical aspects of using const instead of var and what are the human-related aspects of doing so   The technical difference is significant  In compiled languages  a constant will be replaced at compile-time and its use will allow for other optimizations like dead code removal to further increase the runtime efficiency of the code  Recent  loosely used term  JavaScript engines actually compile JS code to get better performance  so using the const keyword would inform them that the optimizations described above are possible and should be done  This results in better performance   The human-related aspect is about the semantics of the keyword  A variable is a data structure that contains information that is expected to change  A constant is a data structure that contains information that will never change  If there is room for error  var should always be used  However  not all information that never changes in the lifetime of a program needs to be declared with const  If under different circumstances the information should change  use var to indicate that  even if the actual change doesn t appear in your code

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For why to use const   Tibos s answer s great   But you said      From what i can tell  it is used to create immutable variables   That is wrong  Mutating a variable is different from reassigning   var hello    world     assigning hello    bonjour      reassigning   With const  you can not do that   const hello    world  hello    bonjour      error   But you can mutate your variable   const marks    92  83  marks push 95  console log marks      92  83  95  - gt  the variable has been mutated    So  any process that changes the variable s value without using the   sign is mutating the variable   Note     for example is     re-assigning   var a   5 a    2    is the same as a   a   2   So  the bottom line is  const doesn t prevent you from mutating variables  it prevents you from re-assigning them

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It provides   a constant reference  eg const x      - the array can be modified  but x can t point to another array  and  block scoping    const and let will together replace var in ecma6 2015  See discussion at https   strongloop com strongblog es6-variable-declarations

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In my experience I use const when I want to set something I may want to change later without having to hunt through the code looking for bits that have been hard coded e g  A file path or server name    The error in your testing is another thing though  you are tring to make another variable called x  this would be a more accurate test   const x    const   x    not-const

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const is not immutable   From the MDN      The const declaration creates a read-only reference to a value  It   does not mean the value it holds is immutable  just that the variable   identifier cannot be reassigned

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const  is an indication to your code that the identifier will not be reassigned  This is a good article about when to use  const    let  or  var  https   medium com javascript-scene javascript-es6-var-let-or-const-ba58b8dcde75  ukgxpfhao

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I am not an expert in the JS compiling business  but it makes sense to say  that v8 makes use from the const flag  Normally a after declaring and changing a bunch of variables  the memory gets fragmented  and v8 is stopping to execute  makes a pause some time of a few seconds  to make gc  or garbage collection   if a variable is declared with const v8 can be confident to put it in a tight fixed size container between other const variables  since it will never change  It can also save the proper operations for that datatypes since the type will not change

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You have great answers  but let s keep it simple   const should be used when you have a defined constant  read as  it won t change during your program execution    For example   const pi   3 1415926535   If you think that it is something that may be changed on later execution then use a var   The practical difference  based on the example  is that with const you will always asume that pi will be 3 14       it s a fact   If you define it as a var  it might be 3 14      or not   For a more technical answer  Tibos is academically right

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var  Declare a variable  value initialization optional   let  Declare a local variable with block scope   const  Declare a read-only named constant   Ex   var a  a   1  a   2   re-initialize possible var a   3   re-declare console log a    3  let b  b   5  b   6   re-initiliaze possible    let b   7    re-declare not possible console log b       const c     c   9      initialization and declaration at same place const c   9     const c   9    re-declare and initialization is not possible console log c    9    NOTE  Constants can be declared with uppercase or lowercase  but a common    convention is to use all-uppercase letters

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The semantics of var and let  var and let are a statement to the machine and to other programmers       I intend that the value of this assignment change over the course of execution  Do not rely on the eventual value of this assignment    Implications of using var and let  var and let force other programmers to read all the intervening code from the declaration to the eventual use  and reason about the value of the assignment at that point in the program s execution   They weaken machine reasoning for ESLint and other language services to correctly detect mistyped variable names in later assignments and scope reuse of outer scope variable names where the inner scope forgets to declare   They also cause runtimes to run many iterations over all codepaths to detect that they are actually  in fact  constants  before they can optimise them  Although this is less of a problem than bug detection and developer comprehensibility   When to use const  If the value of the reference does not change over the course of execution  the correct syntax to express the programmer s intent is const  For objects  changing the value of the reference means pointing to another object  as the reference is immutable  but the object is not    const  objects  For object references  the pointer cannot be changed to another object  but the object that is created and assigned to a const declaration is mutable  You can add or remove items from a const referenced array  and mutate property keys on a const referenced object    To achieve immutable objects  which again  make your code easier to reason about for humans and machines   you can Object freeze the object at declaration assignment creation  like this   const Options   Object freeze   YES    NO      Object freeze does have an impact on performance  but your code is probably slow for other reasons  You want to profile it   You can also encapsulate the mutable object in a state machine and return deep copies as values  this is how Redux and React state work   See Avoiding mutable global state in Browser JS for an example of how to build this from first principles   When var and let are a good match  let and var represent mutable state  They should  in my opinion  only be used to model actual mutable state  Things like  is the connection alive     These are best encapsulated in testable state machines that expose constant values that represent  the current state of the connection   which is a constant at any point in time  and what the rest of your code is actually interested in   Programming is already hard enough with composing side-effects and transforming data  Turning every function into an untestable state machine by creating mutable state with variables just piles on the complexity   For a more nuanced explanation  see Shun the Mutant - The case for const

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Main point is that how to decide which one identifier should be used during development  In java-script here are three identifiers   1  var  Can re-declared  amp  re-initialize  2  const  Can t re-declared  amp  re-initialize  can update array values by using push  3  let  Can re-initialize but can t re-declare        var    At the time of codding when we talk about code-standard then we usually use name of identifier which one easy to understandable by other user developer     For example if we are working thought many functions where we use some input and process this and return some result  like      Example of variable use    function firstFunction input1 input2     var process   input1   2    var result   process - input2   return result      function otherFunction input1 input2     var process   input1   8    var result   process   input2   return result         In above examples both functions producing different-2 results but using same name of variables  Here we can see  process   amp   result  both are used as variables and they should be        Example of constant with variable     const tax   10    const pi   3 1415926535    function firstFunction input1 input2     var process   input1   2    var result   process - input2   result    result   tax  100    return result      function otherFunction input1 input2     var process   input1   8    var result   process   input2   pi   return result         Before using  let  in java-script we have to add    use strict    on the top of js file      Example of let with constant  amp  variable     const tax   10    const pi   3 1415926535    let trackExecution        function firstFunction input1 input2     trackExecution     On firstFunction     var process   input1   2    var result   process - input2   result    result   tax  100    return result      function otherFunction input1 input2     trackExecution     On otherFunction     can add current time   var process   input1   8    var result   process   input2   pi   return result      firstFunction     otherFunction     console log trackExecution        In above example you can track which one function executed when  amp  which one function not used during specific action

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Summary    const creates an immutable binding meaning const variable identifier is not re-assignable    const a    value1     you cannot re-assign it with   a    value2     However  if const identifier holds an object or an array  value of it can be changed as far as we are not re-assigning it   const x     a  1    x a   2    is possible and allowed  const numbers    1  2   numbers push 3     is possible and allowed   Please note that const is a block-scoped just like let which is not same as var   which is function-scoped   In short  When something is not likely to change through re-assignment use const else use let or var depending on the scope you would like to have   It s much easier to reason about the code when it is dead obvious what can be changed through re-assignment and what can t  be  Changing a const to a let is dead simple  And going const by default makes you think twice before doing so  And this is in many cases a good thing

[javascript] Const in JavaScript: when to use it and is it necessary?

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