Are Arrow Functions and Functions equivalent interchangeable

Question

Arrow functions in ES2015 provide a more concise syntax     Can I replace all my function declarations   expressions with arrow functions now   What do I have to look out for    Examples   Constructor function  function User name      this name   name        vs  const User   name   gt      this name   name       Prototype methods  User prototype getName   function       return this name         vs  User prototype getName        gt  this name    Object  literal  methods  const obj       getName  function                           vs  const obj       getName       gt                        Callbacks  setTimeout function                 500       vs  setTimeout      gt                500     Variadic functions  function sum       let args      slice call arguments                  vs const sum       args    gt

User · Answer

Arrow functions   gt  best ES6 feature so far  They are a tremendously powerful addition to ES6  that I use constantly   Wait  you can t use arrow function everywhere in your code  its not going to work in all cases like this where arrow functions are not usable  Without a doubt  the arrow function is a great addition it brings code simplicity  But you can   t use an arrow function when a dynamic context is required  defining methods  create objects with constructors  get the target from this when handling events  Arrow functions should NOT be used because   They do not have this It uses    lexical scoping    to figure out what the value of    this    should be  In simple word lexical scoping it uses    this    from the inside the function   s body   They do not have arguments Arrow functions don   t have an arguments object  But the same functionality can be achieved using rest parameters  let sum       args    gt  args reduce  x  y    gt  x   y  0  sum 3  3  1     output - 7    They cannot be used with new Arrow functions can t be construtors because they do not have a prototype property    When to use arrow function and when not   Don t use to add function as a property in object literal because we can not access this  Function expressions are best for object methods  Arrow functions are best for callbacks or methods like map  reduce  or forEach  Use function declarations for functions you   d call by name  because they   re hoisted   Use arrow functions for callbacks  because they tend to be terser

User · Answer

tl dr  No  Arrow functions and function declarations   expressions are not equivalent and cannot be replaced blindly  If the function you want to replace does not use this  arguments and is not called with new  then yes   As so often  it depends  Arrow functions have different behavior than function declarations   expressions  so let s have a look at the differences first  1  Lexical this and arguments Arrow functions don t have their own this or arguments binding  Instead  those identifiers are resolved in the lexical scope like any other variable  That means that inside an arrow function  this and arguments refer to the values of this and arguments in the environment the arrow function is defined in  i e   quot outside quot  the arrow function    x000D   x000D     Example using a function expression function createObject       console log  Inside  createObject     this foo     return       foo  42      bar  function           console log  Inside  bar     this foo                  createObject call  foo  21   bar       override  this  inside createObject x000D   x000D   x000D    x000D   x000D     Example using a arrow function function createObject       console log  Inside  createObject     this foo     return       foo  42      bar       gt  console log  Inside  bar     this foo           createObject call  foo  21   bar       override  this  inside createObject x000D   x000D   x000D   In the function expression case  this refers to the object that was created inside the createObject  In the arrow function case  this refers to this of createObject itself  This makes arrow functions useful if you need to access the this of the current environment     currently common pattern var that   this  getData function data      that data   data          better alternative with arrow functions getData data   gt      this data   data       Note that this also means that is not possible to set an arrow function s this with  bind or  call  If you are not very familiar with this  consider reading  MDN - this YDKJS - this  amp  Object prototypes  2  Arrow functions cannot be called with new ES2015 distinguishes between functions that are callable and functions that are constructable  If a function is constructable  it can be called with  new  i e  new User    If a function is callable  it can be called without new  i e  normal function call   Functions created through function declarations   expressions are both constructable and callable  Arrow functions  and methods  are only callable  class constructors are only constructable  If you are trying to call a non-callable function or to construct a non-constructable function  you will get a runtime error   Knowing this  we can state the following  Replaceable   Functions that don t use this or arguments  Functions that are used with  bind this   Not replaceable   Constructor functions Function   methods added to a prototype  because they usually use this  Variadic functions  if they use arguments  see below     Lets have a closer look at this using your examples  Constructor function This won t work because arrow functions cannot be called with new  Keep using a function declaration   expression or use class  Prototype methods Most likely not  because prototype methods usually use this to access the instance  If they don t use this  then you can replace it  However  if you primarily care for concise syntax  use class with its concise method syntax  class User     constructor name        this name   name           getName         return this name         Object methods Similarly for methods in an object literal  If the method wants to reference the object itself via this  keep using function expressions   or use the new method syntax  const obj       getName                         Callbacks It depends  You should definitely replace it if you are aliasing the outer this or are using  bind this      old setTimeout function                bind this   500       new setTimeout      gt                500    But  If the code which calls the callback explicitly sets this to a specific value  as is often the case with event handlers  especially with jQuery  and the callback uses this  or arguments   you cannot use an arrow function  Variadic functions Since arrow functions don t have their own arguments  you cannot simply replace them with an arrow  function  However  ES2015 introduces an alternative to using arguments  the rest parameter     old function sum       let args      slice call arguments                  new const sum       args    gt                  Related question   When should I use Arrow functions in ECMAScript 6  Do ES6 arrow functions have their own arguments or not  What are the differences  if any  between ES6 arrow functions and functions bound with Function prototype bind  How to use arrow functions  public class fields  as class methods   Further resources   MDN - Arrow functions YDKJS - Arrow functions

User · Answer

To use arrow functions with function prototype call  I made a helper function on the object prototype        Using       func   function    use this here  or This   gt   use This here    using func        return func call this  this         usage    var obj    f 3  a 2     using This   gt  This f   This a     5   Edit  You don t NEED a helper  You could do   var obj    f 3  a 2   This   gt  This f   This a  call undefined  obj      5

[javascript] Are 'Arrow Functions' and 'Functions' equivalent / interchangeable?

Arrow functions should NOT be used because:

When to use arrow function and when not:

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