Default argument values in JavaScript functions

Question

Possible Duplicate    How do I make a default value for a parameter to a javascript function       in PHP   function func  a   10   b   20        if func   is called with no arguments  a will be 10 and   b  will be 20     How can you do this in JavaScript   I get a error if I try to assign values in function arguments     missing   after formal parameters

User · Accepted Answer

In javascript you can call a function  even if it has parameters  without parameters   So you can add default values like this   function func a  b      if  typeof a     undefined   a   10     if  typeof b     undefined   b   20        your code     and then you can call it like func    to use default parameters   Here s a test   function func a  b      if  typeof a     undefined   a   10     if  typeof b     undefined   b   20      alert  A    a   nB    b       testing func    func 80   func 100 200

User · Answer

function func a  b      if  typeof a     undefined       a   10    if  typeof b     undefined       b   20       do what you want     for example   alert a         b       in shorthand  function func a  b      a    typeof a     undefined   10 a    b    typeof b     undefined   20 b        do what you want     for example   alert a         b

User · Answer

I have never seen it done that way in JavaScript  If you want a function with optional parameters that get assigned default values if the parameters are omitted  here s a way to do it    function a  b          if  typeof a     undefined             a   10                 if  typeof b     undefined             a   20                 alert  a      a     b      b

User · Answer

You cannot add default values for function parameters  But you can do this   function tester paramA  paramB    if  typeof paramA     undefined       paramA   defaultValue      if  typeof paramB     undefined       paramB   defaultValue

User · Answer

You have to check if the argument is undefined   function func a  b        if  a     undefined  a    default value       if  b     undefined  b    default value       Also note that this question has been answered before

User · Answer

ES2015 onwards   From ES6 ES2015  we have default parameters in the language specification  So we can just do something simple like   function A a  b   4  c   5        or combined with ES2015 destructuring   function B  c     c  2    d  e     3  4         For detailed explanation    https   developer mozilla org en docs Web JavaScript Reference Functions default parameters     Default function parameters allow formal parameters to be initialized with default values if no value or undefined is passed    Pre ES2015   If you re going to handle values which are NOT Numbers  Strings  Boolean  NaN  or null you can simply use   So  for Objects  Arrays and Functions that you plan never to send null  you can use   param    DEFAULT VALUE   for example   function X a      a   a    function           Though this looks simple and kinda works  this is restrictive and can be an anti-pattern because    operates on all falsy values      null  NaN  false  0  too - which makes this method impossible to assign a param the falsy value passed as the argument   So  in order to handle only undefined values explicitly  the preferred approach would be   function C a  b      a   typeof a      undefined    DEFAULT VALUE A   a    b   typeof b      undefined    DEFAULT VALUE B   b

[javascript] Default argument values in JavaScript functions

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