How can I declare optional function parameters in JavaScript

Question

Can I declare default parameter like function myFunc  a  b 0         b is my optional parameter    in JavaScript

User · Accepted Answer

With ES6  This is now part of the language   function myFunc a  b   0          function body     Please keep in mind that ES6 checks the values against undefined and not against truthy-ness  so only real undefined values get the default value - falsy values like null will not default      With ES5   function myFunc a b      b   b    0        b will be set either to b or to 0      This works as long as all values you explicitly pass in are truthy  Values that are not truthy as per MiniGod s comment  null  undefined  0  false      It s pretty common to see JavaScript libraries to do a bunch of checks on optional inputs before the function actually starts

User · Answer

Update  With ES6  this is possible in exactly the manner you have described  a detailed description can be found in the documentation   Old answer  Default parameters in JavaScript can be implemented in mainly two ways   function myfunc a  b           use this if you specifically want to know if b was passed     if  b     undefined               b was not passed              use this if you know that a truthy value comparison will be enough     if  b               b was passed and has truthy value       else              b was not passed or has falsy value              use this to set b to a default value  using truthy comparison      b   b     default value       The expression b     default value  evaluates the value AND existence of b and returns the value of  default value  if b either doesn t exist or is falsy   Alternative declaration   function myfunc a        var b          use this to determine whether b was passed or not     if  arguments length    1               b was not passed       else           b   arguments 1      take second argument           The special  array  arguments is available inside the function  it contains all the arguments  starting from index 0 to N - 1  where N is the number of arguments passed    This is typically used to support an unknown number of optional parameters  of the same type   however  stating the expected arguments is preferred   Further considerations  Although undefined is not writable since ES5  some browsers are known to not enforce this  There are two alternatives you could use if you re worried about this   b     void 0  typeof b      undefined      also works for undeclared variables

[javascript] How can I declare optional function parameters in JavaScript?

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