Set JavaScript variable null or leave undefined

Question

When declaring variables at the top of the JavaScript function  is it best practice to set them equal to null  or leave as  undefined   Another way to ask  what circumstances call for each option below   Option A   var a   null      b   null    Option B   var a      b

User · Answer

I usually set it to whatever I expect to be returned from the function.

If a string, than i will set it to an empty string ='', same for object ={} and array=[], integers = 0.

using this method saves me the need to check for null / undefined. my function will know how to handle string/array/object regardless of the result.

User · Answer

Generally  I use null for values that I know can have a  null  state  for example  if jane isManager    false     jane employees   null     Otherwise  if its a variable or function that s not defined yet  and thus  is not  usable  at the moment  but is supposed to be setup later  I usually leave it undefined

User · Answer

Be careful if you use this value to assign some object s property and call JSON stringify later  - nulls will remain  but undefined properties will be omited  as in example below    x000D   x000D  var a  b   null  x000D   x000D  c    a  b   x000D   x000D  console log c   x000D  console log JSON stringify c      a omited x000D   x000D   x000D     or some utility function library that works in similar way or uses JSON stringify underneath

User · Answer

I declare them as undefined when I don t assign a value because they are undefined after all

User · Answer

Generally speak I defined null as it indicates a human set the value and undefined to indicate no setting has taken place

User · Answer

You can use     to declaring NULL variable in Javascript

User · Answer

It depends on the context      undefined  means this value does not exist  typeof returns  undefined   null  means this value exists with an empty value  When you use typeof to test for   null   you will see that it s an object  Other case when you serialize  null  value to backend server like asp net mvc  the server will receive  null   but when you serialize  undefined   the server is unlikely to receive a value

User · Answer

The only time you need to set it  or not  is if you need to explicitly check that a variable a is set exactly to null or undefined   if a     null            is not the same as   if a     undefined         That said  a    null  amp  amp  a    undefined will return true   Fiddle

User · Answer

There are two features of null we should understand    null is an empty or non-existent value  null must be assigned    You can assign null to a variable to denote that currently that variable does not have any value but it will have later on  A null means absence of a value   example -    let a   null   console log a     null

[javascript] Set JavaScript variable = null, or leave undefined?

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