Declaring a boolean in JavaScript using just var

Question

If I declare a JavaScript boolean variable like this   var IsLoggedIn    And then initialize it with either true or 1  is that safe   Or will initializing it with 1 make the variable a number

User · Answer

The variable will become what ever type you assign it   Initially it is undefined   If you assign it  true  it will become a string  if you assign it true it will become a boolean  if you assign it 1 it will become a number   Subsequent assignments may change the type of the variable later

User · Answer

No it is not safe  You could later do var IsLoggedIn    Foo   and JavaScript will not throw an error   It is possible to do  var IsLoggedIn   new Boolean false   var IsLoggedIn   new Boolean true     You can also pass the non boolean variable into the new Boolean   and it will make IsLoggedIn boolean    var IsLoggedIn   new Boolean 0      false var IsLoggedIn   new Boolean NaN      false var IsLoggedIn   new Boolean  Foo       true var IsLoggedIn   new Boolean 1      true

User · Answer

If you want IsLoggedIn to be treated as a boolean you should initialize as follows   var IsLoggedIn true    If you initialize it with var IsLoggedIn 1  then it will be treated as an integer   However at any time the variable IsLoggedIn could refer to a different data type    IsLoggedIn  Hello World     This will not cause an error

User · Answer

Types are dependent to your initialization   var IsLoggedIn1    true     string var IsLoggedIn2   1    integer var IsLoggedIn3   true    bool   But take a look at this example    var IsLoggedIn1    true     string IsLoggedIn1   true    now your variable is a boolean   Your variables  type depends on the assigned value in JavaScript

User · Answer

You can use and test uninitialized variables at least for their  definedness    Like this   var iAmNotDefined  alert  iAmNotDefined     true   or alert   iAmNotDefined     false   Furthermore  there are many possibilites  if you re not interested in exact types use the      operator  or   variable       variable   for comparison  that is what Douglas Crockford calls  truthy  or  falsy  I think   In that case assigning true or 1 or  1  to the unitialized variable always returns true when asked  Otherwise  if you need type safe comparison  use       for comparison   var thisMayBeTrue   thisMayBeTrue   1  alert thisMayBeTrue    true       gt  true alert   thisMayBeTrue       gt  true alert thisMayBeTrue     true       gt  false  thisMayBeTrue    1   alert thisMayBeTrue    true       gt  true  alert   thisMayBeTrue       gt  true alert thisMayBeTrue     true       gt  false    so  in this case  using    or     1  is implicitly     converted to 1 and 1 is implicitly converted to true   thisMayBeTrue   true  alert thisMayBeTrue    true       gt  true alert   thisMayBeTrue       gt  true alert thisMayBeTrue     true       gt  true  thisMayBeTrue    true   alert thisMayBeTrue    true       gt  false alert   thisMayBeTrue       gt  true alert thisMayBeTrue     true       gt  false    so  here s no implicit conversion of the string  true     it s also a demonstration of the fact that the       or    operator tests the  definedness  of a variable    PS  you can t test  definedness  for nonexisting variables though  So   alert   HelloWorld     gives a reference Error   HelloWorld is not defined     is there a better word for  definedness   Pardon my dutch anyway

User · Answer

Variables in Javascript don t have a type  Non-zero  non-null  non-empty and true are  true   Zero  null  undefined  empty string and false are  false    There s a Boolean type though  as are literals true and false

User · Answer

As this very useful tutorial says   var age   0      bad var hasAge   new Boolean age       good var hasAge   Boolean age       good var hasAge     age

User · Answer

How about something like this   var MyNamespace         convertToBoolean  function  value              VALIDATE INPUT         if  typeof value      undefined     value     null  return false             DETERMINE BOOLEAN VALUE FROM STRING         if  typeof value      string                 switch  value toLowerCase                      case  true                   case  yes                   case  1                       return true                  case  false                   case  no                   case  0                       return false                                     RETURN DEFAULT HANDLER         return Boolean value              Then you can use it like this   MyNamespace convertToBoolean  true     true MyNamespace convertToBoolean  no     false MyNamespace convertToBoolean  1     true MyNamespace convertToBoolean 0    false   I have not tested it for performance  but converting from type to type should not happen too often otherwise you open your app up to instability big time

[javascript] Declaring a boolean in JavaScript using just var

Examples related to javascript

Examples related to boolean