boolean in an if statement

Question

Today I ve gotten a remark about code considering the way I check whether a variable is true or false in a school assignment   The code which I had written was something like this   var booleanValue   true   function someFunction        if booleanValue     true           return  something             They said it was better neater to write it like this   var booleanValue   true   function someFunction        if booleanValue           return  something             The remark which I have gotten about the      true  part was that it was not needed and could create confusion   However my idea is that it is better to check whether the variable is a boolean or not  especially since Javascript is a loosetyped language   In the second example a string would also return  something    So my question  Is it neater to loose the      true  part in the future  or is it good practise to check the type of the variable as well   Edit  In my  real  code the boolean represents whether an image has been deleted or not  so the only values boolValue should ever have is true or false   0 and 1 for example shouldn t be in that variable

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In general  it is cleaner and simpler to omit the     true   However  in Javascript  those statements are different     if  booleanValue  will execute if booleanValue is truthy  ndash  anything other than 0  false      NaN  null  and undefined   if  booleanValue     true  will only execute if booleanValue is precisely equal to true

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In Javascript the idea of boolean is fairly ambiguous  Consider this    var bool   0   if bool        evaluates to false   if   uninitialized var    evaluates to false   So when you re using an if statement   or any other control statement   one does not have to use a  boolean  type var  Therefore  in my opinion  the      true  part of your statement is unnecessary if you know it is a boolean  but absolutely necessary if your value is an ambiguous  truthy  var  More on booleans in javscript can be found here

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Since you already initialized clearly as bool  I think     operator is not required

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In the plain  if  the variable will be coerced to a Boolean and it uses toBoolean on the object -      Argument Type   Result      Undefined       false     Null            false     Boolean         The result equals the input argument  no conversion       Number          The result is false if the argument is  0  -0  or NaN                      otherwise the result is true      String          The result is false if the argument is the empty                      String  its length is zero   otherwise the result is true      Object          true    But comparison with     does not have any type coercion  so they must be equal without coercion   If you are saying that the object may not even be a Boolean then you may have to consider more than just true false   if x   true         else if x   false          else

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It depends on your usecase  It may make sense to check the type too  but if it s just a flag  it does not

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The identity       operator behaves identically to the equality      operator except no type conversion is done  and the types must be the same to be considered equal

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First off  the facts   if  booleanValue    Will satisfy the if statement for any truthy value of booleanValue including true  any non-zero number  any non-empty string value  any object or array reference  etc      On the other hand   if  booleanValue     true    This will only satisfy the if condition if booleanValue is exactly equal to true   No other truthy value will satisfy it   On the other hand if you do this   if  someVar    true    Then  what Javascript will do is type coerce true to match the type of someVar and then compare the two variables   There are lots of situations where this is likely not what one would intend   Because of this  in most cases you want to avoid    because there s a fairly long set of rules on how Javascript will type coerce two things to be the same type and unless you understand all those rules and can anticipate everything that the JS interpreter might do when given two different types  which most JS developers cannot   you probably want to avoid    entirely   As an example of how confusing it can be    x000D   x000D  var x  x000D   x000D  x   0  x000D  console log x    true        false  as expected x000D  console log x    false       true as expected x000D   x000D  x   1  x000D  console log x    true        true  as expected x000D  console log x    false       false as expected x000D   x000D  x   2  x000D  console log x    true        false     x000D  console log x    false       false  x000D   x000D   x000D    For the value 2  you would think that 2 is a truthy value so it would compare favorably to true  but that isn t how the type coercion works   It is converting the right hand value to match the type of the left hand value so its converting true to the number 1 so it s comparing 2    1 which is certainly not what you likely intended   So  buyer beware   It s likely best to avoid    in nearly all cases unless you explicitly know the types you will be comparing and know how all the possible types coercion algorithms work     So  it really depends upon the expected values for booleanValue and how you want the code to work   If you know in advance that it s only ever going to have a true or false value  then comparing it explicitly with   if  booleanValue     true    is just extra code and unnecessary and  if  booleanValue    is more compact and arguably cleaner better   If  on the other hand  you don t know what booleanValue might be and you want to test if it is truly set to true with no other automatic type conversions allowed  then   if  booleanValue     true    is not only a good idea  but required     For example  if you look at the implementation of  on   in jQuery  it has an optional return value   If the callback returns false  then jQuery will automatically stop propagation of the event   In this specific case  since jQuery wants to ONLY stop propagation if false was returned  they check the return value explicity for     false because they don t want undefined or 0 or    or anything else that will automatically type-convert to false to also satisfy the comparison   For example  here s the jQuery event handling callback code   ret     specialHandle    handleObj handler   apply  matched elem  args     if   ret     undefined          event result   ret       if   ret     false              event preventDefault             event stopPropagation                You can see that jQuery is explicitly looking for ret     false   But  there are also many other places in the jQuery code where a simpler check is appropriate given the desire of the code   For example      The DOM ready check for Internet Explorer function doScrollCheck         if   jQuery isReady             return

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Revisa https   www w3schools com js js comparisons asp   example   var p 5   p  5   true p   5     true p    5    false       means same type also same value    just same value

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I think that your reasoning is sound   But in practice I have found that it is far more common to omit the     comparison   I think that there are three reasons for that    It does not usually add to the meaning of the expression - that s in cases where the value is known to be boolean anyway  Because there is a great deal of type-uncertainty in JavaScript  forcing a type check tends to bite you when you get an unexpected undefined or null value   Often you just want your test to fail in such cases    Though I try to balance this view with the  fail fast  motto   JavaScript programmers like to play fast-and-loose with types - especially in boolean expressions - because we can    Consider this example   var someString   getInput    var normalized   someString  amp  amp  trim someString        trim   removes leading and trailing whitespace  if  normalized        submitInput normalized       I think that this kind of code is not uncommon   It handles cases where getInput   returns undefined  null  or an empty string   Due to the two boolean evaluations submitInput   is only called if the given input is a string that contains non-whitespace characters   In JavaScript  amp  amp  returns its first argument if it is falsy or its second argument if the first argument is truthy  so normalized will be undefined if someString was undefined and so forth   That means that none of the inputs to the boolean expressions above are actually boolean values   I know that a lot of programmers who are accustomed to strong type-checking cringe when seeing code like this   But note applying strong typing would likely require explicit checks for null or undefined values  which would clutter up the code   In JavaScript that is not needed

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This depends  If you are concerned that your variable could end up as something that resolves to TRUE  Then hard checking is a must  Otherwise it is up to you  However  I doubt that the syntax whatever    TRUE would ever confuse anyone who knew what they were doing

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If the variable can only ever take on boolean values  then it s reasonable to use the shorter syntax   If it can potentially be assigned other types  and you need to distinguish true from 1 or  foo   then you must use     true

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Also can be tested with Boolean object  if you need to test an object error  Boolean errors email

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Since the checked value is Boolean it s preferred to use it directly for less coding and at all it did same   true

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If you write  if x     true    It will be true for only x   true  If you write  if x    it will be true for any x that is not      empty string   false  null  undefined  0  NaN

[javascript] boolean in an if statement

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