What does the construct x x y mean

Question

I am debugging some JavaScript  and can t explain what this    does   function  title  msg      var title   title     Error     var msg     msg     Error on Request       Can someone give me an hint  why this guy is using var title   title     ERROR   I sometimes see it without a var declaration as well

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If title is not set  use  ERROR  as default value   More generic   var foobar   foo    default    Reads  Set foobar to foo or default  You could even chain this up many times   var foobar   foo    bar    something    42

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It means the title argument is optional  So if you call the method with no arguments it will use a default value of  Error    It s shorthand for writing   if   title      title    Error       This kind of shorthand trick with boolean expressions is common in Perl too  With the expression   a OR b   it evaluates to true if either a or b is true  So if a is true you don t need to check b at all  This is called short-circuit boolean evaluation so   var title   title     Error     basically checks if title evaluates to false  If it does  it  returns   Error   otherwise it returns title

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Whilst Cletus  answer is correct  I feel more detail should be added in regards to  evaluates to false  in JavaScript    var title   title     Error   var msg     msg     Error on Request     Is not just checking if title msg has been provided  but also if either of them are falsy  i e  one of the following         false       0  zero        empty string    null    undefined    NaN  a special Number value meaning Not-a-Number        So in the line  var title   title     Error     If title is truthy  i e   not falsy  so title    titleMessage  etc   then the Boolean OR      operator has found one  true  value  which means it evaluates to true  so it short-circuits and returns the true value  title    If title is falsy  i e  one of the list above   then the Boolean OR      operator has found a  false  value  and now needs to evaluate the other part of the operator   Error   which evaluates to true  and is hence returned   It would also seem  after some quick firebug console experimentation  if both sides of the operator evaluate to false  it returns the second  falsy  operator   i e   return        undefined    returns undefined  this is probably to allow you to use the behavior asked about in this question when trying to default title message to     i e  after running  var foo   undefined foo   foo         foo would be set to

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Basically it checks if the value before the    evaluates to true  if yes  it takes this value  if not  it takes the value after the      Values for which it will take the value after the     as far as i remember     undefined false 0     Null or Null string

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And I have to add one more thing  This bit of shorthand is an abomination  It misuses an accidental interpreter optimization  not bothering with the second operation if the first is truthy  to control an assignment  That use has nothing to do with the purpose of the operator  I do not believe it should ever be used   I prefer the ternary operator for initialization  eg    var title   title title  Error     This uses a one-line conditional operation for its correct purpose  It still plays unsightly games with truthiness but  that s Javascript for you

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What is the double pipe operator       The double pipe operator      is the logical OR operator   In most languages it works the following way   If the first value is false  it checks the second value  If that s true  it returns true and if the second value is false  it returns false  If the first value is true  it always returns true  no matter what the second value is   So basically it works like this function  function or x  y      if  x        return true      else if  y        return true      else       return false         If you still don t understand  look at this table          true   false   ------ --------------- true    true   true    false   true   false    In other words  it s only false when both values are false  How is it different in JavaScript  JavaScript is a bit different  because it s a loosely typed language  In this case it means that you can use    operator with values that are not booleans  Though it makes no sense  you can use this operator with for example a function and an object   function             What happens there  If values are not boolean  JavaScript makes implicit conversion to boolean  It means that if the value is falsey  e g  0   quot  quot   null  undefined  see also All falsey values in JavaScript    it will be treated as false  otherwise it s treated as true  So the above example should give true  because empty function is truthy  Well  it doesn t  It returns the empty function  That s because JavaScript s    operator doesn t work as I wrote at the beginning  It works the following way   If the first value is falsey  it returns the second value  If the first value is truthy  it returns the first value   Surprised  Actually  it s  quot compatible quot  with the traditional    operator  It could be written as following function  function or x  y      if  x        return x      else       return y         If you pass a truthy value as x  it returns x  that is  a truthy value  So if you use it later in if clause   function x  y      var eitherXorY   x    y    if  eitherXorY        console log  quot Either x or y is truthy  quot        else       console log  quot Neither x nor y is truthy quot          true    undefined       you get  quot Either x or y is truthy  quot   If x was falsey  eitherXorY would be y  In this case you would get the  quot Either x or y is truthy  quot  if y was truthy  otherwise you d get  quot Neither x nor y is truthy quot   The actual question Now  when you know how    operator works  you can probably make out by yourself what does x   x    y mean  If x is truthy  x is assigned to x  so actually nothing happens  otherwise y is assigned to x  It is commonly used to define default parameters in functions  However  it is often considered a bad programming practice  because it prevents you from passing a falsey value  which is not necessarily undefined or null  as a parameter  Consider following example  function badFunction    boolean   flagA      flagA   flagA    true    console log  quot flagA is set to  quot     flagA    quot true quot     quot false quot        It looks valid at the first sight  However  what would happen if you passed false as flagA parameter  since it s boolean  i e  can be true or false   It would become true  In this example  there is no way to set flagA to false  It would be a better idea to explicitly check whether flagA is undefined  like that  function goodFunction    boolean   flagA      flagA   typeof flagA      quot undefined quot    flagA   true    console log  quot flagA is set to  quot     flagA    quot true quot     quot false quot        Though it s longer  it always works and it s easier to understand   You can also use the ES6 syntax for default function parameters  but note that it doesn t work in older browsers  like IE   If you want to support these browsers  you should transpile your code with Babel  See also Logical Operators on MDN

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To add some explanation to all said before me  I should give you some examples to understand logical concepts   var name   false     Mohsen     name equals to Mohsen var family   true     Alizadeh    family equals to true   It means if the left side evaluated as a true statement it will be finished and the left side will be returned and assigned to the variable  in other cases the right side will be returned and assigned   And operator have the opposite structure like below   var name   false  amp  amp   Mohsen    name equals to false var family   true  amp  amp   Alizadeh    family equals to Alizadeh

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Quote   What does the construct x   x    y mean    Assigning a default value   This means providing a default value of y to x  in case x is still waiting for its value but hasn t received it yet or was deliberately omitted in order to fall back to a default

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Explaining this a little more     The    operator is the logical-or operator   The result is true if the first part is true and it is true if the second part is true and it is true if both parts are true   For clarity  here it is in a table    X   Y   X    Y  --- --- --------  F   F     F     --- --- --------  F   T     T     --- --- --------  T   F     T     --- --- --------  T   T     T     --- --- --------   Now notice something here   If X is true  the result is always true   So if we know that X is true we don t have to check Y at all   Many languages thus implement  short circuit  evaluators for logical-or  and logical-and coming from the other direction    They check the first element and if that s true they don t bother checking the second at all   The result  in logical terms  is the same  but in terms of execution there s potentially a huge difference if the second element is expensive to calculate   So what does this have to do with your example   var title     title     Error     Let s look at that   The title element is passed in to your function   In JavaScript if you don t pass in a parameter  it defaults to a null value   Also in JavaScript if your variable is a null value it is considered to be false by the logical operators   So if this function is called with a title given  it is a non-false value and thus assigned to the local variable   If  however  it is not given a value  it is a null value and thus false   The logical-or operator then evaluates the second expression and returns  Error  instead   So now the local variable is given the value  Error    This works because of the implementation of logical expressions in JavaScript   It doesn t return a proper boolean value  true or false  but instead returns the value it was given under some rules as to what s considered equivalent to true and what s considered equivalent to false   Look up your JavaScript reference to learn what JavaScript considers to be true or false in boolean contexts

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Double pipe stands for logical  OR   This is not really the case when the  parameter not set   since strictly in the javascript if you have code like this   function foo par        Then calls   foo   foo     foo null  foo undefined  foo 0    are not equivalent    Double pipe      will cast the first argument to boolean and if resulting boolean is true - do the assignment otherwise it will assign the right part   This matters if you check for unset parameter    Let s say  we have a function setSalary that has one optional parameter  If user does not supply the parameter then the default value of 10 should be used   if you do the check like this   function setSalary dollars        salary   dollars    10     This will give unexpected result on call like   setSalary 0     It will still set the 10 following the flow described above

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is the boolean OR operator  As in javascript  undefined  null  0  false are considered as falsy values   It simply means  true    true   true false    true   true true    false   true false    false   false     undefined     value     value   value     undefined    value  null     value     value   value     null    value  0     value     value   value     0    value  false     value     value   value     false    value

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double pipe operator  is this example usefull   var section   document getElementById  special    if  section        section   document getElementById  main        can also be  var section   document getElementById  special      document getElementById  main

[javascript] What does the construct x = x || y mean?

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