What does the exclamation mark do before the function

Question

function

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Its another way of writing IIFE  immediately-invoked function expression    Its other way of writing -   function  args           same as    function   args

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is a logical NOT operator  it s a boolean operator that will invert something to its opposite    Although you can bypass the parentheses of the invoked function by using the BANG     before the function  it will still invert the return  which might not be what you wanted  As in the case of an IEFE  it would return undefined  which when inverted becomes the boolean true      Instead  use the closing parenthesis and the BANG     if needed        I m going to leave the closing    in all examples as invoking the function with just   and    takes away from what s happening    function    return false          gt  false    function    return false          gt  true     function    return false          gt  false      function    return false          gt  true      Other Operators that work        function    return false          gt  0  - function    return false          gt  -0    function    return false          gt  -1      Combined Operators         function    return false          gt  1  -  function    return false          gt  -1     function    return false          gt  true   - function    return false          gt  true     function    return false          gt  -2      function    return false          gt  -1     function    return false          gt  -1

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The function   function         returns nothing  or undefined     Sometimes we want to call a function right as we create it  You might be tempted to try this   function           but it results in a SyntaxError   Using the   operator before the function causes it to be treated as an expression  so we can call it    function           This will also return the boolean opposite of the return value of the function  in this case true  because  undefined is true  If you want the actual return value to be the result of the call  then try doing it this way    function

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Its just to save a byte of data when we do javascript minification   consider the below anonymous function  function        To make the above as self invoking function we will generally change the above code as   function           Now we added two extra characters     apart from adding    at the end of the function which necessary to call the function  In the process of minification we generally focus to reduce the file size  So we can also write the above function as   function          Still both are self invoking functions and we save a byte as well  Instead of 2 characters     we just used one character

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It returns whether the statement can evaluate to false  eg    false         true  true          false  isValid       is not valid   You can use it twice to coerce a value to boolean     1       true   0       false   So  to more directly answer your question   var myVar    function    return false           myVar contains true   Edit  It has the side effect of changing the function declaration to a function expression   E g  the following code is not valid because it is interpreted as a function declaration that is missing the required identifier  or function name    function      return false           syntax error

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There is a good point for using   for function invocation marked on airbnb JavaScript guide  Generally idea for using this technique on separate files  aka modules  which later get concatenated  The caveat here is that files supposed to be concatenated by tools which put the new file at the new line  which is anyway common behavior for most of concat tools   In that case  using   will help to avoid error in if previously concatenated module missed trailing semicolon  and yet that will give the flexibility to put them in any order with no worry    function abc         function bca          Will work the same as   function abc         function bca           but saves one character and arbitrary looks better   And by the way any of   -   void operators have the same effect  in terms of invoking the function  for sure if you have to use something to return from that function they would act differently   abcval    function abc   return true        abcval equals false bcaval    function bca   return true        bcaval equals 1 zyxval   -function zyx   return true        zyxval equals -1 xyzval    function xyz   return true        your guess    but if you using IIFE patterns for one file one module code separation and using concat tool for optimization  which makes one line one file job   then construction   function abc   no returns          function bca      no returns        Will do safe code execution  same as a very first code sample    This one will throw error cause JavaScript ASI will not be able to do its work    function abc   no returns          function bca      no returns         One note regarding unary operators  they would do similar work  but only in case  they used not in the first module  So they are not so safe if you do not have total control over the concatenation order   This works    function abc   no returns          function bca      no returns        This not    function abc   no returns          function bca      no returns

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JavaScript syntax 101  Here is a function declaration  function foo       Note that there s no semicolon  this is just a function declaration  You would need an invocation  foo    to actually run the function  Now  when we add the seemingly innocuous exclamation mark   function foo      it turns it into an expression  It is now a function expression  The   alone doesn t invoke the function  of course  but we can now put    at the end   function foo        which has higher precedence than   and instantly calls the function  So what the author is doing is saving a byte per function expression  a more readable way of writing it would be this   function          Lastly    makes the expression return true  This is because by default all immediately invoked function expressions  IIFE  return undefined  which leaves us with  undefined which is true  Not particularly useful

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Exclamation mark makes any function always return a boolean  The final value is the negation of the value returned by the function    function bool     return false         true  function bool     return true         false   Omitting   in the above examples would be a SyntaxError   function bool     return true         SyntaxError   However  a better way to achieve this would be    function bool     return true          true

[javascript] What does the exclamation mark do before the function?

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