How can I check if a var is a string in JavaScript

Question

How can I check if a var is a string in JavaScript   I ve tried this and it doesn t work     var a string    Hello  I m a string     if  a string typeof  string            this is a string

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Following expression returns true    qwe  constructor     String   Following expression returns true   typeof  qwe       string    Following expression returns false  sic     typeof new String  qwe        string    Following expression returns true   typeof new String  qwe   valueOf        string    Best and right way  imho    if  someVariable constructor     String

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You were close   if  typeof a string      string            this is a string       On a related note  the above check won t work if a string is created with new String  hello   as the type will be Object instead  There are complicated solutions to work around this  but it s better to just avoid creating strings  that way  ever

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Combining the previous answers provides these solutions   if  typeof str     string     str instanceof String    or   Object prototype toString call str       object String

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check for null or undefined in all cases a string  if  a string  amp  amp  typeof a string      string            this is a string and it is not null or undefined

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Now days I believe it s preferred to use a function form of typeof   so     if filename     undefined    typeof filename       string     filename              console log  no filename aborted        return

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The typeof operator isn t an infix  so the LHS of your example doesn t make sense    You need to use it like so     if  typeof a string     string            This is a string      Remember  typeof is an operator  not a function  Despite this  you will see typeof var  being used a lot in the wild  This makes as much sense as var a   4    1    Also  you may as well use     equality comparison operator  since both operands are Strings  typeof always returns a String   JavaScript is defined to perform the same steps had I used      strict comparison operator    As Box9 mentions  this won t detect a instantiated String object    You can detect for that with      var isString   str instanceof String    jsFiddle      or     var isString   str constructor    String    jsFiddle   But this won t work in a multi window environment  think iframes    You can get around this with     var isString   Object prototype toString call str       object String      jsFiddle   But again   as Box9 mentions   you are better off just using the literal String format  e g  var str    I am a string     Further Reading

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My personal approach  which seems to work for all cases  is testing for the presence of members that will all only be present for strings   function isString x        return  typeof x     string     typeof x     object   amp  amp  x toUpperCase  amp  amp  x substr  amp  amp  x charAt  amp  amp  x trim  amp  amp  x replace   true   false       See  http   jsfiddle net x75uy0o6   I d like to know if this method has flaws  but it has served me well for years

[javascript] How can I check if a var is a string in JavaScript?

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