Test for array of string type in TypeScript

Question

How can I test if a variable is an array of string in TypeScript   Something like this   function f    string       var a  string       A    B    C         if  typeof a      string                  return  Yes            else              returns no as it s  object          return  No             TypeScript io here  http   typescript io k0ZiJzso0Qg 2  Edit  I ve updated the text to ask for a test for string    This was only in the code example previously

User · Accepted Answer

You cannot test for string   in the general case but you can test for Array quite easily the same as in JavaScript https   stackoverflow com a 767492 390330 If you specifically want for string array you can do something like  if  Array isArray value        var somethingIsNotString   false     value forEach function item         if typeof item      string             somethingIsNotString   true                   if  somethingIsNotString  amp  amp  value length  gt  0         console log  string

User · Answer

Another option is Array isArray    if   Array isArray classNames         classNames    classNames

User · Answer

there is a little problem here because the   if  typeof item      string         return false     will not stop the foreach  So the function will return true even if the array does contain none string values   This seems to wok for me   function isStringArray value  any   value is number       if  Object prototype toString call value        object Array           if  value length  lt  1           return false         else          return value every  d  any    gt  typeof d      string                 return false      Greetings  Hans

User · Answer

You can have do it easily using Array prototype some   as below  const isStringArray    test  any     boolean   gt     return Array isArray test   amp  amp   test some  value    gt  typeof value      string     const myArray     quot A quot    quot B quot    quot C quot   console log isStringArray myArray      will be log true if string array  I believe this approach is better that others  That is why I am posting this answer  Update on  Sebastian Vitters   s comment Here you can use Array prototype every   as well  const isStringArray    test  any     boolean   gt     return Array isArray test   amp  amp  test every  value    gt  typeof value      string

User · Answer

Try this   if  value instanceof Array    alert  value is Array       else   alert  Not an array

User · Answer

Here is the most concise solution so far   function isArrayOfStrings value  any   boolean      return Array isArray value   amp  amp  value every item   gt  typeof item      string        Note that value every will return true for an empty array  If you need to return false for an empty array  you should add value length to the condition clause   function isNonEmptyArrayOfStrings value  any   boolean       return Array isArray value   amp  amp  value length  amp  amp  value every item   gt  typeof item      string        There is no any run-time type information in TypeScript  and there won t be  see TypeScript Design Goals   Non goals  5   so there is no way to get the type of an empty array  For a non-empty array all you can do is to check the type of its items  one by one

User · Answer

I know this has been answered  but TypeScript introduced type guards  https   www typescriptlang org docs handbook advanced-types html typeof-type-guards  If you have a type like  Object     string   and what to do something conditionally based on what type it is - you can use this type guarding   function isStringArray value  any   value is string       if  value instanceof Array        value forEach function item       maybe only check first value        if  typeof item      string             return false                    return true       return false    function join lt T gt  value  string     T        if  isStringArray value         return value join         value is string   here     else       return value map  x    gt  x toString    join         value is T   here         There is an issue with an empty array being typed as string    but that might be okay

[typescript] Test for array of string type in TypeScript

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