TypeScript Creating an empty typed container array

Question

I am creating simple logic game called  Three of a Crime  in TypeScript   When trying to pre-allocated typed array in TypeScript  I tried to do something like this   var arr   Criminal      which gave the error       Check format of expression term     also tried doing this  var arr   Criminal         and this produced  cannot convert any   to  Criminal   what is the  TypeScript  way to do this

User · Answer

I know this is an old question but I recently faced a similar issue which couldn't be solved by this way, as I had to return an empty array of a specific type.

I had

return [];

where [] was Criminal[] type.

Neither return: Criminal[] []; nor return []: Criminal[]; worked for me.

At first glance I solved it by creating a typed variable (as you correctly reported) just before returning it, but (I don't know how JavaScript engines work) it may create overhead and it's less readable.

For thoroughness I'll report this solution in my answer too:

let temp: Criminal[] = [];
return temp;

Eventually I found TypeScript type casting, which allowed me to solve the problem in a more concise and readable (and maybe efficient) way:

return <Criminal[]>[];

Hope this will help future readers!

User · Answer

The existing answers missed an option  so here s a complete list      1  Explicitly declare the type var arr  Criminal             2  Via type assertion var arr    lt Criminal   gt     var arr      as Criminal        3  Using the Array constructor var arr   new Array lt Criminal gt        Explicitly specifying the type is the general solution for whenever type inference fails for a variable declaration  The advantage of using a type assertion  sometimes called a cast  but it s not really a cast in TypeScript  works for any expression  so it can be used even when no variable is declared  There are two syntaxes for type assertions  but only the latter will work in combination with JSX if you care about that  Using the Array constructor is something that will only help you in this specific use case  but which I personally find the most readable  However  there is a slight performance impact at runtime   Also  if someone were crazy enough to redefine the Array constructor  the meaning could change    It s a matter of personal preference  but I find the third option the most readable  In the vast majority of cases the mentioned downsides would be negligible and readability is the most important factor      Fun fact  at the time of writing the performance difference was 60  in Chrome  while in Firefox there was no measurable performance difference

User · Answer

The issue of correctly pre-allocating a typed array in TypeScript was somewhat obscured for due to the array literal syntax  so it wasn t as intuitive as I first thought    The correct way would be  var arr   Criminal           This will give you a correctly typed  empty array stored in the variable  arr   Hope this helps others

User · Answer

For publicly access use like below   public arr  Criminal

User · Answer

Please try this which it works for me  return    as Criminal

User · Answer

Okay you got the syntax wrong here  correct way to do this is  var arr  Criminal          I m assuming you are using var so that means declaring it somewhere inside the func   my suggestion would be use let instead of var  If declaring it as c class property usse acces modifiers like private  public  protected

[javascript] TypeScript: Creating an empty typed container array

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