for-in statement

Question

TypeScript docs say nothing about loop like for or for-in  From playing with the language it seems that only any or string variables are supported in for loop   Why has this decision been made   Why not use the type information and have strongly-typed iteration variable

User · Answer

In Typescript 1 5 and later  you can use for  of as opposed to for  in  var numbers    1  2  3    for  var number of numbers        console log number

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TypeScript isn t giving you a gun to shoot yourself in the foot with   The iterator variable is a string because it is a string  full stop  Observe   var obj       obj  0      quote zero quote   obj 0 0     zero point zero   obj   object Object       literal string   object Object     obj  lt any gt obj     this obj  obj  lt any gt undefined     undefined   obj  lt any gt  undefined      the literal string  undefined     for var key in obj        console log  Type      typeof key       console log key       gt      obj key        How many key value pairs are in obj now  6  more or less  No  3  and all of the keys are strings   Type  string 0   gt  zero point zero Type  string  object Object    gt  this obj   Type  string undefined   gt  the literal string  undefined

User · Answer

edit 2018  This is outdated  js and typescript now have for  of loops  http   www typescriptlang org docs handbook iterators-and-generators html    The book  TypeScript Revealed  says    You can iterate through the items in an array by using either for or for  in loops as demonstrated here      standard for loop for  var i   0  i  lt  actors length  i        console log actors i          for  in loop for  var actor in actors      console log actor          Turns out  the second loop does not pass the actors in the loop  So would say this is plain wrong  Sadly it is as above  loops are untouched by typescript   map and forEach often help me and are due to typescripts enhancements on function definitions more approachable  lke at the very moment   this notes   arr map state   gt  new Note state      My wish list to TypeScript    Generic collections   Iterators  IEnumerable  IEnumerator interfaces would be best

User · Answer

The for-in statement is really there to enumerate over object properties  which is how it is implemented in TypeScript  There are some issues with using it on arrays   I can t speak on behalf of the TypeScript team  but I believe this is the reason for the implementation in the language

[typescript] for-in statement

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