TypeScript for of with index key

Question

As described here TypeScript introduces a foreach loop   var someArray    9  2  5   for  var item of someArray        console log item      9 2 5     But isn t there any index key  I would expect something like   for  var item  key of someArray

User · Answer

Old school javascript  to the rescue  for those who aren t familiar in love of functional programming   for  let i   0  i  lt  someArray length   i        let item   someArray i

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https   developer mozilla org en-US docs Web JavaScript Reference Global Objects Array entries  for  var  key  item  of someArray entries              In TS this requires targeting ES2015 since it requires the runtime to support iterators  which ES5 runtimes don t  You can of course use something like Babel to make the output work on ES5 runtimes

User · Answer

forEach already has this ability   const someArray    9  2  5   someArray forEach  value  index    gt        console log index      0  1  2     console log value      9  2  5       But if you want the abilities of for   of  then you can map the array to the index and value   for  const   index  value   of someArray map  value  index    gt     index  value            console log index      0  1  2     console log value      9  2  5     That s a little long  so it may help to put it in a reusable function   function toEntries lt T gt  a  T          return a map  value  index    gt   index  value  as const      for  const  index  value  of toEntries someArray              etc       Iterable Version  This will work when targeting ES3 or ES5 if you compile with the --downlevelIteration compiler option   function  toEntries lt T gt  values  T     IterableIterator lt T gt         let index   0      for  const value of values            yield  index  value  as const          index              Array prototype entries   - ES6   If you are able to target ES6  environments then you can use the  entries   method as outlined in Arnavion s answer

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You can use the for  in TypeScript operator to access the index when dealing with collections   var test    7 8 9   for  var i in test       console log i          test i         Output    0  7  1  8  2  9   See Demo

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Or another old school solution   var someArray    9  2  5   let i   0  for  var item of someArray        console log item      9 2 5     i

[for-loop] TypeScript for ... of with index / key?

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