tslint codelyzer ng lint error for in statements must be filtered with an if statement

Question

Lint error message       src app detail edit edit component ts 111  5   for      in        statements must be filtered with an if statement   Code snippet  It is a working code  It is also available at angular io form validation section    for  const field in this formErrors             clear previous error message  if any        this formErrors field              const control   form get field          if  control  amp  amp  control dirty  amp  amp   control valid            const messages   this validationMessages field           for  const key in control errors              this formErrors field     messages key                                   Any idea how to fix this lint error

User · Answer

for  const field in this formErrors      if  this formErrors hasOwnProperty field       for  const key in control errors      if  control errors hasOwnProperty key

User · Answer

A neater way of applying  Helzgate s reply is possibly to replace your  for    in  with  for  const field of Object keys this formErrors

User · Answer

use Object keys   Object keys this formErrors  map key   gt      this formErrors key          const control   form get key      if control  amp  amp  control dirty  amp  amp   control valid        const messages   this validationMessages key       Object keys control errors  map key2   gt          this formErrors key     messages key2

User · Answer

I think this message is not about avoiding to use switch  Instead it wants you to check for hasOwnProperty  The background can be read here  https   stackoverflow com a 16735184 1374488

User · Answer

If the behavior of for     in      is acceptable necessary for your purposes  you can tell tslint to allow it   in tslint json  add this to the  rules  section    forin   false   Otherwise   Maxxx has the right idea with   for  const field of Object keys this formErrors

User · Answer

To explain the actual problem that tslint is pointing out  a quote from  the JavaScript documentation of the for   in statement      The loop will iterate over all enumerable properties of the object   itself and those the object inherits from its constructor s prototype    properties closer to the object in the prototype chain override   prototypes  properties     So  basically this means you ll get properties you might not expect to get  from the object s prototype chain    To solve this we need to iterate only over the objects own properties  We can do this in two different ways  as suggested by  Maxxx and  Qwertiy    First solution  for  const field of Object keys this formErrors                 Here we utilize the Object Keys   method which returns an array of a given object s own enumerable properties  in the same order as that provided by a for   in loop  the difference being that a for-in loop enumerates properties in the prototype chain as well    Second solution  for  var field in this formErrors        if  this formErrors hasOwnProperty field                           In this solution we iterate all of the object s properties including those in it s prototype chain but use the Object prototype hasOwnProperty   method  which returns a boolean indicating whether the object has the specified property as own  not inherited  property  to filter the inherited properties out

[angular] tslint / codelyzer / ng lint error: "for (... in ...) statements must be filtered with an if statement"

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