What is not assignable to parameter of type never error in typescript

Question

Code is   const foo    foo  string    gt      const result        result push foo      I get the following TS error       ts  Argument of type  string  is not assignable to parameter of type  never     What am I doing wrong  Is this a bug

User · Answer

I got the same error in ReactJS function component  using ReactJS useState hook  The solution was to declare the type of useState at initialisation   const  items   setItems    useState lt IItem   gt          replace IItem   with your own typing  string  boolean

User · Answer

I was having same error In ReactJS statless function while using ReactJs Hook useState  I wanted to set state of an object array   so if I use the following way  const  items   setItems    useState        and update the state like this    const item     id   new Date   getTime     text    New Text      setItems   item      items       I was getting error   Argument of type    id  number  text  any    is not assignable to parameter of type  never    but if do it like this   const  items   setItems    useState          Error is gone but there is an item at 0 index which don t have any data don t want that    so the solution I found is    const  items   setItems    useState    as any

User · Answer

This seems to be a recent regression or some strange behavior in typescript   If you have the code   const result        Usually it would be treated as if you wrote   const result any          however  if you have both noImplicitAny FALSE  AND strictNullChecks TRUE in your tsconfig  it is treated as   const result never          This behavior defies all logic  IMHO   Turning on null checks changes the entry types of an array    And then turning on noImplicitAny actually restores the use of any without any warnings    When you truly have an array of any  you shouldn t need to indicate it with extra code

User · Answer

The solution i found was   const  files  setFiles    useState    as any

User · Answer

Another way is    const result      as  any

User · Answer

You need to type result to an array of string const result  string

User · Answer

I was able to get past this by using the Array keyword instead of empty brackets   const enhancers  Array lt any gt          Use    if  typeof devToolsExtension      function       enhancers push devToolsExtension

User · Answer

One more reason for the error  if you are exporting after wrapping component with connect     then props may give typescript errorSolution  I didn t explore much as I had the option of replacing connect function with useSelector hook for example    Comp tsx    interface IComp    a  number    const Comp     a  IComp    gt   lt div gt  a  lt  div gt           below line is culprit  you are exporting default the return  value of Connect and there is no types added to that return value of that connect   Comp           export default connect   Comp    --    App tsx    const App        gt         below line gives same error   ts  Argument of type  number  is not assignable to  parameter of type  never      return  lt Comp a  3    gt

User · Answer

Remove  strictNullChecks   true from  compilerOptions  or set it to false in the tsconfig json file of your Ng app  These errors will go away like anything and your app would compile successfully   Disclaimer  This is just a workaround  This error appears only when the null checks are not handled properly which in any case is not a good way to get things done

User · Answer

All you have to do is define your result as a string array  like the following     const result   string           Without defining the array type  it by default will be never  So when you tried to add a string to it  it was a type mismatch  and so it threw the error you saw

[typescript] What is "not assignable to parameter of type never" error in typescript?

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