Typescript No index signature with a parameter of type string was found on type A string

Question

I have some vanilla javascript code that takes a string input  splits the string into characters  and then matches those characters to a key on an object   DNATranscriber          G   C        C    G        T    A        A    U    function toRna sequence       const sequenceArray       sequence       const transcriptionArray   sequenceArray map character   gt           return this DNATranscriber character                return transcriptionArray join         console log toRna  ACGTGGTCTTAA       Returns UGCACCAGAAUU   This works as expected  I d now like to convert this to typescript    class Transcriptor       DNATranscriber            G  C          C   G          T   A          A   U            toRna sequence  string            const sequenceArray       sequence           const transcriptionArray   sequenceArray map character   gt               return this DNATranscriber character                        export default Transcriptor   But I m getting the following error       Element implicitly has an  any  type because expression of type  string   can t be used to index type     A   string        No index signature with a parameter of type  string  was found on type      A   string     ts 7053    I thought that the issue was that I needed my object key to be a string  But converting them to strings didn t work    DNATranscriber             G   C           C    G           T    A           A    U          I m quite confused by this  It says that no index signature with a type of string exists on my object  But I m sure that it does  What am I doing wrong   Edit - I solved this by giving the DNATranscriber object a type of any   DNATranscriber  any          G   C        C   G        T   A        A   U

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Here is the function example trim generic type of array object const trimArrayObject    lt T gt  items  T      gt       items forEach function  o         for  let  key  value  of Object entries o            const keyName    lt keyof typeof o gt key         if  Array isArray value              trimArrayObject value            else if  typeof o keyName       quot string quot              o keyName    value trim

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Don t Use Any  Use Generics    bad const  getKeyValue    key  string    gt   obj  object    gt  obj key           better const  getKeyValue     key  string    gt   obj  Record lt string  any gt     gt  obj key           best const getKeyValue    lt T extends object  U extends keyof T gt  key  U    gt   obj  T    gt        obj key    Bad - the reason for the error is the object type is just an empty object by default  Therefore it isn t possible to use a string type to index     Better - the reason the error disappears is because now we are telling the compiler the obj argument will be a collection of string value  string any  pairs  However  we are using the any type  so we can do better  Best - T extends empty object  U extends the keys of T  Therefore U will always exist on T  therefore it can be used as a look up value  Here is a full example  I have switched the order of the generics  U extends keyof T now comes before T extends object  to highlight that order of generics is not important and you should select an order that makes the most sense for your function  const getKeyValue    lt U extends keyof T  T extends object gt  key  U    gt   obj  T    gt    obj key    interface User     name  string    age  number     const user  User       name   quot John Smith quot     age  20     const getUserName   getKeyValue lt keyof User  User gt   quot name quot   user         gt   John Smith   Alternative syntax const getKeyValue    lt T  K extends keyof T gt  obj  T  key  K   T K    gt  obj key

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This was what I did to solve my related problem interface Map      key  string   string   undefined    const HUMAN MAP  Map       draft   quot Draft quot      export const human    str  string    gt  HUMAN MAP str     str

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Also  you can do this   this DNATranscriber as any  character    Edit  It s HIGHLY recommended that you cast the object with the proper type instead of any  Casting an object as any only help you to avoid type errors when compiling typescript but it doesn t help you to keep your code type-safe  E g  interface DNA       G   quot C quot       C   quot G quot       T   quot A quot       A   quot U quot     And then you cast it like this   this DNATranscriber as DNA  character

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I resolved a similar issue in my getClass function like this   import   ApiGateway   from    api-gateway class   import   AppSync   from    app-sync class   import   Cognito   from    cognito class    export type stackInstances   typeof ApiGateway   typeof  AppSync   typeof Cognito  export const classes       ApiGateway    AppSync    Cognito   as      key  string   stackInstances     export function getClass name  string      return classes name       Typing my classes const with my union type made typescript happy and it makes sense to me

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For those who Google   No index signature with a parameter of type  string  was found on type     most likely your error should read like   Did you mean to use a more specific type such as keyof Number instead of string   I solved a similar typing issue with code like this  const stringBasedKey    SomeCustomString  someVar   as keyof typeof YourTypeHere   This issue helped me to learn the real meaning of the error

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I messed around with this for awhile  Here was my scenario   I have two types  metrics1 and metrics2  each with different properties   type metrics1         a  number      b  number      c  number     type metrics2         d  number      e  number      f  number      At a point in my code  I created an object that is the intersection of these two types because this object will hold all of their properties   const myMetrics  metrics1  amp  metrics2         a  10      b  20      c  30      d  40      e  50      f  60      Now  I need to dynamically reference the properties of that object  This is where we run into index signature errors  Part of the issue can be broken down based on compile-time checking and runtime checking  If I reference the object using a const  I will not see that error because TypeScript can check if the property exists during compile time   const myKey    a   console log myMetrics myKey       No issues  TypeScript has validated it exists   If  however  I am using a dynamic variable  e g  let   then TypeScript will not be able to check if the property exists during compile time  and will require additional help during runtime  That is where the following typeguard comes in   function isValidMetric prop  string  obj  metrics1  amp  metrics2   prop is keyof  metrics1  amp  metrics2        return prop in obj      This reads as  If the obj has the property prop then let TypeScript know that prop exists in the intersection of metrics1  amp  metrics2   Note  make sure you surround metrics1  amp  metrics2 in parentheses after keyof as shown above  or else you will end up with an intersection between the keys of metrics1 and the type of metrics2  not its keys    Now  I can use the typeguard and safely access my object during runtime   let myKey string       myKey    a   if  isValidMetric myKey  myMetrics         console log myMetrics myKey

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You can fix the errors by validating your input  which is something you should do regardless of course  The following typechecks correctly  via type guarding validations const DNATranscriber         G   C       C   G       T   A       A   U      export default class Transcriptor       toRna dna  string            const codons       dna           if   isValidSequence codons                 throw Error  invalid sequence                      const transcribedRNA   codons map codon   gt  DNATranscriber codon            return transcribedRNA           function isValidSequence values  string     values is Array lt keyof typeof DNATranscriber gt        return values every isValidCodon     function isValidCodon value  string   value is keyof typeof DNATranscriber       return value in DNATranscriber     It is worth mentioning that you seem to be under the misapprehention that converting JavaScript to TypeScript involves using classes  In the following  more idiomatic version  we leverage TypeScript to improve clarity and gain stronger typing of base pair mappings without changing the implementation  We use a function  just like the original  because it makes sense  This is important  Converting JavaScript to TypeScript has nothing to do with classes  it has to do with static types  const DNATranscriber         G    C       C    G       T    A       A    U      export default function toRna dna  string        const codons       dna       if   isValidSequence codons             throw Error  invalid sequence              const transcribedRNA   codons map codon   gt  DNATranscriber codon        return transcribedRNA     function isValidSequence values  string     values is Array lt keyof typeof DNATranscriber gt        return values every isValidCodon     function isValidCodon value  string   value is keyof typeof DNATranscriber       return value in DNATranscriber     Update  Since TypeScript 3 7  we can write this more expressively  formalizing the correspondence between input validation and its type implication using assertion signatures  const DNATranscriber         G    C       C    G       T    A       A    U    as const   type DNACodon   keyof typeof DNATranscriber  type RNACodon   typeof DNATranscriber DNACodon    export default function toRna dna  string   RNACodon         const codons       dna       validateSequence codons       const transcribedRNA   codons map codon   gt  DNATranscriber codon        return transcribedRNA     function validateSequence values  string     asserts values is DNACodon         if   values every isValidCodon             throw Error  invalid sequence                function isValidCodon value  string   value is DNACodon       return value in DNATranscriber     You can read more about assertion signatures in the TypeScript 3 7 release notes

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For anyone struggling with similar cases No index signature with a parameter of type  string  was found on type X  trying to use it with simple objects  used as dicts  like  DNATranscriber        G  quot C quot      C   quot G quot      T   quot A quot      A   quot U quot     and trying to dynamically access the value from a calculated key like  const key   getFirstType dnaChain   const result   DNATranscriber key    and you faced the error as shown above  you can use the keyof operator and try something like const key   getFirstType dnaChain  as keyof typeof DNATranscriber   certainly you will need a guard at the result but if it seems more intuitive than some custom types magic  it is ok

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You have two options with simple and idiomatic Typescript    Use index type   DNATranscriber     char  string   string         G   C     C   G     T   A     A   U        This is the index signature the error message is talking about  Reference   Type each property    DNATranscriber    G  string  C  string  T  string  A  string         G   C     C   G     T   A     A   U

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This will eliminate the error and is type safe  this DNATranscriber character as keyof typeof DNATranscriber

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Solved similar issue by doing this  export interface IItem extends Record lt string  any gt        itemId  string      price  number     const item  IItem     itemId   someId   price  200    const fieldId    someid       gives you no errors and proper typing item fieldId

[javascript] Typescript: No index signature with a parameter of type 'string' was found on type '{ "A": string; }

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