Question 259192 in Javascript Question posted in
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Javascript – Get the value of the object using the string index in ts and continue to access it


interface IpPatientAddressDto {
  ...
  addressSeq: number;
}

interface IpPatientInfoDto {
  ...
  localAddress: IpPatientAddressDto;
}

const originalPatient:IpPatientInfoDto  = {
  ...
  localAddress:{
     addressSeq:0001
  }
}

 const createAddrCollectionObj = (prop: keyof IpPatientInfoDto) => {
      return {
         addressSeq: originalPatient[prop].addressSeq,  // error:TS2339      
      }
 }

const a1 = createAddrCollectionObj('localAddress');

How do I need to define prop?

typescript error:TS2339: Property  addressSeq  does not exist on type
string | number | Date | IpPatientAddressDto | IpPatientRelationshipDto[] Property  addressSeq  does not exist on type  string

I know the way to write it, but I don’t want to provide extra ways to get it.

const get = <T extends keyof ObjType>(key: T): ObjType[T] => {
    return obj[key]
}
let res = get("a"); //"1"
Tags:

2

Answers


  1. 0

    You need to add a key check so the type system can know what the property type is. All it knows is you’re passing a key of IpPatientInfoDto but there are other properties that do not have type IpPatientAddressDto.

    const createAddrCollectionObj = (prop: keyof IpPatientInfoDto) => {
    switch (prop)
    {
        case 'localAddress': // originalPatient['localAddress'] is IpPatientAddressDto
            return {
                addressSeq: originalPatient[prop].addressSeq,
            };
        // other cases...
        default:
            return {
                addressSeq: null,
            };
    }
 };
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  2. 0

    Apparently only the property localAdress has an object with an addressSeq property. What I see is that TS is making you avoid a mistake of accesing a property that may not exist.

    What you can do is use a Type Guard to check if the object has the shape you expect to have like this:

    function isIpPatientAddressDto(obj: IpPatientInfoDto[keyof IpPatientInfoDto]): obj is IpPatientAddressDto {
  return obj.hasOwnProperty('addressSeq');
}

const createAddrCollectionObj = (prop: keyof IpPatientInfoDto) => {
  const property = originalPatient[prop];
  if (!isIpPatientAddressDto(property)) {
    return {
      addressSeq: defaultValue, //defaultValue needs to be defined elsewhere   
    };
  }

  return {
    addressSeq: property.addressSeq   
  }
}

const a1 = createAddrCollectionObj('localAddress');

    In that way you check if the property exists first or return a default value if it doesn’t.

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