Validation of object with incorrect child fields using Typeguard

This code snippet validates the 'Discharge' object by checking if it contains the correct children fields.

interface DischargeEntry {
  date: string;
  criteria: string;
}

const isDischargeEntry = (discharge:unknown): discharge is DischargeEntry => {
  
  return (
          (((discharge as DischargeEntry).date) !== undefined) ||
          (((discharge as DischargeEntry).criteria) !== undefined)
         );
  
}

const incorrectDischarge:unknown = {
  criteria: "Thumb has healed."
};

console.log('isDischargeEntry:', isDischargeEntry(incorrectDischarge)) // = true (but should be false)

Playground

Is there a mistake in the boolean evaluation within the return statement or could the 'as' keyword be impacting the logic?

Answer №1

To ensure that both properties are mandatory, they must be combined using the && operator to form a logical AND.

function checkDischargeEntry(discharge: unknown): discharge is DischargeEntry {
  return (
          (((discharge as DischargeEntry).date) !== undefined) 
          &&
          (((discharge as DischargeEntry).criteria) !== undefined)
         );
}

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