FAQ: Union Types - Common Key Value Pairs

This community-built FAQ covers the “Common Key Value Pairs” exercise from the lesson “Union Types”.

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This exercise can be found in the following Codecademy content:

Learn TypeScript

FAQs on the exercise Common Key Value Pairs

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I don’t understand how this part of the function works:

return event.displayname || event.username

if event.displayname has a truthy value, the function returns event.displayname or else it will return event.username

1 Like

I think the example here is misleading

type Goose = { 
  isPettable: boolean; 
  hasFeathers: boolean;
  canThwartAPicnic: boolean;
}
 
type Moose = {
  isPettable: boolean; 
  hasHoofs: boolean;
}
 
const pettingZooAnimal: Goose | Moose = { isPettable: true };
 
console.log(pettingZooAnimal.isPettable); // No TypeScript error
console.log(pettingZooAnimal.hasHoofs); // TypeScript error

The assignment itself will throw a TypeScript error because it doesn’t respect neither Types.
Here’s the code tested on the official TypeScrypt Playground

3 Likes

Was literally just about to say this. Was this allowed in earlier versions of Typescript or was it just an error from the team?

Also, I noticed the properties in the type objects aren’t separated by commas like in normal objects. That was a surprise

1 Like

const batteryStatus: boolean | number = false;

batteryStatus.toString(); // No TypeScript error
batteryStatus.toFixed(2); // TypeScript error

The above is the example given in the exercise.
However it only throws an error since the variable is assigned as a boolean, not because the method does not exist on both booleans and numbers.

when assigning the same variable as a number, there are no errors thrown.