Author: John Au-Yeung

Sometimes, we want to create an object index key type in TypeScript.

In this article, we’ll look at how to create an object index key type in TypeScript.

How to create an object index key type in TypeScript?

To create an object index key type in TypeScript, we can use index signatures.

For instance, we write

interface IDictionary<TValue> {
  [id: string]: TValue;
}

class Test {
  private dictionary: IDictionary<string>;
  //...
}

to create the IDictionary interface that takes the TValue generic type.

Then we can use it as the type for the dictionary variable in the Test class.

As a result, dictionary should be assigned an object with all string property values.

Conclusion

To create an object index key type in TypeScript, we can use index signatures.

Sometimes, we want to add types for rest props in React and TypeScript.

In this article, we’ll look at how to add types for rest props in React and TypeScript.

How to add types for rest props in React and TypeScript?

To add types for rest props in React and TypeScript, we can add an index signature into the prop’s type.

For instance, we write

type Props = {
  id: number;
  name: string;
  [x: string]: any;
};

const MyComponent: React.FC<Props> = (props) => {
  //...
};

to add the Props type that has the

[x: string]: any;

index signature that lets us allow any property in props.

Then we can access any property in addition to id and name without errors.

Conclusion

To add types for rest props in React and TypeScript, we can add an index signature into the prop’s type.

Sometimes, we want to fix the "property does not exist on ‘Object’" error in TypeScript.

In this article, we’ll look at how to fix the "property does not exist on ‘Object’" error in TypeScript.

How to fix the "property does not exist on ‘Object’" error in TypeScript?

To fix the "property does not exist on ‘Object’" error in TypeScript, we can define an interface and use that as the type instead of using the Object type.

For instance, we write

interface Options {
  selector?: string;
  template?: string;
}

const doStuff = (o: Options) => {
  //...
};

doStuff({});
doStuff({ selector: "foo" });

to add the Options interface that has some optional properties inside.

Then we use that as the type for the o parameter in doStuff.

Finally, we can call it with some objects that matches the structure of the Options interface.

Conclusion

To fix the "property does not exist on ‘Object’" error in TypeScript, we can define an interface and use that as the type instead of using the Object type.

Sometimes, we want to fix sort by date not working with TypeScript.

In this article, we’ll look at how to fix sort by date not working with TypeScript.

How to fix sort by date not working with TypeScript?

To fix sort by date not working with TypeScript, we can use the difference in the timestamps between 2 entries in the array.

For instance, we write

const sorted = myArray.sort((a: TaskItemVO, b: TaskItemVO) => {
  return a.dueDate.getTime() - b.dueDate.getTime();
});

to call myArray.sort with a callback that returns the difference of the dueDate timestamps which we get from the getTime method.

The sort callback should return a number and subtracting timestamps would return a number since timestamps are numbers.

Conclusion

To fix sort by date not working with TypeScript, we can use the difference in the timestamps between 2 entries in the array.

Sometimes, we want to merge two enums in TypeScript.

In this article, we’ll look at how to merge two enums in TypeScript.

How to merge two enums in TypeScript?

To merge two enums in TypeScript, we can use the spread operator.

For instance, we write

enum Mammals {
  Humans = "Humans",
  Bats = "Bats",
  Dolphins = "Dolphins",
}

enum Reptiles {
  Snakes = "Snakes",
  Alligators = "Alligators",
  Lizards = "Lizards",
}

const Animals = { ...Mammals, ...Reptiles };
type Animals = typeof Animals;

to create the Mammals and Reptiles enums.

Then we combine them in the Animals object by spreading the entries of Mammals and Reptiles into it.

And finally, we create the Animals type by using the typeof operator on Animals.

Conclusion

To merge two enums in TypeScript, we can use the spread operator.