Author: John Au-Yeung

Sometimes, we want to fix ‘property mock does not exist on type’ error with Jest and TypeScript.

In this article, we’ll look at how to fix ‘property mock does not exist on type’ error with Jest and TypeScript.

How to fix ‘property mock does not exist on type’ error with Jest and TypeScript?

To fix ‘property mock does not exist on type’ error with Jest and TypeScript, we can cast the variable with the error to the jest.MockedFunction<typeof addListener> type.

For instance, we write

const addListenerMock = addListener as jest.MockedFunction<typeof addListener>;

to cast the type with as.

Conclusion

To fix ‘property mock does not exist on type’ error with Jest and TypeScript, we can cast the variable with the error to the jest.MockedFunction<typeof addListener> type.

Sometimes, we want to specify generic object type in TypeScript.

In this article, we’ll look at how to specify generic object type in TypeScript.

How to specify generic object type in TypeScript?

To specify generic object type in TypeScript, we can use the Record type.

For instance, we write

const myObj: Record<string, any> = {
  //...
};

to set myObj to the Record type with string keys and any type for the property values.

Conclusion

To specify generic object type in TypeScript, we can use the Record type.

Sometimes, we want to add enum flags in TypeScript.

In this article, we’ll look at how to add enum flags in TypeScript.

How to add enum flags in TypeScript?

To add enum flags in TypeScript, we can assign the enum entries to binary values.

For instance, we write

enum Traits {
  None = 0,
  Friendly = 1 << 0,
  Mean = 1 << 1,
  Funny = 1 << 2,
  Boring = 1 << 3,
  All = ~(~0 << 4),
}

to assign each entry of Traits to binary values.

Then we can combine them with bitwise operators like

const traits = Traits.Mean | Traits.Funny;

Conclusion

To add enum flags in TypeScript, we can assign the enum entries to binary values.

Sometimes, we want to define an object of objects type in TypeScript.

In this article, we’ll look at how to define an object of objects type in TypeScript.

How to define an object of objects type in TypeScript?

To define an object of objects type in TypeScript, we can use index signatures with the type set to the type for the value object.

For instance, we write

const data: { [name: string]: DataModel } = {
  //...
};

to create a data variable of type { [name: string]: DataModel } where DataModel is an object type.

[name: string] is the index signature that lets us add any property with string keys.

As long as the structure of data matches the structure specified in the type, the type check will pass.

Conclusion

To define an object of objects type in TypeScript, we can use index signatures with the type set to the type for the value object.

Sometimes, we want to return a proper Promise with TypeScript.

In this article, we’ll look at how to return a proper Promise with TypeScript.

How to return a proper Promise with TypeScript?

To return a proper Promise with TypeScript, we can at the type for the resolved value of the promise.

For instance, we write

class C {
  //...
  saveMyClass(updatedMyClass: MyClass) {
    return new Promise<Package>((resolve, reject) => {
      const savedPackage: Package = updatedPackage;
      //...
      setTimeout(() => {
        resolve(savedPackage);
      }, 1500);
    });
  }
  //...
}

to return a promise in the saveMyClass method that has a Package object as the promise’s resolve value.

This is specified with <Package>.

We set savedPackage to the Package type and call resolve with it to make the code work with the type checks.

Conclusion

To return a proper Promise with TypeScript, we can at the type for the resolved value of the promise.