Author: John Au-Yeung

Sometimes, we want to declare a nested class structure in TypeScript.

In this article, we’ll look at how to declare a nested class structure in TypeScript.

How to declare a nested class structure in TypeScript?

To declare a nested class structure in TypeScript, we can create a namespace or module and put our class in it.

For instance, we write

class Outer {}

namespace Outer {
  export class Mid {}

  export module Mid {
    export class Inner {}
  }
}

const a = new Outer();
const b = new Outer.Mid();
const x = new Outer.Mid.Inner();

to create the Outer namespace.

And we put the Mid class in it.

Then we create the Mid module in the same namespace and put the Inner class in it.

Then we can access the 3 classes as we did in the last 3 lines.

Anything before the class name is the namespace or module name.

Conclusion

To declare a nested class structure in TypeScript, we can create a namespace or module and put our class in it.

Sometimes, we want to fix the ‘Jest did not exit one second after the test run has completed’ error with TypeScript.

In this article, we’ll look at how to fix the ‘Jest did not exit one second after the test run has completed’ error with TypeScript.

How to fix the ‘Jest did not exit one second after the test run has completed’ error with TypeScript?

To fix the ‘Jest did not exit one second after the test run has completed’ error with TypeScript, we should make sure we clean up when our test is done.

For instance, we write

beforeAll((done) => {
  done();
});

//...

afterAll((done) => {
  mongoose.connection.close();
  done();
});

to call done after calling mongoose.connection.close(); to close the MongoDB connection after our test is done.

Then Jest knows that the test is done and ends the test.

Conclusion

To fix the ‘Jest did not exit one second after the test run has completed’ error with TypeScript, we should make sure we clean up when our test is done.

Sometimes, we want to fix an enum type not defined at runtime with TypeScript.

In this article, we’ll look at how to fix an enum type not defined at runtime with TypeScript.

How to fix an enum type not defined at runtime with TypeScript?

To fix an enum type not defined at runtime with TypeScript, we can use the const or export keyword before the enum.

For instance, we write

const enum MyEnum {
  One = "one",
  Two = "two",
}

to create the MyEnum enum with the const keyword before it to keep it in the compiled JavaScript code as an object.

We can also write

export enum MyEnum {
  One = "one",
  Two = "two",
}

if we want to export the module and use it in another module.

Conclusion

To fix an enum type not defined at runtime with TypeScript, we can use the const or export keyword before the enum.

Sometimes, we want to create an object with a TypeScript interface as its type.

In this article, we’ll look at how to create an object with a TypeScript interface as its type.

How to create an object with a TypeScript interface as its type?

To create an object with a TypeScript interface as its type, we can use the as keyword to cast an object to the given type.

For instance, we write

const simpleObject = {} as ISimpleObject;

to cast the empty object to the ISimpleObject object type with as.

ISimpleObject is an interface with the list of properties with their types that can be in simpleObject.

Conclusion

To create an object with a TypeScript interface as its type, we can use the as keyword to cast an object to the given type.

Sometimes, we want to initialize a map containing arrays in TypeScript.

In this article, we’ll look at how to initialize a map containing arrays in TypeScript.

How to initialize a map containing arrays in TypeScript?

To initialize a map containing arrays in TypeScript, we can use the Map type.

For instance, we write

const map: Map<string, string[]> = new Map();

to create a Map instance with the Map<string, string[]> type.

The first type is the keys’ types and the 2nd type is the values’ types.

Conclusion

To initialize a map containing arrays in TypeScript, we can use the Map type.