How do you explicitly set a new property on window in typescript

In TypeScript, if you want to add a new property to the window object, you need to extend the Window interface and specify the type of the property you are adding. Since window is a global object, TypeScript requires knowledge of the property's type to ensure type safety. This can be achieved through declaration merging.

Here is an example demonstrating how to add a new property named myNewProperty to the window object:

typescript
// First, extend the existing `Window` interface using declaration merging
interface Window {
  myNewProperty: string;
}

// Then, safely add the new property to `window`
window.myNewProperty = "Hello, World!";

// When using this property, TypeScript knows its type
console.log(window.myNewProperty); // Output: "Hello, World!"

In this example, we first extend the global Window interface by declaring a new Window interface with the same name. The key is to use the interface keyword instead of type. Declaration merging allows us to safely add the myNewProperty property without disrupting the type definitions of other properties or methods on the window object.

Now, TypeScript knows that the window object has a property named myNewProperty with the type string. As a result, when assigning a value to window.myNewProperty, TypeScript will allow any string value and provide error warnings if an incompatible type is attempted.

It's worth noting that directly modifying the window object in actual frontend projects is not a good practice, as it can lead to global state pollution, which is difficult to track and maintain. Typically, we should avoid adding extra state or functionality to the global scope unless it is absolutely necessary.

In TypeScript, adding new properties to the window object requires two steps:

1. Extend the Window Interface: Since TypeScript uses a structural type system, we can extend existing type definitions using declaration merging. For the window object, we need to extend the global Window interface.

2. Set the Property: After extending the interface, we can add properties to the window object as we would in JavaScript.

Below is an example of how to add a new property to the window object:

typescript
// Step 1: Extend the Window interface with the new property
interface Window {
  myCustomProperty: string;
}

// Step 2: Add the property to the window object
window.myCustomProperty = "This is a custom property on the window object";

console.log(window.myCustomProperty); // Output: This is a custom property on the window object

In the above example, we first extend the Window interface to include a string property named myCustomProperty. Then, we set this property on the window object.

It's important to note that this approach is not best practice because directly adding properties to the global object can lead to pollution of the global namespace. In real-world TypeScript projects, we typically avoid this unless it's necessary in specific scenarios, such as when implementing polyfills or integrating with third-party libraries.

Additionally, if your TypeScript compilation options include strict mode or have noImplicitAny set to true, you should ensure that the extension of the window object is performed in the correct scope, such as within a global .d.ts file, to prevent interference with type checking in other files.

In TypeScript, you should generally avoid directly adding new properties to the window object because it can compromise the security and predictability of the global scope. However, if you do need to do this (for example, to integrate with third-party libraries), you can explicitly declare new properties by extending the Window interface.

Here is an example demonstrating how to add a new property named myCustomProperty to the window object in TypeScript:

typescript
// First, use TypeScript's declaration merging feature to extend the global Window interface
declare global {
  interface Window {
    myCustomProperty: string;
  }
}

// Now you can safely set a new property on window
window.myCustomProperty = 'some value';

// You can also read the property
console.log(window.myCustomProperty);

In this example, we first use the declare global statement to extend the global Window interface. Then, we declare myCustomProperty as a string-typed property. This allows the TypeScript compiler to recognize the new property on the window object and its type, enabling safe usage of window.myCustomProperty in other parts of your code without type errors.

Note: If you are working in a module environment (i.e., any file containing top-level import or export declarations), you must wrap declare global in a module declaration; otherwise, it will not be recognized as a global declaration. For example:

typescript
export {}; // This file is now a module

declare global {
  interface Window {
    myCustomProperty: string;
  }
}

window.myCustomProperty = 'some value';

In this case, export {} transforms the file into a module, allowing us to use declare global for global declarations. If you omit export {}, declare global will result in an error because TypeScript will interpret it as an attempt to declare a global variable within the module scope.