Can you explain the true meaning behind this specific type definition?

Since starting to dive into TypeScript recently, I came across an express server written in TS while browsing the Internet. However, I am struggling to comprehend the type definition of the 'middlewares' argument. Despite attempting to research it in the TypeScript documentation, I have not come across anything similar.

<!-- language: lang-ts -->

    function initializeMiddlewares (middlewares: {
            forEach: (arg0: (middleware: any) => void) => void
        }): void {
            middlewares.forEach(middlewares => {
                this.app.use(middlewares);
            })
    }

When I replace it with just 'any' as the type, I ended up with the following code snippet along with an error message:

TS7006: Parameter 'middlewares' implicitly has an 'any' type.
<!-- language: lang-ts -->

    function initializeMiddlewares(middlewares: any): void {
        middlewares.forEach(middlewares => {
            console.log(middlewares)
            this.app.use(middlewares);
        })
    }

Answer №1

The definition provided explains that initializeMiddlewares is a function that operates on an argument called middlewares, which should return void. The middlewares argument is expected to have a method called forEach, which is a function that takes another function as an argument (arg0). This inner function accepts an any parameter named middleware and returns nothing (void). The outer function also returns void.

If you encounter confusion after making changes, it may be due to the shadowing of the name middlewares, which refers to the inner variable within the forEach function. To enable implicit any, consider modifying your tsconfig.json file with "noImplicitAny": false.

Breaking down the type definition into smaller parts can aid in grasping complex structures:

function initializeMiddlewares (
  // Middlewares argument
  middlewares: {
    // Expected forEach method on the middlewares argument
    forEach: 
      // Inner function
      arg0: (middleware: any) => void)
    // Return type of forEach
    => void,
  }
// Return type of initializeMiddlewares
): void
{
  middlewares.forEach(middleware => {
    this.app.use(middleware);
  })
}

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