Monkey patching in Typescript cannot be applied to the String data type

I have developed a new interface for String that incorporates additional utility methods by utilizing Monkey-patching.

interface String {
  toCamelCase(): string;
}

String.prototype.toCamelCase = function (): string {
    return this.replace(/[^a-z ]/gi, '').replace(
      /(?:^\w|[A-Z]|\b\w|\s+)/g,
      (match: any, index: number) => {
        return +match === 0
          ? ''
          : match[index === 0 ? 'toLowerCase' : 'toUpperCase']();
      },
    );
  };
  

When calling the new function in my controller, toCamelCase :

    const str: string = 'this is an example';
    const result = str.toCamelCase();
    console.log(result);

An error arises with the following message:

[Nest] 35664 - ERROR [ExceptionsHandler] str.toCamelCase is not a function TypeError: str.toCamelCase is not a function

What might be causing this issue in the implementation?

Answer №1

Instead of cluttering the String prototype, I propose creating a separate function called camelCase(str: string): string that takes in the desired string and returns it in camel case format. Here is an example:

export const camelCase = (str: string): string => {
  return str.replace(/[^a-z ]/gi, '').replace(
    /(?:^\w|[A-Z]|\b\w|\s+)/g,
    (match: any, index: number) => {
      return +match === 0
        ? ''
        : match[index === 0 ? 'toLowerCase' : 'toUpperCase']();
      },
    );
}

This function can then be imported using

import { camelCase } from './utilties';
and utilized by calling camelCase('Hello World!');

By following this approach, the function remains independent, more reliable, and easier to test compared to modifying the String prototype. Modifying prototypes is also not commonly practiced anymore.

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