Tips for creating a string extension in TypeScript

Adding a custom method to a string in TypeScript can be achieved like so:

const myPoint : string = "23";
const customNumber = myPoint.convertUsingMyCustomImplementation();

Attempting to define the method with a capital 'S' results in the following errors:

// eslint-disable-next-line no-extend-native
String.prototype.convertUsingMyCustomImplementation = function() : number {
  return parseFloat(this);
};

However, there are two errors that occur:

https://i.sstatic.net/LiZZ0.png

Property 'convertUsingMyCustomImplementation' does not exist on type 'String'.

Argument of type 'String' is not assignable to parameter of type 'string'. 'string' is a primitive, but 'String' is a wrapper object. Prefer using 'string' when possible.

Another attempt with the following code:

// eslint-disable-next-line no-extend-native
string.prototype.convertUsingMyCustomImplementation = function() : number {
  return parseFloat(this);
};

Also results in an error:

https://i.sstatic.net/4zbDt.png

To address these errors and successfully add a custom method in TypeScript, the correct approach should be considered.

Answer №1

It seems like your prototype method was correct. To ensure that the string will have this method, you just need to specify it in a .d.ts file for universal access. Alternatively, you can also include it in a regular ts file as a quick solution.

declare global { 
    interface String { 
        applyCustomMethod(): number;
    } 
}

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