TypeScript Monkey patch a function into an existing class


Sometimes it's useful to be able to extend a class with new functions. For example let's suppose that a string should be converted to a camel case string. So we need to tell TypeScript, that String contains a function called toCamelCase, which returns a string.

interface String {
    toCamelCase(): string;

Now we can patch this function into the String implementation.

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

If this extension of String is loaded, it's usable like this:

"This is an example".toCamelCase();    // => "thisIsAnExample"