TypeScript add static helper to prototype of existing class

Question

This is mainly a question on how to add/extend any existing type with static custom methods.

I want to have the String prototype extended with a function, e.g. isNullOrEmpty which should be invoked the C# way:

if(!String.isNullOrEmpty(myStringToCheck)){
    // do things as of myStringToCheck is set to something
}

In plain javascript I could do something like

String.isNullOrEmpty = function (s) {
    if (s == null || s === "")
        return true;

    return false;
}

But, when calling it inside a TypeScript it tells me

The property 'isNullOrEmpty' does not exist on value of type '{ prototype: String; fromCharCode(...codes: number[]): string; (value?: any): string; new(value?: any): String; }'.

How can this be done so it is known by TypeScript?

Edit #1

How is String.fromCharCode() implemented which is already known by TypeScript?

Edit #2

Because of other dependencies in the project I'm currently only allowed to use TypeScript 1.0!

Edit #3

String.d.ts

interface StringConstructor {
    isNullOrEmpty(): boolean;
}

interface String {
    format(...args: any[]): string;
    isNullOrEmpty(): boolean;
}

and my String.ts

/// <reference path="../../../typings/String.d.ts"/>

String.prototype.format = function (): string {
    var formatted = this;
    for (var i = 0; i < arguments.length; i++) {
        var regexp = new RegExp("\\{" + i + "\\}", "gi");
        formatted = formatted.replace(regexp, arguments[i]);
    }
    return formatted;
}

String.isNullOrEmpty = function(s) { <-- here is the exception
    if (s == null || s === "")
        return true;
    return false;
}

Solution #1 (TypeScript version > 1.0?)

see first answer by MartyIX

Solution #2 (workaround for TypeScript version 1.0)

see second answer

You should mark your answer with the green tick. :)

MartyIX
– MartyIX

2015-10-23 06:28:31 +00:00
Commented Oct 23, 2015 at 6:28 — MartyIX
– MartyIX, Commented Oct 23, 2015 at 6:28

MartyIX · Accepted Answer · 2015-10-14 08:25:48Z

2

// String.ts
interface StringConstructor {
    isNullOrEmpty(text: string): boolean;
}

interface String {
    isNullOrEmpty(text: string): boolean;
}

String.isNullOrEmpty = (s:string):boolean => {
    return (s == null || s === "");
};

// OtherFile.ts
///<reference path="path/to/String.ts" />

if(!String.isNullOrEmpty("test")){
    // Do something
}

var isEmpty = String.fromCharCode(100).isNullOrEmpty("Nah");

This works for me. The interface extends StringConstructor.

edited Oct 14, 2015 at 8:25

answered Oct 14, 2015 at 8:04

MartyIX

28.8k33 gold badges143 silver badges218 bronze badges

Sign up to request clarification or add additional context in comments.

11 Comments

KingKerosin Over a year ago

And how would I implement the method? I have a String.ts where I have some more extensions already implemented (but currently only new functions instead of static functions).

MartyIX Over a year ago

Please see the edited answer. You don't need to do anything fancy here. Just do what you would do in plain JavaScript.

KingKerosin Over a year ago

This is not working. Is still says

The property 'isNullOrEmpty' does not exist on value of type '{ prototype: String; fromCharCode(...codes: number[]): string; (value?: any): string; new(value?: any): String; }'.

Maybe an version-issue?

MartyIX Over a year ago

What about the latest version?

MartyIX Over a year ago

Well, if you my code to typescriptlang.org/Playground, it works. Unfortunately, I don't have TypeScript 1.0, it's rather old.

|

KingKerosin · Accepted Answer · 2015-10-14 08:50:06Z

1

As by comments from @MartyIX, it seems to be a problem with TypeScript 1.0. As a workaround I've added a method to the prototype of String:

String.d.ts (definition)

interface String {
    isNullOrEmpty(text: string): boolean;
}

String.ts (implementation)

String.prototype.isNullOrEmpty = function(text) => {
    if (text == null || text === "")
        return true;

    return false;
}

which can be used

if(!String.prototype.isNullOrEmpty(myStringToCheck)){
    // do things as of myStringToCheck is set to something
}

answered Oct 14, 2015 at 8:50

KingKerosin

3,8815 gold badges44 silver badges87 bronze badges

Collectives™ on Stack Overflow

TypeScript add static helper to prototype of existing class

2 Answers 2

11 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

11 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related