How to use the string Constructor in typescript 2

Question

StringConstructor is working in Typescript 1.8:

interface StringConstructor {
    trim(msg: string): string;
}
String.trim = function (msg: string) {
                if (msg)
                    return msg.trim();
                return msg;
            }
 String.trim("Url)

but not working in Typescript 2 and I get the error:

Property 'trim' does not exist on type 'StringConstructor'.

This code, when copied to playground and then runs in the console, works well. — Nitzan Tomer
– Nitzan Tomer, Commented Nov 9, 2016 at 13:31

Community · Accepted Answer · 2017-05-23 10:33:00Z

0

Try this:

 module String {
  export function trim(msg: string): string {
    if (msg)
      return msg.trim();
    return msg;
  }
}
String.trim("Url")

(Note that you missed the string closing quotation mark)

Explanation In short: TypeScript does something called declaration merging, which is explained in the docs.

For more info see Jeffery's answer

edited May 23, 2017 at 10:33

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answered Nov 9, 2016 at 14:07

yuval.bl

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8 Comments

Nitzan Tomer Over a year ago

This seems like an overkill. Why take this approach when there's a simpler one like the one the OP is trying?

yuval.bl Over a year ago

I couldn't make this approach work with my Typescript 2 compiler. Would love to see working examples of this approach :) For example - this supposed to be a working example, yet did not pass my compiler.

Nitzan Tomer Over a year ago

The code in the answer you linked to work great in playground, just like the code the OP posted.

yuval.bl Over a year ago

I've saw that before. I'm not sure which typescript compiler/version playground uses. I've checked it locally using ver 2.0.6 & 2.0.8 and got the same errors as @Hossein. In other words, although you're right, I don't see how it help solving the problem.

Nitzan Tomer Over a year ago

Have you tried putting it in declare global { ... }?

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