Prototype String with Typescript

Question

The following prototype method for a JavaScript String in Typescript 2.0.3:

interface String {
    splice(start: number, delCount: number, newSubStr: string): string;
}

String.prototype.splice = function(idx: number, rem: number, str: string): string {
    return this.slice(0, idx) + str + this.slice(idx + Math.abs(rem));
};

throws the error:

error TS2339: Property 'splice' does not exist on type 'String'.

despite my interface. It seems to work fine in the playground. I am just running tsc on that file with no options. Why isn't this working?

I ran tsc (2.0.3 and 1.8.10) on a file containing only your code and it worked just fine. Are you sure that this is all? — Nitzan Tomer
– Nitzan Tomer, Commented Oct 4, 2016 at 23:10

basarat · Accepted Answer · 2016-10-05 04:18:33Z

4

It seems to work fine in the playground.

That because you probably have an import / export in your file. Fix:

declare global {
    interface String {
        splice(start: number, delCount: number, newSubStr: string): string;
    }
}

String.prototype.splice = function(idx: number, rem: number, str: string): string {
    return this.slice(0, idx) + str + this.slice(idx + Math.abs(rem));
};

More

This is covered here : https://basarat.gitbooks.io/typescript/content/docs/types/lib.d.ts.html#modifying-native-types

answered Oct 5, 2016 at 4:18

basarat

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