Question

Following code snippet in typescript does not work the way I mean. It should be self-explanatory:

declare interface Date {
    toUrlString(): string;
}

Date.prototype.toUrlString = () => {
    return this.toISOString().substring(0, 10);
};

document.write(
    new Date().toUrlString()
    // Error: Object [object Window] has no method 'toISOString'
);

Compiled code is:

var _this = this;
Date.prototype.toUrlString = function () {
    return _this.toISOString().substring(0, 10);
};
document.write(new Date().toUrlString());

How can I fix this?

Answer 1

The => ‘fat arrow’ notation invokes lexical scoping rules. Use a traditional function if you don’t want that:

Date.prototype.toUrlString = function() {
    return this.toISOString().substring(0, 10);
};