Hands-On Functional Programming with TypeScript

Laziness

Many functional programming languages feature lazy-evaluated APIs. The idea behind lazy evaluation is that operations are not computed until doing so can no longer be postponed. The following example declares a function that allows us to find an element in an array. When the function is invoked, we don't filter the array. Instead, we declare a proxy and a handler:

function lazyFind<T>(arr: T[], filter: (i: T) => boolean): T {    let hero: T | null = null;        const proxy = new Proxy(        {},        {            get: (obj, prop) => {                console.log("Filtering...");                if (!hero) {                    hero = arr.find(filter) || null;                }                return hero ? (hero as any)[prop] : null;            }        }    );    return proxy as any;}

It is only later, when one of the properties in the result is accessed, that the ...