Learning TypeScript 2.x - Second Edition

Maybe

The following Maybe data type is a Functor and an Applicative, which means it contains a value and implements the map method. The main difference with the preceding implementation of Functor is that in the Maybe Functor, the value contained by the data type is optional:

class MayBe<T> { 
    public static of<TVal>(val?: TVal) { 
        return new MayBe(val); 
    } 
    private _value!: T; 
    public constructor(val?: T) { 
        if (val) { 
            this._value = val; 
        } 
    } 
    public isNothing() { 
        return (this._value === null || this._value === undefined); 
    } 
    public map<TMap>(fn: (val: T) => TMap) { 
        if (this.isNothing()) { 
            return new MayBe<TMap>(); 
        } else { 
            return new MayBe<TMap>(fn(this._value)); 
        } 
    } 
} 

As we can see in the preceding implementation of the map method, the mapping ...