You can't write fold in TypeScript (doesn't compile)

nofold.ts

interface Combinable<T> {
    plus(T): T,
    zero(): T    
}

class Max implements Combinable<Max> {
    private value: number
    constructor(num: number) {
        this.value = num;
    }
    plus(m) {
        if (this.value > m.value)
            return this;
        else
            return m;
    }
    zero() {
        return new Max(Number.MIN_VALUE);
    }
}

function fold<T extends Combinable<T>>(array: T[], fn: (left: T, right: T) => T) : T {
    return reduce((left, right) => left.plus(right)) T.zero() array 
}


function add(x: number, f: (left :number) => number): number {
    return f(x);
}