javascript - TypeError: Cannot read property 'toArray' of null at Context.<anonymous> in Typescript -

when try use toarray function newly creates list keep getting error "typeerror: cannot read property 'toarray' of null @ context".

i'm trying create linked list implementation. toarray function must not change i'm pretty sure problem has in reduce function, if not maybe fromarray function

type selector<t> = (head:t, rest?:cons<t>)=> t|cons<t>; type cons<t> = (selector: selector<t>) => t|cons<t>;  function cons<t>(head:t, rest?: cons<t>): cons<t> {     return (selector: selector<t>) => selector(head, rest); }  function head<t>(list:cons<t>):t {     return <t>list((head, rest?) => head); }  function rest<t>(list:cons<t>):cons<t> {     return <cons<t>>list((head, rest?) => rest); }  function fromarray<t>(arr:t[]):cons<t>{     function aux(array:t[], i:number):cons<t>{         if(i<array.length){             return cons(array[i], aux(array.slice(i+1), i))         }         else{             return null         }     }     return aux(arr, 0); }  function reduce<t>(f:(x:t, y:t)=>t, initial:t, list:any):t {     if(list){         return reduce(f, f(initial, head(list)), rest(list));     }     else{         return initial;     } }  class list<t> {     private head: cons<t>;      constructor(list: t[] | cons<t>) {         if (list instanceof array) {                     this.head = fromarray(list);         } else {             this.head = list;         }     }      toarray(): t[] {         return reduce((a,t)=>(a.push(t), a), [], this.head).reverse();     } 

you should turn on strictnullchecks compiler option, since there's lot of undefined , null floating around code without flagging errors. main issue here in implementation, empty list undefined (or maybe null), , you're not checking in enough places. following debugged being; don't know if bugs gone, toarray() seems work now. added comments changed things:

type selector<t> = (head: t, rest?: cons<t>) => t | cons<t>; // list can undefined, added it: type cons<t> = undefined | ((selector: selector<t>) => t | cons<t>);  function cons<t>(head: t, rest?: cons<t>): cons<t> {   return (selector: selector<t>) => selector(head, rest); }  // head function guaranteed return t on non-empty list, // overloaded it: function head(list: undefined): undefined; function head<t>(list: cons<t>): t; function head<t>(list: cons<t>): t | undefined {   if (!list) return; // watch out empty list   return <t>list((head, rest?) => head); }  function rest<t>(list: cons<t>): cons<t> {   if (!list) return; // watch out empty list   return <cons<t>>list((head, rest?) => rest); }  function fromarray<t>(arr: t[]): cons<t> {   function aux(array: t[], i: number): cons<t> {     if (i < array.length) {       return cons(array[i], aux(array.slice(i + 1), i))     }     else {       return; // use undefined instead of null, easier way     }   }   return aux(arr, 0); }  // note want reduce return t[] instead of t in toarray() // means f cannot (x:t, y:t) => t.  add u parameter // type of initial, return value, , return type of f function reduce<t, u>(f: (x: u, y: t) => u, initial: u, list: cons<t>): u {   if (list) {     return reduce(f, f(initial, head(list)), rest(list));   }   else {     return initial;   } }  class list<t> {   private head: cons<t>;    constructor(list: t[] | cons<t>) {     if (list instanceof array) {       this.head = fromarray(list);     } else {       this.head = list;     }   }    toarray(): t[] {     // push puts things @ end; you're trying cons, right?       // that's unshift     return reduce<t, t[]>((a, t) => (a.unshift(t), a), [], this.head).reverse();   } } 

hope helps; luck!