recursing implementation function reduce javascript

Question

I want to write a recursive version of reduce function.

Array.prototype.reduce2 =
function reduce(fn, initial) {
 var head = this[0];
 var tail = this.slice(1);
 var result = fn(initial, head, 0, tail);
 return reduce(tail, fn, result);
}
var a = [1, 2, 3, 4];
function add(a, b) { return a + b } ;
function mul(a, b) { return a * b } ;
function foo(a, b) { return a.concat(b) };
console.log(a.reduce(add), a.reduce2(add)); // 10 10
console.log(a.reduce(add, 10), a.reduce2(add, 10)) ; // 20 20
console.log(a.reduce(add, undefined), a.reduce2(add, undefined)); // NaN NaN
console.log(a.reduce(mul), a.reduce2(mul)); // 24 24
console.log(a.reduce(foo, ''), a.reduce2(foo, '')); // 1234 123

The result was:

10 [Function: add]
20 [Function: add]
NaN [Function: add]
24 [Function: mul]
1234 function foo(a, b) { return a.concat(b) }

Yes, I know that this seems like a lot of topics, but I couldn't find answer.

• Kraylog: Some things in your code don't make sense. Why do you call fn with 4 parameters when it only accepts 2? Your recursion has no stopping clause. You're calling reduce with 3 parameters when it accepts 2.

Answer 1

You should avoid adding methods to native objects.

You can do the same with a simple function in a simpler way using destructuring of the arguments.

function reduce(fn, initial, [head, ...tail]) {
 return tail.length
 ? reduce(fn, fn(initial, head), tail)
 : fn(initial, head)
}
Array.prototype.reduce2 = function(fn, initial) {
 const head = this[0]
 const tail = Array.prototype.slice.call(this, 1)
 return tail.length
 ? tail.reduce2(fn, fn(initial, head))
 : fn(initial, head)
}
const a = [1, 2, 3, 4];
const add = (a, b) => a + b
const mul = (a, b) => a * b
const foo = (a, b) => a.concat(b)
console.log(
 reduce(add, 0, a),
 a.reduce(add, 0),
 a.reduce2(add, 0)
)
console.log(
 reduce(mul, 1, a),
 a.reduce(mul, 1),
 a.reduce2(mul, 1)
)
console.log(
 reduce(foo, '', a),
 a.reduce(foo, ''),
 a.reduce2(foo, '')
)
console.assert(
 a.reduce2(add, 0) === 10,
 'reduce2 can sum numbers'
)
console.assert(
 a.reduce2(mul, 1) === 24,
 'reduce2 can multiply numbers'
)
console.assert(
 a.reduce2(foo, '') === '1234',
 'reduce2 can contatinate strings'
)

<script src="https://codepen.io/synthet1c/pen/KyQQmL.js?tab=assert"></script>