How to fix ‘TypeError: arr.map is not a function’ error in Javascript

Question

I am doing a Hackerrank challenge 'Manasa and Stones'

I have already done an Looping solution but it took to much time solving tree levels and I need a recursive solution I guess.

function stones(n, a, b) {


    var arr = [0,0];
    var rresult = recursive(0,a,b,arr,n)

    return rresult;
}
function recursive(n,a,b,arr,end){
    if (n == end){ return arr }
    else {
        let arr2 = arr.map(function(x) {
   return x * 2;
});
        arr = arr.map(function(x) {
   return x * 2;
});
        arr = arr.join(arr2)
        recursive(n,a,b,arr,end)
    }

}

It should be working as expected to solve https://www.hackerrank.com/contests/microverse-coding-challenges/challenges/manasa-and-stones/problem (I don't from expect you to do a solution I need to know why my issue is there * It doesn't make sense)

all my code => https://github.com/Macatuz/MHackerrankSamples/blob/master/Manasa_and_Stones.js

since the else code in recursive does not return anything, chances are rresult will more often than not be undefined (except when stones is called with first argument == 0) - in all other cases however, I believe you'll end up with an infinite recursion, since n == end will never be true (once you fix the issue as noted in the answer below) — Jaromanda X
– Jaromanda X, Commented Jun 26, 2019 at 0:13

ggorlen · Accepted Answer · 2019-06-26 00:27:46Z

2

arr = arr.join(arr2) is not doing what you think it does--the .join method joins the elements in an array into a string delimited by the parameter. When you pass this string arr into the recursive function call, you'll get a crash on the next stack frame, because strings don't have a map function.

You probably meant .concat, which puts the elements from the parameter array on the back of the instance array. Here's a snippet to illustrate what's going on.

const arr1 = ["apples", "bananas"];
const arr2 = ["celery", "asparagus"];

const joined = arr1.join(arr2);
const concatted = arr1.concat(arr2);

console.log(joined, typeof joined);  // whoops?
console.log(concatted);              // ah. much better.

Note that this only solves the query in your title but doesn't produce working code that solves the challenge--that's an exercise for the reader.

edited Jun 26, 2019 at 0:27

answered Jun 25, 2019 at 23:45

ggorlen

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4 Comments

Jaromanda X Over a year ago

note: once you change .join to .concat, the code in the question will result in infinite recursion

ggorlen Over a year ago

Sure. The OP has more work to do to solve the problem beyond this immediate bug, so I'm not contending this will give the correct solution, only that it resolves the question asked in the post title.

Jaromanda X Over a year ago

true - I wasn't saying your answer was wrong in any way, just warning the OP :p

SotoArmando Over a year ago

i finally got to solve the issue, his answer leaded me there basically.

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