如何修复Javascript中的“ TypeError：arr.map不是函数”错误

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

Answer 1

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.