JavaScript Error - concat_1

There was an Error message while entering to recursive function.

Error Message : Uncaught TypeError : reverse(...).concat is not a function

In this case, I want to build an array by using a concat function. But there was an error. So I have to find out what's the problems.

first I have to find which variable is coming in front of the concat. It has to be an array.

When I check the base, it wasn't. it was a number. So I switch to an array.

function reverse(arr) {

    const head = arr[0];

    if(arr.length === 1){

      return arr[0]; // Error

    }

    else if(arr.length > 1){

      return reverse(arr.slice(1)).concat(arr[0]);

    }

    

  }

  function reverse(arr) {

      const head = arr[0];

      if(arr.length === 1){

        return arr; // Solve the Error

      }

      else if(arr.length > 1){

        return reverse(arr.slice(1)).concat(arr[0]);

      }

      

    }