5kyu_ISBN-10Validation.js

// 5kyu - ISBN-10 Validation

// Description:

// ISBN-10 identifiers are ten digits. The first nine digits are on the range 0 to 9. 
// The last digit can be either on the range 0 to 9 or the letter 'X' used to indicate 
// a value of 10.

// For an ISBN-10 to be valid, the sum of the digits multiplied by their position has to 
// equal zero modulo 11. For example, the ISBN-10: 1112223339 is valid because:

// (((1*1)+(1*2)+(1*3)+(2*4)+(2*5)+(2*6)+(3*7)+(3*8)+(3*9)+(9*10)) % 11) == 0
// Complete the validISBN10() function.

// validISBN10('1112223339') ; should return true
// validISBN10('1234554321') ; should return true
// validISBN10('1234512345') ; should return false


let validISBN10 = function(isbn) {
    if(!/[0-9]{9}[0-9X]/.test(isbn)) { return false; }
    isbn = isbn.split("").map((a,i) => ((a==="X") ? 10 : a) *(i+1));
    return isbn.reduce((x,y)=>x+y) % 11 === 0;
}