Please, explain 4th line

Question

Don't know, how it works(called) and where i can read about it.
https://code.sololearn.com/WQxKZm5NReTK/?ref=app

Answer 1

c[0] == 7;
c[c[0]] == c[7];

https://www.sololearn.com/learn/JavaScript/1241/

Answer 2

The concat command do joined the two arrays, the first one which is B with the second which is A, in that order.

C in position Zero is value Seven.
Position seven in C is equal Four.

var a = [1,3,4,6,8];
var b = [7,3,2,9,5];
var c = b.concat(a); // c = ([7,3,2,9,5] + [1,3,4,6,8]) == [7,3,2,9,5,1,3,4,6,8]
console.log(c[c[0]]); // c[0] == First position in array equals is 7 in c is 8 position 7+1
// Logic positions
[0,1,2,3,4,5,6,7] = "Positions of c"
[7,3,2,9,5,1,3,4,6,8] = "Values of c"

Answer 3

Vladislav Semeniuk
Нет, это не двумерный массив.
Определить индекс массива Вы можете как угодно:
"с[с[с[0]]] == 5".

Answer 4

Vasiliy вон оно как, спасибо.

Answer 5

Vasiliy я шарю, что такое конкатенация. Не знал, как двойной аррай работает. Уже понял, спасибо.