adding and subtracting in a 'for' loop

I’d like to start by saying that I am still new to JavaScript and this is a CodeWars Kata Number of people in the Bus.

Also I know there is a simpler way of completing this task but If I just googled the answer, I feel that I wont have learned anything so here goes:

Hi All,
at the end of a loop, how do I get it to add a, minus b on repeat? what is that called?

i.e.

[ 10, 0, 3, 5, 5, 8 ]

should work math like:

[ 10 - 0 + 3 - 5 + 5 - 8 ]

it’s a codewars kata and I know there is a simple way of doing it but I have gone around it the long way.

Here is the code I am up to (and the console.log that is the test case)

var number = function (busStops) {
  let newBusStops = [];
  for (let i = 0; i < busStops.length; i++) {
    newBusStops = newBusStops.concat(busStops[i]);
  }
  //   return newBusStops;
  let passengers = 0;
  for (let i = 0; i < newBusStops.length; i++) {
    passengers += newBusStops[i];
  }
  return passengers;
};
// var number = function (busStops) {
//   let passengers = 0;
//   for (let i = 0; i < busStops.length; i++) {
//     passengers += parseInt(number[i]);
//     busStops.toString();
//     return busStops;
//   }
// };

// var number = function (busStops) {
//   for (let i = 0; i < busStops.length; i++) {
//     return busStops[i][0] - busStops[i][1];
//   }
// };
// return busStops[0][0];

console.log(
  number([
    [10, 0],
    [3, 5],
    [5, 8],
  ])
);
console.log(
  number([
    [3, 0],
    [9, 1],
    [4, 10],
    [12, 2],
    [6, 1],
    [7, 10],
  ])
);

I’ve managed to flatten the 2d array but I am only able to add the flattened array, I can’t figure out how to do add a minus b. I don’t know what that is called so I can search it

>Solution :

I would go through the array with a counting index (like you do). Depending on this index, I would add or substract the value from the last result.

First Hint: Loop over an array with key-value pairs like this

for (const [key, value] of array) {console.log(key, value);}

Second Hint: A subtraction is a addition with a negativ number

5-3 == 5+(3*-1)

Third Hint: Have a look at the modul Operator %

console.log(1%2);
console.log(2%2);
console.log(3%2);

Solution:

var number = function (busStops) {

var total = 0;

for (const [i, value] of busStops){

total += value*(i%2?1:-1);

}

return total;

}