Reduce

Reduce

/* 
  1.
  
  a: NaN
  b: 0123
  c: 6
*/
var transactions = [{ id: 1, value: 1 }, { id: 2, value: 2 }, { id: 3, value: 3 }];

var total = transactions.reduce(function(prev, transaction) { 
    return (prev + parseInt(transaction.value, 10)); 
}, '0');

console.log(result);


/* 
  2.
  
  a: [0, 1, 3, 2, 5, 4]
  b: [1, 0, 3, 2, 5, 4]
  c: [0, 1, 3, 2, 4, 5]
*/
var result = [[0, 1], [2, 3], [4, 5]].reduce(function(a, b) {
    return a.concat(b.reverse());
});

console.log(result);


/* 
  3.
  
  a: [0, 1, 0, 0, 1, 1] 
  b: [0, 1, 0, 1, 0, 1]
*/
var result = [0, 1].reduce(function(a, b, index, arr) {
    return a.concat(arr).concat(b);
}, []);

console.log(result);