JS - Condense arrays with reduce, concatenate arrays

jsArrayConcat.js

var oldArray = [1,2,3];
var newArray = [];

var concatMe = [4,5,6];

newArray = oldArray.concat(concatMe);

jsArrayCondense.js

var array = [4,5,6,7,8];
var singleVal = 0;

/* Call reduce with a callback that includes an accumulator (previousVal in this case) and the current value (currentVal)
Note that reduce() has an optional second argument to set the initial value of the accumulator. If left out, the
initial value will be the 1st array element, and currentVal will begin with the 2nd array element.
*/

singleVal = array.reduce(function(previousVal, currentVal) {
  return previousVal + currentVal;
}, 0);

// Another example with all four arguments (previousValue, currentValue, currentIndex, array):
[0, 1, 2, 3, 4].reduce(function(previousValue, currentValue, currentIndex, array) {
  return previousValue + currentValue;
});

To flatten an array, you can use reduce as follows:

var flattened = [[0, 1], [2, 3], [4, 5]].reduce(function(a, b) {
  return a.concat(b);
}, []);
// flattened is [0, 1, 2, 3, 4, 5]

Also, you can use an arrow function to get a one-liner for a simple evaluation:

[0, 1, 2, 3, 4].reduce( (prev, curr) => prev + curr );