What does the reduce() method do? code example

Example 1: how to flatten array with reduce in javascript

let flattened = [[0, 1], [2, 3], [4, 5]].reduce(
  function(accumulator, currentValue) {
    return accumulator.concat(currentValue)
  },
  []
)
// flattened is [0, 1, 2, 3, 4, 5]

Example 2: js reduce a array of straing

var authors = [{name: 'some author'},{name: 'another author'},{name: 'last author'}]
var authorString = authors.map(function(author){
    return author.name;
}).join(",");
console.log(authorString);