Callbag operator that puts the previous value alongside the current in an array.
[currentValue, previousValue]
The first output will also have true as a third item, to allow you to distinguish from the previous item being undefined and the first value.
[firstValue, undefined, true]
npm install callbag-with-previous
const fromIter = require('callbag-from-iter');
const forEach = require('callbag-for-each');
const withPrevious = require('callbag-with-previous');
const source = withPrevious(fromIter([1,2,3]));
forEach(x => console.log(x))(source); // [1, undefined, true]
// [2, 1]
// [3, 2]