An implementation of hysteresis in JavaScript

$ npm install hysteresis

$ bower install littlebits/hysteresis

There are a variety of use-cases for hysteresis. One use-case at littleBits is that we use it to avoid jittery event inference in the cloudBit's data stream.

Imagine detecting a rising-edge event in a jittery but generally upward-growing value. As the value crosses our target threshold value, it may actually go slightly above and then below multiple times in quick succession, triggering many events instead of just one.

|                               /
|                        /\    /
|=======================/==\==/=============
|       /\         /\  /    \/
|------/--\-------/--\/---------------------
|   /\/    \_____/
|  /
| /
|-------------------------------------------
                        ✓     ×

✓ rising-edge event detected
× ignore insignificant threshold cross

= target threshold
- hyteresis buffer

Here is a naive example:

var Hysteresis = require('hysteresis')
var createServer = require('net').createServer

var server = createServer(9500, function(socket){
  var check = Hysteresis([68,70])
  socket.on('data', function(data){
    var didCross = check(Number(data))
    if (didCross) socket.emit(['release', 'ignite'][didCross - 1], data)
  })
})

Hysteresis(threshold, config) -> (check(number) -> 0 | 1 | 2)`

Instantiate a hysteresis instance. You must provide a threshold and may optionally provide a config object that tweaks behaviour.

config exposes the following options:

• initialSide – May be 1 or 0, defaults to null causing side bootstrapping to be resolved using first received number
• initialBias – May be 1 or 0, defaults to 1
• initialIsChange – May be Boolean, defaults to true
• checkType – May be 'crosses' or 'crossesOrMeets', defaults to 'crossesOrMeets'

The constructor returns a check function that accepts number and returns one of the following codes:

check(number) -> 0 | 1 | 2

• 0 – the number did not cross the threshold
• 1 – the number fell below the threshold
• 2 – the number rose above the threshold

For more details please read the annotated source code and review the tests.