cappedarray

Basic usage

var cappedArray = require('cappedarray')

// Create array with a length cap of 3
var log = cappedArray(3)  // []

// log.lengthCap === 3


// Add some items
log.push(1)               // [1]         
log.push(2)               // [1, 2]       
log.push(3)               // [1, 2, 3]


// Add more items than the length cap
log.push(4)               // [2, 3, 4] - Too many items, The 1 is pushed off the front
log.push(5)               // [3, 4, 5]
log.unshift(6)            // [6, 3, 4] - The 5 is unshifted off the back

// Add a lot more than the cap length
for (var i = 0; i < 50; i++) {
	log.push(i)
}

// Still 3 items
console.log(log.length)
> 3

console.log(log)
> [47, 48, 49]

Installation

$ npm install cappedarray

How it works

The capped array is just a normal array with the push and unshift functions overridden and an extra lengthCap property. The functions and lengthCap are set to enumerable false so they do not show up in forEach loops, Object.keys, etc...

var recentItems = cappedArray(5)

recentItems.push('foo')
recentItems.push('bar')

var util = require('util')
util.isArray(recentItems)   // true
Array.isArray(recentItems)  // true

Object.keys(recentItems)    // ['0', '1']
JSON.stringify(recentItems) // '["foo","bar"]'

Further Examples

Multiple arguments

// Create array with a length cap of 5
var log = cappedArray(5)                    // []

// Multiple arguments to push and unshift work as expected.
log.unshift('a', 'b', 'c', 'd', 'e', 'f')   // ['a', 'b', 'c', 'd', 'e'] - Too long, 'f' has already popped off the end.
                                            // If this was a 'push' the 'f' would have stayed and the 'a' would have gone.

log.unshift('h')                            // ['h', 'a', 'b', 'c', 'd']
log.unshift.apply(log, ['i', 'j', 'k'])     // ['i', 'j', 'k', 'h', 'a']
log.push('l', 'm')                          // ['k', 'h', 'a', 'l', 'm']

The length cap can be changed on the fly.

// Create array with a length cap of 5
var recentItems = cappedArray(5)

recentItems.push(1, 2, 3, 4, 5, 6, 7)   // [3, 4, 5, 6, 7]

// Need to keep more recent items
recentItems.lengthCap = 8               // [3, 4, 5, 6, 7] Still only 5 items

recentItems.push(8, 9, 10, 11)          // [4, 5, 6, 7, 8, 9, 10, 11]

When changing the length cap to a lower value than the existing item count, the extra items will not be automatically removed. It is unknown whether the first or last items should be removed until push or unshift is called afterward.

// I've changed my mind. There's too many items now, lower the cap.
recentItems.lengthCap = 6              // [4, 5, 6, 7, 8, 9, 10, 11]

// More items than the lengthCap?
console.log(recentItems.length)
> 8
console.log(recentItems.lengthCap)
> 6

Passing no arguments to either push or unshift will reduce the array to the capped length.

// I want to keep the end of the array.
recentItems.push()                     // [6, 7, 8, 9, 10, 11]

// That has sorted it!
console.log(recentItems.length)
> 6
console.log(recentItems.lengthCap)
> 6

Capping existing Arrays

var cap = require('cappedarray').cap

var messages = [{msg: 'First!'}, {msg: 'Second'}, {msg: 'Third'}]

// Cap the array to 5 items
cap(messages, 5)

messages.push({msg: 'Forth'}, {msg: 'Fith'}, {msg: 'Sixth'})

console.log(log)
> [ { msg: 'Second' },
  { msg: 'Third' },
  { msg: 'Forth' },
  { msg: 'Fith' },
  { msg: 'Sixth' } ]

If the second argument is missing the cap is set to the length of the array or the lengthCap property if it exists.

var numerosEnFrancais = cap(['un', 'deux', 'trois'])

// Both the cap and length are the same
console.log(numerosEnFrancais.length + ' === ' numerosEnFrancais.lengthCap)
> 3 === 3

// This can be used to loop through an array repeatedly
numerosEnFrancais.push(numerosEnFrancais[0])  // ['deux', 'trois', 'un']
numerosEnFrancais.push(numerosEnFrancais[0])  // ['trois', 'un', 'deux']
numerosEnFrancais.push(numerosEnFrancais[0])  // ['un', 'deux', 'trois']



var top5 = ['High Fidelity']

// This...
top5.lengthCap = 5
cap(top5)  // Uses the existing lengthCap property for the length cap

// ...is the same as
cap(top5, 5)

Like changing the lengthCap above, capping an existing array with more items than the length cap will not automatically remove the extra items. Use push or unshift with no arguments to reduce the array to the capped length.

var alphabet = ['a', 'b', 'c', 'd', etc..., 'x', 'y', 'z']
console.log(alphabet.length)
> 26

// Cap it to 13 items...
cap(alphabet, 13)

// ...but it still has 26 items.
console.log(alphabet.length)
> 26
console.log(alphabet.lengthCap)
> 13

// removes extra items from the front.
cap.push()        

console.log(alphabet.length)
> 13
console.log(alphabet)
> ['n', 'o', 'p', 'q', 'r', 's', etc..., 'x', 'y', 'z']


// Lets try that again with unshift
var alphabet = ['a', 'b', 'c', 'd', etc..., 'x', 'y', 'z']

// cap returns the array so we can do this. Unshift removes extra items from the back.
cap(alphabet, 13).unshift()

console.log(alphabet.length)
> 13
console.log(alphabet)
> ['a', 'b', 'c', 'd', 'e', 'f', etc..., 'k', 'l', 'm']

Uncap

Use uncap to turn the array back to normal.

var cap   = require('cappedarray').cap
var uncap = require('cappedarray').uncap

var cycleNumbers = cap([1, 2, 3, 4, 5]) // Sets lengthCap to length: 5

cycleNumbers.push(cycleNumbers[0])      // [2, 3, 4, 5, 1]
cycleNumbers.push(cycleNumbers[0])      // [3, 4, 5, 1, 2]
cycleNumbers.push(cycleNumbers[0])      // [4, 5, 1, 2, 3]

uncap(cycleNumbers)

// The cycle is broken
cycleNumbers.push(cycleNumbers[0])      // [4, 5, 1, 2, 3, 4]
cycleNumbers.push(cycleNumbers[0])      // [4, 5, 1, 2, 3, 4, 4]
cycleNumbers.push(cycleNumbers[0])      // [4, 5, 1, 2, 3, 4, 4, 4]

Clone

Copy the values into a new capped array with the same lengthCap

var cap   = require('cappedarray').cap
var clone = require('cappedarray').clone

var capped1 = cap([1, 2, 3])
var capped2 = clone(capped1)

capped2.push(4)

console.log(capped1, capped2)
> [1, 2, 3] [2, 3, 4]

// Clone is just a wrapper for
cap(capped.slice(), capped.lengthCap)

Gotchas

• Behavior maybe unexpected if a capped array is passed to a library for modification.
• lengthCap must be a non negative integer. Other values will throw 'Invalid array lengthCap'. This mimics the behavior of assigning array.length.
• push and unshift return the length of the array, not the array, in line with the existing behavior.

// array1 is not an array
var array1 = cap([1, 2, 3, 4, 5, 6], 3).push()
console.log(array1)
> 3

// This works
var array2 = cap([1, 2, 3, 4, 5, 6], 3)
array2.push()
console.log(array2)
> [4, 5, 6]

// or this
var array3
( array3 = cap([1, 2, 3, 4, 5, 6], 3) ).push()
console.log(array3)
> [4, 5, 6]

• Arrays can be made longer than lengthCap by using splice, assigning to a numbered key or using the Array prototype functions with call or apply.

var array1 = cap([1, 2, 3], 3)       // [1, 2, 3]                length = 3, lengthCap = 3
array1.splice(1, 0, 1.1, 1.2, 1.3)   // [1, 1.1, 1.2, 1.3, 2, 3] length = 6, lengthCap = 3


var array2 = cap([1, 2, 3], 3)       // [1, 2, 3]    length = 3, lengthCap = 3
array2[3] = 4                        // [1, 2, 3, 4] length = 4, lengthCap = 3


var array3 = cap([1, 2, 3], 3)       // [1, 2, 3]          length = 3, lengthCap = 3
Array.prototype.push.call(array3, 4) // [1, 2, 3, 4]       length = 4, lengthCap = 3
[].push.apply(array3, [5, 6])        // [1, 2, 3, 4, 5, 6] length = 6, lengthCap = 3