Simple object pool with fixed array size

objectpool.js

//@flow

export default class ObjectPool {
  size: number
  head: number
  items: Array<any>

  renewCallback: ?Function
  releaseCallback: ?Function

  constructor(size: number, init: Function, renewCallback: ?Function, releaseCallback: ?Function) {
    this.size = size
    this.items = new Array(size)
    this.head = -1

    this.renewCallback = renewCallback
    this.releaseCallback = releaseCallback

    // Init objects
    for (let idx = 0; idx < size; idx++) { this.release(init()) }
  }

  // The length of active objects taken from the pool
  get length() : number {
    return this.size - this.head - 1
  }

  // Take an object from the pool
  renew() {
    if (this.head >= 0) {
      const object = this.items[this.head--]
      if (typeof this.renewCallback === 'function') { this.renewCallback(object) }
      return object
    }
  }

  // Release an object back into the pool
  release(object: any) {
    if (this.head < this.size - 1) {
      this.items[++this.head] = object
      if (typeof this.releaseCallback === 'function') { this.releaseCallback(object) }
    }
  }

  // Peak at the next object in the pool
  peekNext() {
    if (this.head >= 0) {
      return this.items[this.head]
    }
  }

  // Peak at the last object in the pool
  peekLast() {
    if (this.head >= 0) {
      return this.items[0]
    }
  }
}