lib/bind_keys.js
function pop () {
    var i = this.length - 1
    var previousValue = previousValues[i]
    var value = Array.prototype.pop.call(this)

    removeNode(null, previousValue, i)
    previousValues.length = activeNodes.length = this.length

    return value
  }
function pop () {
    var i = this.length - 1
    var previousValue = previousValues[i]
    var value = Array.prototype.pop.call(this)

    removeNode(null, previousValue, i)
    previousValues.length = activeNodes.length = this.length

    return value
  }
Below are optimized array mutator methods. They have to exist within this closure. Note that the native implementations of these methods do not trigger setter functions on array indices.