transition.js

const field = this._leaving ? 'leave' : 'enter'
const lastField = '_lastDuration_' + field
const duration = this.duration
const explicitDuration = parseDuration((
  duration !== null &&
  typeof duration === 'object' &&
  duration[field]
) || duration)

const el = child.elm

// Did explicit duration change?
if (el && explicitDuration !== this[lastField]) {
  this[lastField] = explicitDuration

  const cb = this._leaving ? el._leaveCb : el._enterCb
  if (el._transitionTimer) {
    clearTimeout(el._transitionTimer)
  }
  if (typeof explicitDuration !== 'undefined') {
    const lastedDuration = Date.now() - el._transitionStarted
    const remainingDuration = explicitDuration - lastedDuration
    // console.log('remainingDuration', remainingDuration)
    if (el._offTransitionEnds) {
      el._offTransitionEnds()
      el._offTransitionEnds = null
    }
    el._transitionTimer = setTimeout(cb, remainingDuration)
  } else {
    el._transitionTimer = null
    el._offTransitionEnds = whenTransitionEnds(el, this.type, cb)
  }
}