js 实现获取微秒的方法

'use strict'

let now

if (global.process && process.hrtime) {
  const hrtime = process.hrtime

  now = () => {
    const hr = hrtime()
    return (hr[0] * 1e9 + hr[1]) / 1e3
  }
} else if (global.performance && performance.now) {
  const timing = performance.timing
  const start = (timing && timing.navigationStart) || Date.now()

  now = () => (start + performance.now()) * 1e3
} else {
  now = () => Date.now() * 1e3
}

module.exports = now