once函数，简约不简单的

module.exports = once

once.proto = once(function () {
Object.defineProperty(Function.prototype, 'once', {
value: function () {
return once(this)
},
configurable: true
})
})

function once (fn) {
var f = function () {
if (f.called) return f.value
f.called = true
return f.value = fn.apply(this, arguments)
}
f.called = false
return f
}