你不知道的arguments

arguments.callee.caller.arguments[0]；

你知道这行代码是什么意思吗？

一开始看到我也是懵逼的，这是什么鬼？

然后就查了下资料，写了个demo，运行了下：

function foo(a){
    console.log("arg 1>>: ", arguments);
    console.log("arg 2>>: ", arguments.callee);
    console.log("arg 3>>: ", arguments.callee.caller);
    console.log("arg 4>>: ", arguments.callee.caller.arguments);
}

function bar(b){
    console.log("arguments >>: ", arguments);
    foo(2)
}
bar(1)

输出如下：
arguments >>:  [Arguments] { '0': 1 }
arg 1>>:  [Arguments] { '0': 2 }
arg 2>>:  function foo(a){
    console.log("arg 1>>: ", arguments);
    console.log("arg 2>>: ", arguments.callee);
    console.log("arg 3>>: ", arguments.callee.caller);
    console.log("arg 4>>: ", arguments.callee.caller.arguments);
}
arg 3>>:  function bar(b){
    console.log("arguments >>: ", arguments);
    foo(2)
}
arg 4>>:  [Arguments] { '0': 1 }

可以看出：

　　1.arguments就是调用者传入的参数集合，注意这个集合并不是数组二十一个类数组，可以通过Array.from(arguments)转换成数组，

　　2.arguments.callee就是当前函数方法

　　3.arguments.callee.caller就是当前函数调用者

　　4.arguments.callee.caller.arguments就是当前函数调用者的参数集合

看了mozilla上提到箭头函数，然后又写了几行试试：

let foo = (a) => {
    console.log("arg 1>>: ", arguments);
    console.log("arg 2>>: ", arguments.callee);
    console.log("arg 3>>: ", arguments.callee.caller);
    console.log("arg 4>>: ", arguments.callee.caller.arguments);
}

let bar = (b)=>{
    console.log("arguments >>: ", arguments);
    foo(2)
}
bar(1)

输出如下：
arguments >>:  [Arguments] {
  '0': {},
  '1':
   { [Function: require]
     resolve: { [Function: resolve] paths: [Function: paths] },
     main:
      Module {
        id: '.',
        exports: {},
        parent: null,
        filename: 'd:\docs\demo\day1126\tempCodeRunnerFile.js',
        loaded: false,
        children: [],
        paths: [Array] },
     extensions:
      [Object: null prototype] { '.js': [Function], '.json': [Function], '.node': [Function] },
     cache:
      [Object: null prototype] { 'd:\docs\demo\day1126\tempCodeRunnerFile.js': [Module] } },
  '2':
   Module {
     id: '.',
     exports: {},
     parent: null,
     filename: 'd:\docs\demo\day1126\tempCodeRunnerFile.js',
     loaded: false,
     children: [],
     paths:
      [ 'd:\docs\demo\day1126\node_modules',
        'd:\docs\demo\node_modules',
        'd:\docs\node_modules',
        'd:\node_modules' ] },
  '3': 'd:\docs\demo\day1126\tempCodeRunnerFile.js',
  '4': 'd:\docs\demo\day1126' }
arg 1>>:  [Arguments] {
  '0': {},
  '1':
   { [Function: require]
     resolve: { [Function: resolve] paths: [Function: paths] },
     main:
      Module {
        id: '.',
        exports: {},
        parent: null,
        filename: 'd:\docs\demo\day1126\tempCodeRunnerFile.js',
        loaded: false,
        children: [],
        paths: [Array] },
     extensions:
      [Object: null prototype] { '.js': [Function], '.json': [Function], '.node': [Function] },
     cache:
      [Object: null prototype] { 'd:\docs\demo\day1126\tempCodeRunnerFile.js': [Module] } },
  '2':
   Module {
     id: '.',
     exports: {},
     parent: null,
     filename: 'd:\docs\demo\day1126\tempCodeRunnerFile.js',
     loaded: false,
     children: [],
     paths:
      [ 'd:\docs\demo\day1126\node_modules',
        'd:\docs\demo\node_modules',
        'd:\docs\node_modules',
        'd:\node_modules' ] },
  '3': 'd:\docs\demo\day1126\tempCodeRunnerFile.js',
  '4': 'd:\docs\demo\day1126' }
arg 2>>:  function (exports, require, module, __filename, __dirname) {
let foo = (a) => {
    console.log("arg 1>>: ", arguments);
    console.log("arg 2>>: ", arguments.callee);
    console.log("arg 3>>: ", arguments.callee.caller);
    console.log("arg 4>>: ", arguments.callee.caller.arguments);
}

let bar = (b)=>{
    console.log("arguments >>: ", arguments);
    foo(2)
}
bar(1)
}
arg 3>>:  null
d:docsdemoday1126	empCodeRunnerFile.js:5
    console.log("arg 4>>: ", arguments.callee.caller.arguments);
                                                     ^

TypeError: Cannot read property 'arguments' of null
    at foo (d:docsdemoday1126	empCodeRunnerFile.js:5:54)
    at bar (d:docsdemoday1126	empCodeRunnerFile.js:10:5)
    at Object.<anonymous> (d:docsdemoday1126	empCodeRunnerFile.js:12:1)
    at Module._compile (internal/modules/cjs/loader.js:776:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:787:10)
    at Module.load (internal/modules/cjs/loader.js:653:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:593:12)
    at Function.Module._load (internal/modules/cjs/loader.js:585:3)
    at Function.Module.runMain (internal/modules/cjs/loader.js:829:12)
    at startup (internal/bootstrap/node.js:283:19)

不太一样！

over！