例1：var a = 1function fn1(){ function fn2(){ console.log(a) } function fn3(){ var a = 4 fn2() } var a = 2 return fn3}var fn = fn1()fn() //输出多少调用过程：调用fn()=>fn1(),fn1()返回fn3，调用fn3，fn3中调用了fn2，输出2。分析：fn2中没有a变量，fn2被定义在fn1中，获得fn1中的变量a=2例2：var a = 1function fn1(){ function fn3(){ var a = 4 fn2() } var a = 2 return fn3}function fn2(){ console.log(a)}var fn = fn1()fn() //输出多少调用过程：fn()=>fn1()=>fn3()=>fn2()=>consolo.log(a)=>输出分析：fn2中没有a变量，在fn1作用域中寻找，任没有，在全局作用域寻找，输出1例3：var a = 1function fn1(){ function fn3(){ function fn2(){ console.log(a) } var a fn2() a = 4 } var a = 2 return fn3}var fn = fn1()fn() //输出多少调用过程：fn()=>fn1()=>fn3()=>fn2()=>console.log(a)=>输出undefined分析: fn2内部没有a变量，fn3中寻找，var a已经定义，但未赋值，所以输出undefined总结：作用域链不难，先找局部变量，再往定义该函数的作用域寻找变量，直到全局作用域即可。==============================================================================================if中的表达式一定要用==或者！=。单用一个=，一定会出错==========================================================================================
来源：https://www.cnblogs.com/hy96/p/11368892.html