JS面试题，关于eval与new Function

Question

var a,b,c;

(function(){

  eval('var b = 2');

  (1, eval)('var c = 3');

  (new Function('var a = 4'))();

  document.write('<br>a: ' + a);

  document.write('<br>b: ' + b);

  document.write('<br>c: ' + c);

})()

document.write('<br>a: ' + a);

document.write('<br>b: ' + b);

document.write('<br>c: ' + c);

执行此段代码后，得到如下结果

a: undefined

b: 2

c: 3

a: undefined

b: undefined

c: 3

Answer 1

eval能访问上下文，new Function只能构建自己的一个私有作用域。更新，注释一下var a, b, c;(function() {&nbsp; // eval('var b = 2');&nbsp; // 调用当前作用域&nbsp; var b = 2;&nbsp; // (1, eval)('var c = 3');&nbsp; // 逗号操作符，括号表达式，返回的是最后一个挂载在window上的eval&nbsp; window.eval('var c = 3');&nbsp; // (new Function('var a = 4'))();&nbsp; // 申明一个匿名函数&nbsp; (function() {&nbsp; &nbsp; var a = 4;&nbsp; })&nbsp; document.write('<br>a: ' + a);&nbsp; document.write('<br>b: ' + b);&nbsp; document.write('<br>c: ' + c);})()document.write('<br>a: ' + a);document.write('<br>b: ' + b);document.write('<br>c: ' + c);