JS闭包问题

Question

for (var i = 1; i < 5; i++) {

  setTimeout(function(i) {

    console.log(i);

  }, 1000 * i);

}

上面的代码，打印出来为什么是四个undefined,变量i的值为什么传不进去

Answer 1

跟闭包无关，改成这样就可以了：for (var i = 1; i < 5; i++) {&nbsp; setTimeout(function(i) {&nbsp; &nbsp; console.log(i);&nbsp; }, 1000 * i,i);}setTimeout(function[, delay, param1, param2, ...]) 你没有给 function 传 param，所以 i 是 undefinedMDN

Answer 2

因为你&nbsp;function(i)&nbsp;这里把&nbsp;i&nbsp;的声明覆盖了啊。

Answer 3

简单的道理，你调用函数没传参数但在函数内用到了参数for (var i = 1; i < 5; i++) {&nbsp; setTimeout(function(i) {&nbsp; &nbsp; console.log(i);&nbsp; }, 1000 * i， i);}

Answer 4

for (var i = 0; i < 5; i++) {&nbsp; &nbsp; setTimeout(function(i) {&nbsp; &nbsp; &nbsp; &nbsp; return function() {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; console.log(i)&nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; }(i), 1000 * i)}&nbsp;&nbsp;可以这样通过闭包来实现，你的代码没有传参