javascript - 为什么 Nodejs callback() 不能访问回调范围外的变量？

Question

总的来说，我对 NodeJS 和 JavaScript 还比较陌生。这是我的脚本:

var fs = require('fs') ;

var temp = "???";

var test = function (){
    fs.readdir("./", function(err,result){
    temp = result;            // i change the temp's value
    console.log("inter result ....."+temp);        //  temp's value changed
    setTimeout(pr,1000,"inter setTimeout: "+temp);  //  temp's value changed 
    });
}

var pr = function (str){
    console.log("Print str: "+ str);
} ;


test();

setTimeout(pr,1000,"Out setTimeout print: "+temp);  //  Why here temp's value not change???

如何在回调之外更改为 var temp 的值？

Answer 1

setTimeout(pr,1000,"Out setTimeout print: "+temp);

与

相同

var str = "Out setTimeout print: " + temp;
setTimeout(pr, 1000, str);

此时 temp 仍然是 "???"。你必须使用

setTimeout(function() {
  pr("Out setTimeout print: "+temp);
}, 1000);