For循环不允许setTimeout执行 [英] For loop does not allow setTimeout to execute

问题描述

我具有下面的功能，该功能可以在给定的时间将一个字符记录在传递给该功能的句子中，看看下面的功能:

I have the below function that logs a character in a sentence passed to a function at a certain given time, have a look at the function below:

function print_string(param , timer) {

  var strt_point = 0,
      str_len = param.length,
      self = this;


  //typewritter();

  typeit();

  function typeit() {

    setTimeout(function(){

      console.log(param.substring(strt_point).charAt(0)); 

      if(strt_point < str_len ) {
        strt_point++; 
        typeit()
      }
    } , timer);
  } 

}

var str = print_string("hey there , whats up , hows the weather in Manhatten" , 50);
console.log(str);

如果我在上述程序I.E中添加一个for循环，则上述程序员现在可以正常工作了.在for循环中包装setTimeout ，

The above programmer works fine, now if i add a for loop to the above programme I.E. WRAP THE setTimeout in a for loop ,

function print_string(param , timer) {

  var strt_point = 0,
      str_len = param.length,
      self = this;

  for(var i = 0 ; i < str_len ; i++) {

  //typewritter();

  typeit();

  function typeit() {

    setTimeout(function(){

      console.log(param.substring(strt_point).charAt(0)); 

      if(strt_point < str_len ) {
        strt_point++; 
        typeit()
      }
    } , timer);
  } 

}

var str = print_string("hey there , whats up , hows the weather in Manhatten" , 50);
console.log(str);

所有字符都立即打印出来，为什么?

All the characters are printed at once , Why ?

为什么for循环不遵守 setTimeout 间隔?有人可以解释吗?

Why is it that the for loop does not honor the setTimeout interval ? can anybody explain ?

推荐答案

如果您希望 timer 参数充当实际间隔，请使用以下方法:

If you want the timer argument to act as an actual interval, use this:

function print_string(param, timer) {

  for(var i = 0 ; i < param.length ; i++) {

    setTimeout((function(char){

      return function () {

        console.log(char);

      }

    })(param.charAt(i)), timer * i);  
  }
}

var str = print_string("hey there , whats up , hows the weather in Manhatten" , 500);

这是小提琴.

您的困惑是for循环立即发生，或者在处理器允许的速度下发生.当 setTimeout 处理程序执行时，它的作用域与您可能期望的范围不同.作用域不再位于for循环内(因为发生在处理器的不同滴答声中)，因此打印变量丢失了.为了解决这个问题，我使用了所谓的闭包.我正在动态构建一个函数，以打印所需的特定变量，并将其作为参数传递给 setTimeout .

The confusion for you is that a for loop happens immediately, or as fast as the processor will allow it. When the setTimeout handler executes, it has a different scope to what you're probably expecting. The scope is no longer within the for loop (because that happened in a different tick of the processor) so the print variable is lost. In order to get around this, I'm using what is called a closure. I'm building a function on the fly, to print the specific variable that I need, and passing it as an argument to setTimeout.

在此处了解更多信息.

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