用回调编写函数

Question

Lets imagine that I have some code:

var someString = "";
function do1(){
    doA();
    doB();
}

function doA(){
    // some process that takes time and gets me a value
    someString = // the value I got in prior line

function doB(){
    //do something with someString;
}

What is the correct way to make sure somestring is defined by doB tries to use it? I think this is a situation that calls for a callback, but I'm not sure how to set it up?

Answer 1

Usually, I have solved this problem like following code by callback parameter. However, I don't know this is correct answer. In my case, it's done well.

var someString = "";
function do1(){ 
    doA(doB);
}

function doA(callback){
    // some process that takes time and gets me a value
    someString = // the value I got in prior line
    callback();
}

function doB(){
    //do something with someString;
}

Answer 2

I usually write these such that the function can be called with, or without, the callback function. You can do this by calling the callback function only if typeof callback === 'function' . This allows the function which includes the possibility of a callback to be a bit more general purpose. The call to the callback() , obviously, needs to be from within the callback of whatever asynchronous operation you are performing. In the example below, setTimeout is used as the asynchronous action.

 var someString = ""; function do1() { doA(doB); //Call doA with doB as a callback. } function doA(callback) { setTimeout(function() { //setTimeout is an example of some asynchronous process that takes time //Change someString to a value which we "received" in this asynchronous call. someString = 'Timeout expired'; //Do other processing that is normal for doA here. //Call the callback function, if one was passed to this function if (typeof callback === 'function') { callback(); } }, 2000); } function doB() { //do something with someString; console.log(someString); } do1(); 

You can, of course, do this without using a global variable:

 function do1() { doA(doB); //Call doA with doB as a callback. } function doA(callback) { setTimeout(function() { //setTimeout is an example of some asynchronous process that takes time //Simulate a result var result = 'Timeout expired'; //Do other processing that is normal for doA here. //Call the callback function, if one was passed to this function if (typeof callback === 'function') { callback(result); } }, 2000); } function doB(result) { console.log(result); } do1(); 

Answer 3

function someFunctionA(callback){
  var someString = "modify me";
  callback(someString);
}

function someFunctionB(someString){
  // do something
}

function main() {
   someFunctionA(somefunctionB);
 }