详解JavaScript的回调函数

//Note that the item in the click method's parameter is a function, not a variable.
//The item is a callback function
$("#btn_1").click(function() {
 alert("Btn 1 Clicked");
});

var friends = ["Mike", "Stacy", "Andy", "Rick"];

friends.forEach(function (eachName, index){
console.log(index + 1 + ". " + eachName); // 1. Mike, 2. Stacy, 3. Andy, 4. Rick
});

//The anonymous function is not being executed there in the parameter. 
//The item is a callback function
$("#btn_1").click(function() {
 alert("Btn 1 Clicked");
});


// global variable
var allUserData = [];

// generic logStuff function that prints to console
function logStuff (userData) {
  if ( typeof userData === "string")
  {
    console.log(userData);
  }
  else if ( typeof userData === "object")
  {
    for (var item in userData) {
      console.log(item + ": " + userData[item]);
    }

  }

}

// A function that takes two parameters, the last one a callback function
function getInput (options, callback) {
  allUserData.push (options);
  callback (options);

}

// When we call the getInput function, we pass logStuff as a parameter.
// So logStuff will be the function that will called back (or executed) inside the getInput function
getInput ({name:"Rich", speciality:"JavaScript"}, logStuff);
// name: Rich
// speciality: JavaScript

//Global variable
var generalLastName = "Clinton";

function getInput (options, callback) {
  allUserData.push (options);
// Pass the global variable generalLastName to the callback function
  callback (generalLastName, options);
}

function getInput(options, callback) {
  allUserData.push(options);

  // Make sure the callback is a function
  if (typeof callback === "function") {
  // Call it, since we have confirmed it is callable
    callback(options);
  }
}

// Define an object with some properties and a method
// We will later pass the method as a callback function to another function
var clientData = {
  id: 094545,
  fullName: "Not Set",
  // setUserName is a method on the clientData object
  setUserName: function (firstName, lastName) {
    // this refers to the fullName property in this object
   this.fullName = firstName + " " + lastName;
  }
}

function getUserInput(firstName, lastName, callback) {
  // Do other stuff to validate firstName/lastName here

  // Now save the names
  callback (firstName, lastName);
}

getUserInput ("Barack", "Obama", clientData.setUserName);

console.log (clientData.fullName);// Not Set

// The fullName property was initialized on the window object
console.log (window.fullName); // Barack Obama

//Note that we have added an extra parameter for the callback object, called "callbackObj"
function getUserInput(firstName, lastName, callback, callbackObj) {
  // Do other stuff to validate name here

  // The use of the Apply function below will set the this object to be callbackObj
  callback.apply (callbackObj, [firstName, lastName]);
}

// We pass the clientData.setUserName method and the clientData object as parameters. The clientData object will be used by the Apply function to set the this object

getUserInput ("Barack", "Obama", clientData.setUserName, clientData);

// the fullName property on the clientData was correctly set
console.log (clientData.fullName); // Barack Obama

function successCallback() {
  // Do stuff before send
}

function successCallback() {
  // Do stuff if success message received
}

function completeCallback() {
  // Do stuff upon completion
}

function errorCallback() {
  // Do stuff if error received
}

$.ajax({
  url:"http://fiddle.jshell.net/favicon.png",
  success:successCallback,
  complete:completeCallback,
  error:errorCallback

});

var p_client = new Db('integration_tests_20', new Server("127.0.0.1", 27017, {}), {'pk':CustomPKFactory});
p_client.open(function(err, p_client) {
  p_client.dropDatabase(function(err, done) {
    p_client.createCollection('test_custom_key', function(err, collection) {
      collection.insert({'a':1}, function(err, docs) {
        collection.find({'_id':new ObjectID("aaaaaaaaaaaa")}, function(err, cursor) {
          cursor.toArray(function(err, items) {
            test.assertEquals(1, items.length);

            // Let's close the db
            p_client.close();
          });
        });
      });
    });
  });
});

// First, setup the generic poem creator function; it will be the callback function in the getUserInput function below.
function genericPoemMaker(name, gender) {
  console.log(name + " is finer than fine wine.");
  console.log("Altruistic and noble for the modern time.");
  console.log("Always admirably adorned with the latest style.");
  console.log("A " + gender + " of unfortunate tragedies who still manages a perpetual smile");
}

//The callback, which is the last item in the parameter, will be our genericPoemMaker function we defined above.
function getUserInput(firstName, lastName, gender, callback) {
  var fullName = firstName + " " + lastName;

  // Make sure the callback is a function
  if (typeof callback === "function") {
  // Execute the callback function and pass the parameters to it
  callback(fullName, gender);
  }
}

getUserInput("Michael", "Fassbender", "Man", genericPoemMaker);
// Output
/* Michael Fassbender is finer than fine wine.
Altruistic and noble for the modern time.
Always admirably adorned with the latest style.
A Man of unfortunate tragedies who still manages a perpetual smile.
*/

function greetUser(customerName, sex) {
  var salutation = sex && sex === "Man" ? "Mr." : "Ms.";
 console.log("Hello, " + salutation + " " + customerName);
}

// Pass the greetUser function as a callback to getUserInput
getUserInput("Bill", "Gates", "Man", greetUser);

// And this is the output
Hello, Mr. Bill Gates