Arrow_onmouseover is attached with the 'onmouseover' event of an object. I get a 'timeline is not defined' JS error when the onmouseover event occurs. I know this relates to the variable scope. How can I correct this.
function startloop()
{
var changer = setInterval("changecontents();", 2000);
var timeline = setInterval("change();", 2000);
}
window.onload = startloop();
function arrow_onmouseover()
{
window.clearInterval(timeline);
window.clearInterval(changer);
}
Also what is the difference between a function called as Function(); and Function; . I believe this may also be relevant here since it greatly affects the scope of a variable.
ie what is the difference between
onclick="js();" and onclick="js;" or onclick="js"
where JS is a defined Java Script function.
Change your code like this.
var timeline,changer;
function startloop()
{
changer = setInterval("changecontents();", 2000);
timeline = setInterval("change();", 2000);
}
window.onload = startloop();
function arrow_onmouseover()
{
window.clearInterval(timeline);
window.clearInterval(changer);
}
The difference between
onclick="js();" and onclick="js;" or onclick="js"
if your js function doesn't have any parameters to receive you can simple avoid () like if you want to get the date you can call new Date(); but instead you can simply call new Date;
Related
My application is a simple mysql client used from command line - it connects to database and makes few queries to get information from database. Mysql functionality is encapsulated in a class and problem is since calls to mysql server is async (understandably) - the code flow reaches end of application.
And I am unable to refer to 'this'(Mysql) inside a method of Mysql class.
How do I get around this problem ?
Below is my code.
//CLASS
function Mysql(config) {
//...
}
//METHOD
Mysql.prototype.getDbInfo = function (cbk) {
this.showTables(function(e,r) {
// >>>>>>>>>> PROBLEM HERE using 'this' <<<<<<<<<<<
console.log(this.configVar);
});
}
module.exports = Mysql;
//CLASS OBJECT
var test = new Mysql(config);
//METHOD INVOKE
test.getDbInfo(function (err,results) {
//...
});
Every time that you jump into a callback function you are loosing the scope of the this object. There are different ways to work around it.
Assign this to another variable
The first solution is to assign the this object to another variable (e.g.: that, self). When you assign one variable to another and the first variable is an object then you keep the reference to the original object and you can use it within the callback. Something like that:
Mysql.prototype.getDbInfo = function (cbk) {
var self = this;
self.showTables(function(e,r) {
// >>>>>>>>>> PROBLEM HERE using 'this' <<<<<<<<<<<
console.log(self.configVar);
});
}
Bind the this object to the function
You can bind the this object to the function and like that you set the this keyword set to the provided value (in your case the scope outside of showTables function). You can read the documentation of this and you will be able to understand more:
Mysql.prototype.getDbInfo = function (cbk) {
this.showTables(function(e,r) {
// >>>>>>>>>> PROBLEM HERE using 'this' <<<<<<<<<<<
console.log(self.configVar);
}.bind(this));
}
Use es6 arrow functions
It is more or less the same solution like the first one. If you use a transpiler you will find out that it is translated like the first solution:
Mysql.prototype.getDbInfo = function (cbk) {
self.showTables((e,r) => {
// >>>>>>>>>> PROBLEM HERE using 'this' <<<<<<<<<<<
console.log(this.configVar);
});
}
i'm very familiar with the javascript console.log(), and the php_dump() functions that allows us to see what's in a variable, i want to know if there is some function like this in hogan.js that let us inspect the content of a variable.
add some method to your data and include it at the loctation you need to inspect the scope
var data = {
...
// your vars,
...
inspect: function () {
return function () {
console.log(this);
}
}
};
template.render(data);
anywhere you use {{inspect}} it will log the current render context in the console
I slightly modified it to add the function to the data packet that is passed to Hogan in a centralized position, which, in my code, is a function called render().
Thank you for this clever trick.
function render(template, data, destination) {
data.inspect = function() {
return function() {
console.log("inspect:")
console.log(this);
};
};
// localized strings
data.strings = app.strings;
var tmpl = Hogan.compile(template);
var content = tmpl.render(data);
document.querySelector(destination).innerHTML = content;
}
My WinJS app uses the single navigation model. There is some common code that I would like to apply to every page in the app. Instead of placing the code in each page's ready function, I would like to be able to able to define a "global" ready function that will be executed when a page's ready event is fired. Any ideas?
you can define a Mixin object with utility function used for all pages.
utils.js:
PageMixin = {
ready: function ready(element, options)
{
this.element = element;
this.options = options;
this.initialize();
this.onready();
},
initialize: function initialize()
{
// write common initialize code here
}
};
page.js:
var Page = WinJS.UI.Pages.define('/pages/mypage/page.html',
{
onready: function onready()
{
// page specific initialization code here
}
});
// this will make all PageMixin util methods available on Page.
WinJS.Class.mix(Page, PageMixin);
refer WinJS.Class.mixin for details.
Currently I have a Node.js module with the following form :
var events = require('events');
var emitter = new events.EventEmitter();
function emitSomething() {
emitter.emit("event");
}
exports.emitSomething = emitSomething;
exports.on = emitter.on;
However any callback registered through on do not get called when I call emitSomething.
I can get around this by changing the last line to
exports.on = function(event, callback) { emitter.on(event, callback); };
Is there a reason I can't delegate the on function to a function defined in another module?
This is a common JS mistake. Note that the on method depends on this (which is emitter in your example). However, all you export is the function itself, so it's context (emitter) is lost when you call it later.
Here is a small example for this issue:
var someObj = {
doSth: function() { console.log(this) }
};
someObj.doSth(); // "Object { ..."
var doSth = someObj.doSth;
doSth(); // "Window { ..."
When I call doSth as method of someObj, this references someObj as expected. After I copied the function to doSth, this references it's new context, in the browser the global context Window. This is what happens in your case.
As you already mentioned, you have to bind the emitter context to the function. This can also be done like this:
exports.on = emitter.on.bind(emitter);
I started using backbone.js recently for a game. I used model to create Timer as follows:
var Timer = Backbone.Model.extend({
defaults: {
'hh':00, 'mm':05, 'ss':00
},
initialize: function() {
},
countDownOnce: function() {
// Count down the time by 1 sec
},
run1: function() {
this.countDownOnce();
}
run2: function() {
setInterval(this.countDownOnce, 1000);
}
});
The countDownOnce function, if called directly as in run1, works fine.
But if the function is passed as an argument to some built-in function, say setInterval as in the run2 function, the value of this is lost.
How to propagate this pointer to the built-in functions?
The Backbone docs have a useful section on binding "this".
Basically, Underscore provides a couple of very useful functions, _.bind and _.bindAll, which help you more easily manage 'this's context.
...
initialize: function() {
_.bindAll(this, 'countDownOnce' //, and whatever other functions you want bound)
}),
...
This will make sure that no matter what context it's called from, the 'this' inside countDownOnce refers to your Timer instance.
One way to do this is using call and apply. For example:
run2: function() {
setInterval(this.countDownOnce.call, 1000, this);
}
Also note that passing a closure/function reference with arguments to setTimeout() and setInterval() requires some manual hacks to work correctly in IE.
this is how javascript was designed (pun intended :)
Basically 'this' binds to the current context so if in a function/method call like this:
var Data = function(name) {
this.name = name;
}
var myData = new Data("viky");
would refer to the name element "inside" Data (i.e., myData in this case)
Now if you had the following:
var Data = function(name) {
function morphName(anotherName) {
//and if you call this like this ;)
alert(this.name); //undefined
}
}
the inner function morphName is bound to the 'current' context i.e., the outer function. In javascript everything is an object, including functions. So the outer (anonymous) function doesn't have a member called name! (makes sense?)
In order to do that most people use the following (by convention):
var Data = function(name) {
var that = this; //capture current context. Some prefer self = this;
function morphName(anotherName) {
//then you access name like that ;)
alert(that.name);
}
}
I suggest you read about javascript closures and scopes to understand this and that :D