JavaScript Global Var Hoisting Issue - javascript

Here is my basic issue...
<script>
var window.my_Global = false;
</script>
<script>
if(my_Global){...} //my_Global here is undefined, not false??
</script>
I know there must be a simple solution but I'm pretty stuck...

Remove var:
window.my_Global = false;
You don't need the var statement to set properties of an object. In fact, that's invalid syntax:
<script>
window.my_Global = true;
</script>
<script>
console.log(my_Global);
if(my_Global){
alert("Hello world!");
}
</script>

Related

Socket JS not works inside onload method

I have this js piece which I use for django channels:
var chatSocketSender1 = new WebSocket(
'ws://' + window.location.host + '/ws/my_socket1/'
)
function send() {
var msg = "some message"
var receiver_id = 111
window['chatSocketSender1'].send( JSON.stringify({
'msg': msg,
'receiver_id': receiver_id
}) )
}
document.querySelector('#send_button').onclick = function() {
send();
}
The problem is that this js code only works if I put that after latest element of HTML:
<head>
...
</head>
<body>
// whole html content here
<script>
// js piece here
</script>
</body>
But if I use jquery's on load method like:
<head>
<script>
$(window).on('load', function () {
// js piece here
});
</script>
</head>
<body>
// whole html content here
</body>
Then it gives: Uncaught TypeError: window[("chatSocketSender1")] is undefined
Any suggestion please, why this not works with "onload" ?
The reason your code doesn't work is because in your first code var chatSocketSender1 creates a "global" variable (that's defined at the window level) that you later reference as window("chatSocketSender1"), but in the second code, var chatSocketSender1 is scoped to the anonymous event handler function, so is not available as window("chatSocketSender1").
Put another way:
<script>
var x = 1;
function foo() {
console.log(x)
}
</script>
works fine, but
<script>
function foo() {
var x = 1;
}
foo();
console.log(x);
</script>
will give an undefined variable error as x only exists inside foo. This isn't exactly what you've done, but is essentially the same concept.
Now, if your code was:
chatSocketSender1.send(
then it would have worked fine as your variable chatSocketSender1 is defined within the function() { onload event callback.
or instead of
var chatSocketSender1 =
you could do
window.chatSocketSender1 =
to define the variable globally, or you could:
<script>
var chatSocketSender1 = new WebSocket(...
function send() { ... }
$(window).on('load', function () {
document.querySelector('#send_button').onclick = ...
as you generally only need the event binding within the onload.

What is the scope of a JavaScript function? [duplicate]

I'm not quite sure what goes on between <script> tags. For example, the following gives a reference error and type error in Chrome:
<html>
<head>
<script type="text/javascript">
T.prototype.test = function() {
document.write("a");
}
</script>
<script type="text/javascript">
function T() {}
var v = new T();
v.test();
</script>
</head>
<body>
</body>
</html>
But this works:
<html>
<head>
<script type="text/javascript">
</script>
<script type="text/javascript">
T.prototype.test = function() {
document.write("a");
}
function T() {}
var v = new T();
v.test();
</script>
</head>
<body>
</body>
</html>
The upper script is executed first in the first example, so it doesn't know about T yet, hence the error.
In the second example, T is well defined and known anywhere as soon as the script tag is executed. This is due to function declarations being hoisted to the top, no matter what the order is, they are always available. Function declaration hoisting is more deeply explained here
The second example after hoisting is applied:
var v,
T = function(){}; /* using comma like this is short-hand for: var v; var T = function(){}; */
T.prototype.test = function() {
document.write("a");
};
v = new T();
v.test();
They are each parsed in the global context, but in order. The entire first script tag is parsed and executed before the second one is considered. As part of parsing a script, function declarations are recognized ahead of time, which is why the second one works while the first one doesn't.

Javascript function not executing when another function is above it

So, the function "txtLoad()" will not execute while "txtFunc()" is being called by an onLoad function.
<!DOCTYPE html>
<html>
<head>
<title>Page</title>
</head>
<body>
<textarea id="Text" rows="20" cols="70"></textarea>
<script>
function txtFunc(){
var q=1;
}
function txtLoad() {
document.getElementById("Text").innerHTML = "Hello";
}
onload=function(){
txtLoad()
}
onload=function(){
txtFunc()
}
</script>
</body>
</html>
The JavaScript engine says there is nothing wrong with the code inside the "script" element. Does anyone know why the function isn't executing?
your second onload= replaced the function that calls txtLoad, if you want to run both:
onload = function () {
txtLoad();
txtFunc();
};
Also there seems to be extra "}" after the txtFunc.
How can two functions be defined to the onload. For eg:
var x = 1
var x = 2
What do you think the value of x is going to be? Obviously 2.
You are overwriting the onload. Its only going to execute the stuff which you assigned at last.
You could do this to achieve what you want.
onload = init()
function init() {
textLoad();
textFunc();
}
simply because the first onload was overwritten by the las onload.
function txtFunc(){
var q=1;
}
function txtLoad() {
document.getElementById("Text").innerHTML = "Hello";
}
onload=test();
function test()
{
txtLoad();
txtFunc();
}

Why JavaScript function visibility depends on declaration in the same or different script block in HTML

If I implement the following JavaScript code block in HTML markup:
<script type="text/javascript">
function MyFunc() {
alert(1);
}
var f1 = MyFunc;
</script>
<script type="text/javascript">
function MyFunc() {
alert(2)
}
</script>
<script type="text/javascript">
f1();
</script>
I get an alert message '1'.
However, if I use the following code:
<script type="text/javascript">
function MyFunc() {
alert(1);
}
var f1 = MyFunc;
function MyFunc() {
alert(2)
}
</script>
<script type="text/javascript">
</script>
<script type="text/javascript">
f1();
</script>
I get '2'. Why?
Tested in IE10, latest FF, Chrome.
This is caused by hoisting. var and function declarations are hoisted to the top of the script block they are in.
This means that your first script essentially becomes:
var f1;
function MyFunc() {
alert(1);
}
f1 = MyFunc;
// new script block
function MyFunc() {
alert(2)
}
// new script block
f1(); // alert 1
Meanwhile, your second script becomes:
var f1;
function MyFunc() {
alert(1);
}
function MyFunc() { // overwrite previous MyFunc
alert(2)
}
f1 = MyFunc;
// new script block
f1(); // alert 2
I hope this makes sense - just in general avoid overwriting functions XD
Each script element is parsed and executed in order. In the first case, the assignment to f1 is made before the second declaration is processed.
In the second case, both declarations are parsed before the assignment is made (because declarations are processed before any code is executed), so the second declaration replaces the first before the assignment to f1.
I hope I don't confuse anybody, but, I suspect the answer has something to do with scopes.
Here is my argument:
For the 1st case:
<script type="text/javascript">
function MyFunc() {
alert(1);
}
var f1 = MyFunc;
window.foo1 = MyFunc;
</script>
<script type="text/javascript">
function MyFunc() {
alert(2)
}
window.foo2 = MyFunc;
</script>
<script type="text/javascript">
f1();
alert(window.foo2 === window.foo1);
</script>
The second alert will issue a false.
Now the 2nd case.
<script type="text/javascript">
function MyFunc() {
alert(1);
}
var f1 = MyFunc;
window.foo1 = f1;
function MyFunc() {
alert(2)
}
window.foo2 = MyFunc;
</script>
<script type="text/javascript">
</script>
<script type="text/javascript">
f1();
alert(window.foo1 === window.foo2);
</script>
The second alert will issue a true.
It kind of shows that (for 1st case) MyFunc is redefined and is a totally different "object" when the 2nd script block is processed. f1 holds a reference to the first "object" in the first script block. Hence f1() alerts 1.
For the first part with alert message 1 the declaration var f1 = MyFunc; becomes similar to a local wrt different <script> blocks.Hence the <script> block in which var f1 is defined it performs the function contained in that block.
For the second part with alert message 2 , 1st MyFunc() is overwritten by the second.

How to store a url in a variable in javascript?

I'm trying this rough script idea. But it is not working.
<script>
function storeurl() {
var varurl = document.URL;
}
document.onclick = storeurl;
document.write(varurl);
</script>
varurl is set as the actual url using document.URL function.
with broogle
then on click i would like varurl to be set to #2 and then echo.
In a perfect world this script would echo
http://url/#2
when clicking on the link
Any help?
Thx
Your varurl variable is scoped at method (function) level. This means it is not visible to code which runs outside of the function.
Also, the document.write code will execute when the script first runs i.e. before the click (should the click ever happen).
If you don't need to use varurl other than to write it to the document you can move the document.write code into the function and retain the narrow scope of varurl:
<script>
function storeurl() {
var varurl = document.URL;
document.write(varurl);
}
document.onclick = storeurl;
</script>
Otherwise move the variable definition out of the function so that it (the variable) becomes a global:
<script>
var varurl;
function storeurl() {
varurl = document.URL;
document.write(varurl);
}
document.onclick = storeurl;
</script>
The var makes it a local variable to the function's scope. Plus you are trying to read it before it is even set.
You have made varurl location to the function it is declared in with var so it isn't visible from outside that function.
var varurl;
function storeurl() {
varurl = document.URL;
}
You are also writing its value immediately instead of doing so in the click event, so it won't have been set by the time you write().
function storeurl() {
var varurl = document.URL;
document.write(varurl);
}
document.onclick = storeurl;
It should work,
<script>
function storeurl() {
varurl = document.URL; // it should be Global variable, so remove var
document.write(varurl);//when you're declaring this outside of the function
}
document.onclick = storeurl;
</script>
change your code to
var varurl;
function storeurl() {
varurl = window.location.href;
}
For simply storing a URL in a variable, be it an external URL or the URL of the current page, and then either display it or do something else with it, you can follow what is shown in the code below:
<html>
<body>
<button onclick="go()">GO TO GOOGLE</button><br/>
<button onclick="show()">CLICK TO SHOW CURRENT URL</button><br/>
<p id="showhere"></p>
<script>
function go(){
var u = "http://www.google.com" ;
window.location.href = u; //takes you to google.com
}
function show(){
var x = window.location.href;
document.getElementById("showhere").innerHTML = x;
//shows URL of current page below the buttons
}
</script>
</body>
</html>

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