Currently, I am diving into Ajax and exploring this specific example. I am struggling to comprehend the syntax variable = function(){; how does this code actually work? How can a function be assigned to a variable?
xmlhttp.onreadystatechange=function()
{
if (xmlhttp.readyState==4 && xmlhttp.status==200)
{
document.getElementById("txtHint").innerHTML=xmlhttp.responseText;
}
}
While it is commonly referred to as a callback, the answer to your query may become clearer if we compare it to more familiar code.
function myFunction()
{
...
}
Calling myFunction() will execute that code.
In Javascript, functions can be declared in various ways. For example:
var myFunction = function()
{
...
}
This declaration performs the same task as the initial example, allowing you to call the function using myFunction().
When introducing a callback into the equation, consider the following:
xmlhttp.onreadystatechange = function()
{
...
}
This assignment involves placing a function with specific instructions within the onreadystatechange property of the xmlhttp object. Consequently, the code within this function will be triggered whenever there is a state change in the xmlhttp object.
When the status of readiness changes, and the request is completed with a response ready (readyState==4) and the document loads correctly (HTTP Status Code 200 = OK!), then add the response text to the element with the id #txtHint.
onreadystatechange holds a function (or its name) that will be called automatically every time the readyState property changes.
readyState indicates the status of the XMLHttpRequest, transitioning from 0 to 4:
status corresponds to HTTP response codes:
onreadystatechange is a callback that is activated when a specific event occurs. This callback, onreadystate, triggers when the request's ready state changes.
In short, onreadystate
Stores a function (or reference to a function) that is automatically executed whenever the readyState property changes
Now for the line
xmlhttp.readyState==4 && xmlhttp.status==200readyState : Represents the status of the XMLHttpRequest. Changes from 0 to 4:
0: Request not initialized
1: Server connection established
2: Request received
3: Processing request
4: Request finished and response is ready
And status Represents status
200: "OK"
404: Page not found
Therefore, the condition
xmlhttp.readyState==4 && xmlhttp.status==200xmlhttp.responseText holds the server's response.
So
document.getElementById("txtHint").innerHTML=xmlhttp.responseText;txtHint to display the received response.
I hope all of the above information was clear!
When it comes to callbacks, imagine this scenario:
Imagine you're trying to rent a newly released movie on VHS from Blockbuster. The attendant is swamped with customers clamoring for "Labyrinth," unable to check if the film is available. Instead of making you wait, he takes your phone number to follow up later when things calm down. Eventually, he calls back to let you know that the movie is sold out but recommends another title like "Dark Crystal."
In the same way, callbacks are like providing your contact information for delayed results when dealing with tasks that require time, such as communicating with remote servers.
Does this analogy help clarify the concept of callbacks for you?
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