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JavaScript Asynchronous Programming with JavaScript Asynchronous JavaScript with Callbacks Implement a Callback

Begana Choi
PLUS
Begana Choi
Courses Plus Student 13,126 Points
Posted by Begana Choi
Begana Choi
Courses Plus Student 13,126 Points

why this video used onload event instead of onreadystatechange?

and what's the difference between onroad and onreadystatechange?

2 Answers

Zimri Leijen
Zimri Leijen
11,835 Points

the ready state will change several times before it actually finishes loading.

Onload only triggers once, when it's done loading.

ready state will trigger each time it goes up one level. These are the possible levels:

0 UNSENT Client has been created. open() not called yet.
1 OPENED open() has been called.
2 HEADERS_RECEIVED send() has been called, and headers and status are available.
3 LOADING Downloading; responseText holds partial data.
4 DONE The operation is complete.

Zimri Leijen
Zimri Leijen
11,835 Points

glad to help

Jennifer Ciaccio
seal-mask
.a{fill-rule:evenodd;}techdegree
Jennifer Ciaccio
UX Design Techdegree Student 11,137 Points
Jennifer Ciaccio
.a{fill-rule:evenodd;}techdegree
Jennifer Ciaccio
UX Design Techdegree Student 11,137 Points

Before we had seen it as this done as if(xhr.onreadystatechange === 4){ code to be triggered when information is received back from server }

This should also trigger the function to trigger only once, when the AJAX call receives a response from the server correct? Or would that not be true in this instance? Would you be able to use either or?

Pablo Cafiso
Pablo Cafiso
4,184 Points

You are correct Jennifer. The catch though is that the code does execute on each state change, and then the if() acts as a filter to run the code we want only once. So, yes the code inside the if does execute once, but the if had to run several times, one for each state change which is less efficient in terms of performance than the onload event, which triggers only once. End result is the same as you had noticed, but performance-wise, onload is a better choice.