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Execution sequence of functions in Ajax

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An actual example of w3school [1]:

<HTML> XMLHTTP. onreadystatechange = function () {If (XMLHTTP. readystate = 4 & XMLHTTP. status = 200) {document. getelementbyid ("mydiv "). innerhtml = XMLHTTP. responsetext ;}} XMLHTTP. open ("get", "/ajax/test1.txt", true); XMLHTTP. send ();} </SCRIPT> 

Pay attention to the code order of the red part of the subscript. If you change the code to the following order:


<HTML> XMLHTTP. open ("get", "/ajax/test1.txt", true); XMLHTTP. send (); XMLHTTP. onreadystatechange = function () {If (XMLHTTP. readystate = 4 & XMLHTTP. status = 200) {document. getelementbyid ("mydiv "). innerhtml = XMLHTTP. responsetext ;}}} </SCRIPT> 

　　


<HTML> XMLHTTP. open ("get", "/ajax/test1.txt", true); XMLHTTP. onreadystatechange = function () {If (XMLHTTP. readystate = 4 & XMLHTTP. status = 200) {document. getelementbyid ("mydiv "). innerhtml = XMLHTTP. responsetext ;}} XMLHTTP. send ();} </SCRIPT> 

Test results show that the execution results of the three code sequences are the same. This is because the following xhlhttp. onreadystatechange = Function


　　


xmlhttp.onreadystatechange=function()  {  if (xmlhttp.readyState==4 && xmlhttp.status==200)    {    document.getElementById("myDiv").innerHTML=xmlhttp.responseText;    }  }


It's just a function declaration (or registration) [In JS, it allows nested declaration of functions]. It just declares the onreadystatechange "attribute" (actually a function) of xhlhttp, not immediately executed. While


XMLHTTP. Open ("get", "/ajax/test1.txt", true); XMLHTTP. Send ();
Is executed immediately. If you want xhlhttp. onreadystatechange = function to be executed immediately, you only need to declare this module as "immediate execution ".


( xmlhttp.onreadystatechange=function()  {  if (xmlhttp.readyState==4 && xmlhttp.status==200)    {    document.getElementById("myDiv").innerHTML=xmlhttp.responseText;    }  }) ();


 


Reference


[1] http://www.w3school.com.cn/tiy/t.asp? F = ajax_async_true


Execution sequence of functions in Ajax

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