Analysis of the immediate execution function of javascript

This article mainly introduces the analysis of javascript's t immediate execution function. It has certain reference value. Now I share it with you. Friends in need can refer to it.

Concept:
Immediate execution As the name suggests, the function is executed immediately after the function is defined.
Function expression method:
Add parentheses () after the function expression to execute the function immediately.

var xmlhttpUtil = function () { function getText(url,callback){ alert("dog"); }();

Anonymous function method:
Anonymous functions are not allowed to appear directly in javascript as a separate statement, so if you want to execute the function immediately,
must make some grammatical changes: Method 1, It assigns an anonymous function to a variable, which becomes a function expression and can be executed immediately by adding parentheses after it.
Method 2: Put a bracket around the anonymous function (this is allowed by js grammar), which is equivalent to a function expression, and then add brackets after it to execute it immediately.

(function(b){ alert(b); })(321);

Modularization:
Because variables inside the function will not pollute the global scope, immediate execution of functions can be used for modularization. Many third-party libraries have adopted this approach.

var xmlhttpUtil = function () { function getText(url,callback){ var request=new XMLHttpRequest(); request.open("GET",url); request.send(null); request.onreadystatechange=function(){ if(request.readyState===4 && request.status===200){ var type=request.getResponseHeader("Content-Type"); if(type.match(/^text/)) callback(request.responseText); alert(request.responseText); } else { } } } let xmlHttpRequestObj={}; xmlHttpRequestObj.getText=getText; return xmlHttpRequestObj; }();

Execution:

xmlhttpUtil.getText("/dog",function () { alert("xmlhttprequest请求响应！"); });

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