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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