遇到函数javascript中的真假语句，有什么意义？

Question

I was ripping apart some code from sharepoint and noticed that they have random true false statements in their functions. My first thought was that they would stop the next statement from being ran, but when I made a simple test, the console log showed it ran no matter what.

Could someone let me know what this is for, what the point is, and why you would do it.

here is an example I came across, and will post my test.

 function _SubmitFormPost(a, d, c) {
    if (typeof MSOWebPartPageFormName != "undefined") {

        var b = document.forms[MSOWebPartPageFormName];

        if (null != b)
        {
            if (d != undefined && d == true || typeof b.onsubmit == "undefined" || b.onsubmit == null || b.onsubmit() != false) {
                //HERE IS A RANDOM EXAMPLE OF T/F 
                typeof window.WebForm_OnSubmit == "function" && window.WebForm_OnSubmit();
                if (ajaxNavigate.get_search().match(new RegExp("[?&]IsDlg=1")) != null)
                a += a.indexOf("?") == -1 ? "?IsDlg=1" : "&IsDlg=1";
                if (FV4UI())
                try {
                    var e = SP.Ribbon.PageManager.get_instance().get_ribbon().get_selectedTabId();
                    if (Boolean(e)) {
                        a = StURLSetVar2(a, "InitialTabId", escapeProperly(e));
                        a = StURLSetVar2(a, "VisibilityContext", "WSSTabPersistence")
                    }
                } catch (f) {}
                if (c != undefined && c == true) {
                    a = DemoteIntoFormBody(b, a, "owsfileref");
                    a = DemoteIntoFormBody(b, a, "NextUsing")
                }
                b.action = STSPageUrlValidation(a);
                b.method = "POST";
                if (isPortalTemplatePage(a))
                b.target = "_top";
                !bValidSearchTerm && _ClearSearchTerm("");
                b.submit()
            }
        }
    }
}

and here was my test, the console window was hitting all the console.logs that I thought it wouldn't

function s(){
    console.log('in s')
    //is true
    if(1==1){
        //is true
        if(2==2){
            console.log(' if 2==2 and eval of t/f is ',typeof(1) =="function")
            //is false -- expexted to skip console.log
            typeof(1) =="function"
            console.log('2 is 2')
        }
        //is true
        2==3
        if(3==3){
            console.log('3==3')
        }
    }
}
s()

Answer 1

&& doesn't just return a value. It also has a (flow control) effect: Its second operand is only evaluated if its first operand is true. That is, A && B doesn't run B if A is already false . (This is known as short-circuit evaluation .)

In other words,

typeof window.WebForm_OnSubmit == "function" && window.WebForm_OnSubmit();

behaves like

if (typeof window.WebForm_OnSubmit == "function")
    window.WebForm_OnSubmit();

but is slightly shorter.

The point is not to return true or false (the result is ignored); the point is to only call window.WebForm_OnSubmit if it is defined as a function.

Similarly,

!bValidSearchTerm && _ClearSearchTerm("");

works like:

if (!bValidSearchTerm)
    _ClearSearchTerm("");