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JavaScript determines whether a function exists

Source: Internet
Author: User
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Judging method

The code is as follows Copy Code

typeof (Test) = = "function"

Boolean (Window.loginok instanceof Function)

Boolean (window.parent.loginOk instanceof function)//judging functions in the parent page

And then I sorted out a

The code is as follows Copy Code


Window.onload=function () {
try{

if (test&&typeof (TEST) = = "function") {
Test ();
}else{
Alert ("nonexistent function");
}
}catch (e) {
}
}
function Test () {
Alert ("Function execution ...");
}

If it does not exist, an exception is thrown, so add try......catch.

Example

It's okay to use typeof even if it doesn't exist, which is the test code:

The code is as follows Copy Code

    <script type= "Text/javascript"
        //Window scope
        alert (typeof (AAAAA));

       //Custom scope
        var Type = function () {}
        type.prototype.test = function () {alert (typeof bb BB)); };

        var obj = new type ();
        obj.test ();
    </script>
<body>
</body>

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