测试 value 是否为函数

我需要测试一个表单的 onsubmit的值是否是一个函数。这种格式通常是 onsubmit="return valid();"。有没有办法判断这是否是一个函数，以及它是否可调用？使用 typeof 只返回一个字符串，这对我没什么帮助。

编辑 : 当然，我理解“ return valid() ;”是一个字符串。我已经将它的 replaced 值降低到“ valid() ;”，甚至是“ valid()”。我想知道这两个是否是函数。

编辑 : 这里有一些代码，可能有助于解释我的问题:

$("a.button").parents("form").submit(function() {
var submit_function = $("a.button").parents("form").attr("onsubmit");
if ( submit_function && typeof( submit_function.replace(/return /,"") ) == 'function' ) {
return eval(submit_function.replace(/return /,""));
} else {
alert("onSubmit is not a function.\n\nIs the script included?"); return false;
}
} );

编辑2 : 这是新的代码。看起来我仍然需要使用 eval，因为调用 form.subit ()并不会触发现有的 on 提交。

var formObj = $("a.button").parents("form");
formObj.submit(function() {
if ( formObj[0].onsubmit && typeof( formObj.onsubmit ) == 'function' ) {
return eval(formObj.attr("onsubmit").replace(/return /,""));
} else {
alert("onSubmit is not a function.\n\nIs the script included?");
return false;
}
} );

关于如何可能做得更好的建议？

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A simple check like this will let you know if it exists/defined:

if (this.onsubmit)
{
// do stuff;
}
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Make sure you are calling typeof on the actual function, not a string literal:

function x() {
console.log("hi");
}


typeof "x"; // returns "string"


typeof x; // returns "function"
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What browser are you using?

alert(typeof document.getElementById('myform').onsubmit);

This gives me "function" in IE7 and FireFox.

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You can try modifying this technique to suit your needs:

 function isFunction() {
var functionName = window.prompt('Function name: ');
var isDefined = eval('(typeof ' + functionName + '==\'function\');');
if (isDefined)
eval(functionName + '();');
else
alert('Function ' + functionName + ' does not exist');
}
function anotherFunction() {
alert('message from another function.');
}
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Try 

if (this.onsubmit instanceof Function) {
// do stuff;
}
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Well, "return valid();" is a string, so that's correct.

If you want to check if it has a function attached instead, you could try this:

formId.onsubmit = function (){ /* */ }


if(typeof formId.onsubmit == "function"){
alert("it's a function!");
}
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If it's a string, you could assume / hope it's always of the form 

return SomeFunction(arguments);

parse for the function name, and then see if that function is defined using 

if (window[functionName]) {
// do stuff
}
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  if ( window.onsubmit ) {
//
} else {
alert("Function does not exist.");
}
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You could simply use the typeof operator along with a ternary operator for short:

onsubmit="return typeof valid =='function' ? valid() : true;"

If it is a function we call it and return it's return value, otherwise just return true

Edit:

I'm not quite sure what you really want to do, but I'll try to explain what might be happening.

When you declare your onsubmit code within your html, it gets turned into a function and thus its callable from the JavaScript "world". That means that those two methods are equivalent:

HTML: <form onsubmit="return valid();" />
JavaScript: myForm.onsubmit = function() { return valid(); };

These two will be both functions and both will be callable. You can test any of those using the typeof operator which should yeld the same result: "function".

Now if you assign a string to the "onsubmit" property via JavaScript, it will remain a string, hence not callable. Notice that if you apply the typeof operator against it, you'll get "string" instead of "function".

I hope this might clarify a few things. Then again, if you want to know if such property (or any identifier for the matter) is a function and callable, the typeof operator should do the trick. Although I'm not sure if it works properly across multiple frames.

Cheers

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form.onsubmit will always be a function when defined as an attribute of HTML the form element. It's some sort of anonymous function attached to an HTML element, which has the this pointer bound to that FORM element and also has a parameter named event which will contain data about the submit event.

Under these circumstances I don't understand how you got a string as a result of a typeof operation. You should give more details, better some code.

Edit (as a response to your second edit):

I believe the handler attached to the HTML attribute will execute regardless of the above code. Further more, you could try to stop it somehow, but, it appears that FF 3, IE 8, Chrome 2 and Opera 9 are executing the HTML attribute handler in the first place and then the one attached (I didn't tested with jQuery though, but with addEventListener and attachEvent). So... what are you trying to accomplish exactly?

By the way, your code isn't working because your regular expression will extract the string "valid();", which is definitely not a function.

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I think the source of confusion is the distinction between a node's attribute and the corresponding property.

You're using:

$("a.button").parents("form").attr("onsubmit")

You're directly reading the onsubmit attribute's value (which must be a string). Instead, you should access the onsubmit property of the node:

$("a.button").parents("form").prop("onsubmit")

Here's a quick test:

<form id="form1" action="foo1.htm" onsubmit="return valid()"></form>
<script>
window.onload = function () {
var form1 = document.getElementById("form1");


function log(s) {
document.write("<div>" + s + "</div>");
}


function info(v) {
return "(" + typeof v + ") " + v;
}


log("form1 onsubmit property: " + info(form1.onsubmit));
log("form1 onsubmit attribute: " + info(form1.getAttribute("onsubmit")));
};
</script>

This yields:

form1 onsubmit property: (function) function onsubmit(event) { return valid(); }
form1 onsubmit attribute: (string) return valid()
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最佳答案

I'm replacing a submit button with an anchor link. Since calling form.submit() does not activate onsubmit's, I'm finding it, and eval()ing it myself. But I'd like to check if the function exists before just eval()ing what's there. – gms8994

<script type="text/javascript">
function onsubmitHandler() {
alert('running onsubmit handler');
return true;
}
function testOnsubmitAndSubmit(f) {
if (typeof f.onsubmit === 'function') {
// onsubmit is executable, test the return value
if (f.onsubmit()) {
// onsubmit returns true, submit the form
f.submit();
}
}
}
</script>


<form name="theForm" onsubmit="return onsubmitHandler();">
<a href="#" onclick="
testOnsubmitAndSubmit(document.forms['theForm']);
return false;
"></a>
</form>

EDIT : missing parameter f in function testOnsubmitAndSubmit

The above should work regardless of whether you assign the onsubmit HTML attribute or assign it in JavaScript:

document.forms['theForm'].onsubmit = onsubmitHandler;
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You can always use one of the typeOf functions on JavaScript blogs such as Chris West's. Using a definition such as the following for the typeOf() function would work:

function typeOf(o){return {}.toString.call(o).slice(8,-1)}

This function (which is declared in the global namespace, can be used like this:

alert("onsubmit is a " + typeOf(elem.onsubmit));

If it is a function, "Function" will be returned. If it is a string, "String" will be returned. Other possible values are shown here.

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// This should be a function, because in certain JavaScript engines (V8, for
// example, try block kills many optimizations).
function isFunction(func) {
// For some reason, function constructor doesn't accept anonymous functions.
// Also, this check finds callable objects that aren't function (such as,
// regular expressions in old WebKit versions), as according to EcmaScript
// specification, any callable object should have typeof set to function.
if (typeof func === 'function')
return true


// If the function isn't a string, it's probably good idea to return false,
// as eval cannot process values that aren't strings.
if (typeof func !== 'string')
return false


// So, the value is a string. Try creating a function, in order to detect
// syntax error.
try {
// Create a function with string func, in order to detect whatever it's
// an actual function. Unlike examples with eval, it should be actually
// safe to use with any string (provided you don't call returned value).
Function(func)
return true
}
catch (e) {
// While usually only SyntaxError could be thrown (unless somebody
// modified definition of something used in this function, like
// SyntaxError or Function, it's better to prepare for unexpected.
if (!(e instanceof SyntaxError)) {
throw e
}


return false
}
}
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using a string based variable as example and making use instanceof Function You register the function..assign the variable...check the variable is the name of function...do pre-process... assign the function to new var...then call the function. 

function callMe(){
alert('You rang?');
}


var value = 'callMe';


if (window[value] instanceof Function) {
// do pre-process stuff
// FYI the function has not actually been called yet
console.log('callable function');
//now call function
var fn = window[value];
fn();
}
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Isn't typeof xxx === 'function' the best and the fastest?

I made an bench in wich you can try it out, compared to instanceof and _underscore

  1. Its just seems to be faster than instanceof (using chrome)
  2. It won't trow an error if the variable is not defined

Here a bench: https://jsbench.me/qnkf076cqb/1

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Beware that es6 class is also a function but not callable

class C {}


typeof C === "function"  // true
C instanceof Function    // true


C()       // error
C.call()  // error
new C()   // okay
new C     // okay
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Checking the call method on the value seems to be a good enough test. e.g., val.call && val()

> a = () => {}
[Function: a]
> function b() {}
undefined
> c = function(){}
[Function: c]
> d = 2
2
> e = []
[]
> f = {}
{}
> a.call
[Function: call]
> b.call
[Function: call]
> c.call
[Function: call]
> d.call
undefined
> e.call
undefined
> f.call
undefined

Note: Except when it's a class.


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