Is "clear" a reserved word in Javascript?

Question

Welcome To Ask or Share your Answers For Others

Is "clear" a reserved word in Javascript?

posted Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

Is "clear" a reserved word in Javascript?

I just spent a long time figuring out that I shouldn't use clear() as the name of a function in Javascript:

<head>
    <script type="text/javascript" src="Array.js"></script>
</head>
<body>
    Hello!!!!<br>
    <button type="button" onClick="clear()" id="ppp">Shoo!</button><br>
    <button type="button" onClick="add()" id="add">Add a few elements</button><br>
    <button type="button" onClick="check()" id="check">Check the array</button><br>
    <p id="results">Results will appear here.</p>
    <script type="text/javascript">
initialize();
    </script>
</body>

Here's Array.js:

var results;

function initialize(){
 results = document.getElementById("results");
}

function add() {
results.firstChild.data="add";    
}

function clear() {
results.firstChild.data = "Hello?";
}

function check() {
results.firstChild.data = "check";
}

Symptoms: Clicking the 'add' and 'check' buttons gives me the result I expect, but clicking the 'clear' button does nothing.

If I rename clear() to clearxyz(), it works fine.

My questions:

Is "clear" a reserved word? I don't see it on the list: https://developer.mozilla.org/en/JavaScript/Reference/Reserved_Words
Is there a debugging trick I should be using to figure this kind of thing out in the future? It took me a long time (I'm a noob!) to figure out that the name of the function was my problem.

Many thanks. Edit: I'm using Firefox 6.0, and I added a line break to show where Array.js starts.

Question&Answers:os

与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…

1 Reply

深蓝 · Answer 1 · 2021-10-16T21:18:15+0000

As the others said, clear is not a reserved keyword. It seems that the called function is document.clear ^[MDN]. Invoking

console.log(clear === document.clear);

inside the event handler returns true.

DEMO

So it seems, document is in the scope chain of the event handler.... the question now is why.

JavaScript: The Definitive Guide says:

In an event handler as HTML attribute, the Document object is in the scope chain before the Window object (...)

As your method is global, meaning it is a property of the window object, it is not found in the scope chain, as document.clear comes earlier in the scope chain.

I haven't found any specification for this. The guide also says that one should not rely on that, so I assume this is nothing official.

If you have form elements inside a form, then even the corresponding form element will be in the scope chain (not sure whether this holds for all browsers though). This is another reason for confusion.

There are two (not exclusive) ways to avoid such situations:

Don't use inline event handlers. It is considered bad practice as it is mixing logic and presentation. There are other ways to attach event handlers.
Don't pollute the global namespace. Create one object in global scope (with a name you are sure of does not collide with any window or document properties or ids of HTML elements) and assign the functions as properties of this object. Whenever you call a function, you reference it through this object. There also other ways to namespace your code.

Categories

Is "clear" a reserved word in Javascript?

Is "clear" a reserved word in Javascript?

Please log in or register to add a comment.

Please log in or register to reply this article.

1 Reply

Please log in or register to add a comment.

Just Browsing Browsing

Most popular tags