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Answer 1

+1 A:

In the interests of avoiding confusing myself with what function does what, I would always start with a closure, until I needed to reduce memory usage:

var dofocus = function(){element.focus()};
dofocus();

If you're in a loop and element is changing, you can use a function that returns another function: (function(e,m){return function(){e[m]()}})(element, method)

Edit: On a reread of the question, I'm thinking the reason your code doesn't work is because you're passing in the value of some variable named 'focus' and not the string method name 'focus', or rather the focus function (is it defined elsewhere?)

Edit: This works:

<html><title>test</title><script>
function foo (id, method, args) {
    var d = {"id": id, "method": method, "args": args};
    // ... time passes
    var e = document.getElementById(d.id); // test exists
    var f = e[d.method]; // test exists
    var a = d.args; // test is an array
    f.apply(e, a);
}
</script><body id="thebody">
<p>
    <button onclick="foo('bar', 'setSelectionRange', [4,6]);">select</button>
    <button onclick="foo('bar', 'focus',             []);">focus</button>
    <input type="text" id="bar" value="foo bar baz">
<p>
    <button onclick="foo('thebody', 'appendChild', [document.createTextNode(new Date())]);">body</button>
</body></html>

Edit: And if passing functions was the issue, you can always pass an optional method name and test whether the method is a string or a function, then be able to use 'focus' or a custom function without having to place it on the element.

Anonymous 2010-01-23 17:31:14

No the focus method I'm referring to is the native javascript method.

Jacco 2010-01-23 17:44:18

This works due to the reason described in my answer - you're no longer passing the method, but a string, and then f becomes a reference to the method *as it exists on the dom element*

jvenema 2010-01-23 19:51:34

You function works as expected in modern browsers... It is just not functioning in IE6 so I'll have to write some other solution :(

Jacco 2010-01-23 21:33:23

`(function(e,m){return function(){e[m]()}})(element, method)` should be `(function(e,m){return function(){e[m]();};})(element, method)`

Justin Johnson 2010-01-24 00:48:35

@Justin Johnson: Semicolons are optional on the last statement in a block in strict Javscript (eg, Firefox javascript.options.strict=true), which can be annoying or introduce careless bug... unless the whole thing's on one line, anyway, like here.

Anonymous 2010-01-24 04:01:51

I realize that it's not the end-all, but run it through jslint and see what you get. It's a bad practice imho. Also, I checked the ECMA specification but I didn't see any mention of this option. Would you happen to know the section for reference?

Justin Johnson 2010-01-24 07:49:36

@Justin Johnson: "When [...] a token [...] is encountered that is not allowed by any production of the grammar, then a semicolon is automatically inserted before [it if] the offending token is }", page 26 of ECMA-262, Edition 5 (http://www.ecmascript.org/docs.php). --- Contrariwise, JS Lint does not limit itself to syntax errors: "The problem is not necessarily a syntax error, although it often is. JSLint looks at some style conventions as well as structural problems." In other words, it enforces style, not sytnax.

Anonymous 2010-02-02 03:26:57

Answer 2

+1 A:

Read the thread here.

From that thread, we learn that using just "focus" doesn't work, like so:

focus.apply(document.getElementById('someElement'));

(a simple case of what you're attempting) because "focus" has to have an element to which it's bound. Otherwise, it's not a bound function, but rather just a native call with no real reference. My guess is this'll be the case for any element-specific functions.

What you need to do is grab the focus element from either the target element, or another DOM element that has a focus function. For example, this will work:

var fn = document.createElement('input').focus;
fn.apply( document.getElementById('someElement') );

jvenema 2010-01-23 17:48:57

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