Problem with eval

[santel_helvis]

Hi,

Here is my code.

function CallDisplay()
{
nodeobj=new mynode();
x="displayId("+nodeobj+")";
eval(x);

}
function displayId(node)
{
alert(node.myid);
}
function mynode()
{
this.myid="aa";
}

In CallDisplay() I am getting the error as "Microsoft JScript
Compilation error ']' expected". Any one can suggest me how to
overcome this.

[Duncan Booth]

wrote:

Hi,

Here is my code.

function CallDisplay()
{
nodeobj=new mynode();
x="displayId("+nodeobj+")";
eval(x);

}
function displayId(node)
{
alert(node.myid);
}
function mynode()
{
this.myid="aa";
}

In CallDisplay() I am getting the error as "Microsoft JScript
Compilation error ']' expected". Any one can suggest me how to
overcome this.

Try adding 'alert(x);' just before the eval and you should see your problem
pretty quickly. x is the string "displayId([object Object])" which is
obviously rubbish. You cannot put an arbitrary object into a string for use
by eval.

How to overcome this? Well, the code you posted doesn't show any reason for
using eval at all. So just use:

function CallDisplay()
{
nodeobj=new mynode();
displayId(nodeobj);
}

You will find there are very few cases where you actually want to use eval.
Usually, if you think you want eval, you are mistaken and your code will be
cleaner and simpler avoiding it.

[santel_helvis]

Hi, Thanks for the reply.

Actually I will be passing the function name (displayId) in CallDisplay
function.

function CallDisplay(fnName)
{
nodeobj=new mynode();
x=fnName+"("+nodeobj+")";
eval(x);
}

I will be passing different names to this CallDisplay. So only I used
eval. Am I going wrong? I don't find the different way. Plz suggest

[Duncan Booth]

wrote:

Hi, Thanks for the reply.

Actually I will be passing the function name (displayId) in CallDisplay
function.

function CallDisplay(fnName)
{
nodeobj=new mynode();
x=fnName+"("+nodeobj+")";
eval(x);
}

I will be passing different names to this CallDisplay. So only I used
eval. Am I going wrong? I don't find the different way. Plz suggest

Is there any reason you need to pass in the name of the function? Why not
just pass the function itself in directly?

function CallDisplay(fn)
{
nodeobj=new mynode();
fn(nodeobj);
}
function displayId(node)
{
alert(node.myid);
}
function mynode()
{
this.myid="aa";
}

CallDisplay(displayId);

[santel_helvis]

Oh that's great. Thanks for your help. I passed the displayId within
the single quote. Now got it.

[Lasse Reichstein Nielsen]

sa***********@sify.com writes:

Please quote a little of the message you reply to.

Actually I will be passing the function name (displayId) in CallDisplay
function.
Why not pass the function itself. It's less fragile and more easily
readable.

function callDisplay(fn) {
var nodeobj = new MyNode():
fn(nodeobj);
}

If you *really* need to pass the function name, and you are certain
that a function is available as a variable of that name in the current
scope, then you should only eval as much as necessary:

function callDisplay(fnName) {
var nodeobj = new MyNode();
var fn = eval(fnName);
fn(nodeobj);
}

or the shorter:
function callDisplay(fnName) {
eval(fnName)(new MyNode());
}

If you want to create the call as a string (and there is no good
reason for that at all), you should make sure that the expression you
create is correct. Generally, only simple values can be converted to a
string representation of themselves by mere conversion to string
(undefined, null, numbers and booleans). Objects do not have a string
representation that preserves identity, and strings need quoting and
escaping. It's much easier to just embed a variable name:
function CallDisplay(fnName)
{
nodeobj=new mynode();
x=fnName+"("+nodeobj+")";
var x = fnName+"(nodeobj)";
eval(x);
} I will be passing different names to this CallDisplay. So only I used
eval. Am I going wrong?

Most likely. Just pass the functions directly.

/L
--
Lasse Reichstein Nielsen - lr*@hotpop.com
DHTML Death Colors: <URL:http://www.infimum.dk/HTML/rasterTriangleDOM.html>
'Faith without judgement merely degrades the spirit divine.'

[Dr John Stockton]

JRS: In article <Xn*************************@127.0.0.1>, dated Mon, 17
Apr 2006 09:37:28 remote, seen in news:comp.lang.javascript, Duncan
Booth <du**********@invalid.invalid> posted :

You will find there are very few cases where you actually want to use eval.
Usually, if you think you want eval, you are mistaken and your code will be
cleaner and simpler avoiding it.

Except sometimes (treating the use of a new Function wrapper as morally
equivalent); I sometimes use eval for reading input controls.

That allows input by expression, so that where a distance in kilometres
is called for a user can enter a distance of 5 miles as 5*1.609 (or
something more complex to be exact), and 1/-0 can be used for -Infinity.

If the user chooses to put unreasonable code in, the consequences are
his problem, not mine.

Example in <URL:http://www.merlyn.demon.co.uk/$rnd.htm>.

--
© John Stockton, Surrey, UK. ?@merlyn.demon.co.uk Turnpike v4.00 IE 4 ©
<URL:http://www.jibbering.com/faq/> JL/RC: FAQ of news:comp.lang.javascript
<URL:http://www.merlyn.demon.co.uk/js-index.htm> jscr maths, dates, sources.
<URL:http://www.merlyn.demon.co.uk/> TP/BP/Delphi/jscr/&c, FAQ items, links.

[Randy Webb]

Dr John Stockton said the following on 4/17/2006 2:49 PM:

JRS: In article <Xn*************************@127.0.0.1>, dated Mon, 17
Apr 2006 09:37:28 remote, seen in news:comp.lang.javascript, Duncan
Booth <du**********@invalid.invalid> posted :

You will find there are very few cases where you actually want to use eval.
Usually, if you think you want eval, you are mistaken and your code will be
cleaner and simpler avoiding it.

Except sometimes (treating the use of a new Function wrapper as morally
equivalent); I sometimes use eval for reading input controls.

If you "sometimes use eval for reading input controls" then you are
misusing eval. Now, if you use eval to execute the value of that control
- that is different.

--
Randy
comp.lang.javascript FAQ - http://jibbering.com/faq & newsgroup weekly
Javascript Best Practices - http://www.JavascriptToolbox.com/bestpractices/
Pedantic people should be careful about how they word things.

[Duncan Booth]

Dr John Stockton wrote:

JRS: In article <Xn*************************@127.0.0.1>, dated Mon,
17 Apr 2006 09:37:28 remote, seen in news:comp.lang.javascript, Duncan
Booth <du**********@invalid.invalid> posted :

You will find there are very few cases where you actually want to use
eval. Usually, if you think you want eval, you are mistaken and your
code will be cleaner and simpler avoiding it.

Except sometimes (treating the use of a new Function wrapper as
morally equivalent); I sometimes use eval for reading input controls.

That's why I said 'very few cases' rather than 'no cases'. If you have a
situation where you want to evaluate a string entered from outside your
code as an expression, then you may need to use eval or its equivalent, but
that isn't a common situation.

Most questions about eval seem to involve trying to lookup a value which
could be accessed far more easily by just using the object directly, or by
using the name to access a property of another object. The problem is that
eval is a hammer, and as such you can use it to drive a screw home, but you
really shouldn't.