javascript setTimeout() first argument: expression error

Question

function Timer() {
 this.initialTime = 0;
 this.timeStart = null;

 this.getTotalTime = function() {
  timeEnd = new Date();
  diff = timeEnd.getTime() - this.timeStart.getTime();

  return diff+this.initialTime;
 };

 this.formatTime = function() {
  interval = new Date(this.getTotalTime());

  return  interval.getHours() + ":" +  interval.getMinutes() + ":" + interval.getSeconds();
 };

 this.start = function() {
  this.timeStart = new Date();

  setTimeout("this.updateTime()", 1000);
 };

 this.updateTime = function() {
  alert(this.formatTime());
  setTimeout("this.updateTime()", 1000);
 };
}


timer = new Timer();
timer.start();

I am getting an error:

this.updateTime is not a function

Any ideas?

Thanks

I wonder if we can/should change this question to be a very generic one. All the code in the question didn't help me, I just went straight to the answer :). — Gezim
– Gezim, Commented Oct 13, 2011 at 14:49
Turns out, setTimeout with a string as its first argument behaves like an indirect eval. — Sebastian Simon
– Sebastian Simon, Commented Sep 30, 2021 at 3:37

SLaks · Accepted Answer · 2010-04-09 01:47:55Z

2

Your string is not evaluated in the context of your object, so this doesn't refer to what you think it does.

You should not be passing a string argument to setTimeout. Instead, you should pass an anonymous function that calls your method with a saved copy of this.

For example:

var self = this;
setTimeout(function() { self.updateTime(); }, 1000);

The self variable is necessary because setTimeout's callback is also not evaluated in the context of your object.

answered Apr 9, 2010 at 1:47

SLaks

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7 Comments

Jonah Over a year ago

this works but.. is there a more elegant way? Without defining the global variable?

SLaks Over a year ago

That's a local, not a global. You could use a proxy function.

Jonah Over a year ago

Ok.. I'm not a JS expert by any means. Thanks!

SLaks Over a year ago

self is a regular local variable. Because the anonymous function is in the same scope, it saves references to all the local variables. This is an extremely powerful feature called a closure.

deceze Over a year ago

Actually, if you define self inside the updateTime function, it's not guaranteed to refer to the Timer object either. There is no difference between your code and a direct setTimeout(this.updateTime). this/self have the same context. :)

|

user187291 · Accepted Answer · 2010-04-09 01:47:21Z

1

try

var me = this;
setTimeout(function() { me.updateTime() }, 1000);

answered Apr 9, 2010 at 1:47

user187291

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Comments

bobince · Accepted Answer · 2010-04-09 10:01:02Z

is there a more elegant way?

Yes, in ECMAScript Fifth Edition:

setTimeout(this.updateTime.bind(this), 1000);

However, until all browsers support Fifth Edition (which they don't yet by a long measure), you should add your own implementation of Function.bind as fallback. eg.:

// Add ECMA262-5 method binding if not supported natively
//
if (!('bind' in Function.prototype)) {
    Function.prototype.bind= function(owner) {
        var that= this;
        if (arguments.length<=1) {
            return function() {
                return that.apply(owner, arguments);
            };
        } else {
            var args= Array.prototype.slice.call(arguments, 1);
            return function() {
                return that.apply(owner, arguments.length===0? args : args.concat(Array.prototype.slice.call(arguments)));
            };
        }
    };
}

You might also prefer to use setInterval to avoid the repeated setTimeout calls. Finally, you should remember to declare all your variables (timeEnd, diff, etc.) as var, otherwise you're getting accidental globals, which can cause you horrible debugging heartache.

deceze · Accepted Answer · 2010-04-09 02:00:41Z

0

If you supply a string to setTimeout, this string will be executed literally. The thing is, it will be executed sometime later in the global context, outside your Timer object, so this means something completely different.

Just pass the function itself like so:

function Timer() {
  var self = this;   // make reference to this that won't change with context

  this.updateTime = function() {
    alert(self.formatTime());
    setTimeout(self.updateTime, 1000);
  };

}

edited Apr 9, 2010 at 2:00

answered Apr 9, 2010 at 1:48

deceze♦

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