01.06.07

5 things you didn’t know about Javascript alert()

Posted in Coding at 2:47 pm by David Kellogg

1. alert() is a native method
2. alert() is part of window
3. alert() is part of this
4. alert() can be rewritten
5. alert() can be saved and reused

alert() is a dull function, useful for debugging and annoying users with messages that their password must contain at least one non-alphanumeric character. Let’s spice things up a bit while we find the true essence of this old companion.

1. Alert is a native method. It is not made up simply of other Javascript. This can be confirmed by typing javascript:alert(alert.toString()) into the location bar. We want to rewrite this, but luckily, the native code part will not stop us.

2. Alert is a function of window.

window.alert(”hello!”);

is the same as the non-window.alert() version.

3. alert() is part of ‘this’.

this.alert(”alert from this”);

This agrees with window being at the top level.

4. alert() can be rewritten.

function alert(message) {
document.writeln(message+”<br>”);
}

5. alert() can be saved and reused.

Sometimes while writing extensions, I want to dump some data through alert, but I want to limit the number of times alert can be called. Here is how to do this.

var temp_alert = alert;
alert = function(msg) {
  if(alert.count++ < alert.max_alerts) {
      temp_alert("Note from alert "+alert.count+": "+msg);
  } else {
      document.writeln("Max alerts reached!");
  }
}
alert.count=0;
alert.max_alerts = 2;
alert("dude");
alert("dude");
alert("dude");
alert("dude");

Result:

Max alerts reached!
Max alerts reached!

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