Mozilla Developer Network
Non-standard
This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
Summary
Watches for a property to be assigned a value and runs a function when that occurs.
Method of Object |
|
|---|---|
| Implemented in | JavaScript 1.2 |
| ECMAScript Edition | none |
Syntax
object.watch(prop, handler)Parameters
-
prop - The name of a property of the object on which you wish to monitor changes.
-
handler - A function to call when the specified property's value changes.
Description
watch() and unwatch() when possible. These two methods are implemented only in Gecko, and they're intended primarily for debugging use. In addition, using watchpoints has a serious negative impact on performance, which is especially true when used on global objects, such as window. You can usually use setters and getters or proxies instead. See Compatibility for details.Watches for assignment to a property named prop in this object, calling handler(prop, oldval, newval) whenever prop is set and storing the return value in that property. A watchpoint can filter (or nullify) the value assignment, by returning a modified newval (or by returning oldval).
If you delete a property for which a watchpoint has been set, that watchpoint does not disappear. If you later recreate the property, the watchpoint is still in effect.
To remove a watchpoint, use the unwatch() method. By default, the watch method is inherited by every object descended from Object.
The JavaScript debugger has functionality similar to that provided by this method, as well as other debugging options. For information on the debugger, see Venkman.
In Firefox, handler is only called from assignments in script, not from native code. For example, window.watch('location', myHandler) will not call myHandler if the user clicks a link to an anchor within the current document. However, window.location += '#myAnchor' will call myHandler.
watch() on an object for a specific property overrides and previous handler attached for that property.Examples
Example: Using watch and unwatch
var o = { p: 1 };
o.watch("p", function (id, oldval, newval) {
console.log( "o." + id + " changed from " + oldval + " to " + newval );
return newval;
});
o.p = 2;
o.p = 3;
delete o.p;
o.p = 4;
o.unwatch('p');
o.p = 5;
This script displays the following:
o.p changed from 1 to 2 o.p changed from 2 to 3 o.p changed from undefined to 4
Example: Using watch to validate an object's properties
You can use watch to test any assignment to an object's properties. This example ensures that every Person always has a valid name and an age between 0 and 200.
Person = function(name,age) {
this.watch("age", Person.prototype._isValidAssignment);
this.watch("name", Person.prototype._isValidAssignment);
this.name = name;
this.age = age;
}
Person.prototype.toString = function() {
return this.name + ", " + this.age;
};
Person.prototype._isValidAssignment = function(id, oldval, newval) {
if (id === "name" && (!newval || newval.length > 30)) {
throw new RangeError("invalid name for " + this);
}
if (id === "age" && (newval < 0 || newval > 200)) {
throw new RangeError("invalid age for " + this);
}
return newval;
}
will = new Person("Will", 29);
print(will); // Will, 29
try {
will.name = "";
} catch (e) {
print(e);
}
try {
will.age = -4;
} catch (e) {
print(e);
}
This script displays the following:
Will, 29 RangeError: invalid name for Will, 29 RangeError: invalid age for Will, 29
Compatibility
- This Polyfill that offers watch to all ES5 compatible browser
- Using a Proxy enables you do that even deeper changes to how property assignments work