Sometimes you need to temporarily change the value of an object property. Maybe you need to stub something for a test; or maybe you need to revert to the prior value for that property. You can do this imperatively, of course, and that is fine. But I want a more generic solution.
I want a function that I pass an object, the property to override, and the new value for the property, and have it return me an undo function to restore the object’s prior state:
var undo = revalue(obj, prop, newValue);
// do whatever
undo();
// hey, my original obj is back!
Here’s a first try that stores the original value in the closure of the returned undo function:
var revalue = function(obj, prop, newValue) {
var undo, orig = obj[prop];
obj[prop] = newValue;
undo = function() {
obj[prop] = orig;
return orig;
};
return undo;
};
That pretty much does the trick. Then a colleague asked: “If the value is a function, why not add a restore property to it?” OK, it’s a bit redundant, since we are returning the undo function anyways, but WTH.
var revalue = function(obj, prop, newValue) {
var undo, orig = obj[prop], isFn = typeof newValue === "function";
obj[prop] = isFn ?
function() { return newValue.apply(obj, arguments); } :
newValue;
undo = function() {
obj[prop] = orig;
return orig;
};
if (isFn) {
obj[prop].restore = undo;
}
return undo;
};
Note that we have to wrap the newValue function inside a lambda:
obj[prop] = isFn ? function() { return newValue.apply(obj, arguments); } : newValue;
If we don’t wrap newValue this way, then the restore property will be attached to newValue and it will leak out of the revalue function. We don’t want that. It’s safe to attach the restore property to the lambda:
if (isFn) {
obj[prop].restore = undo;
}
And since anything worth doing is worth overdoing, the final version extends undo to take a callback function. That callback can be passed in as an optional fourth argument to revalue, or as an optional argument to the undo function itself. It’s up to the user to make the callback return something useful:
var revalue = function(obj, prop, newValue, undoCallback) {
var undo, orig = obj[prop], isFn = typeof newValue === "function";
obj[prop] = isFn ? function() { return newValue.apply(obj, arguments); } : newValue;
undo = function(cb) {
obj[prop] = orig;
cb = isFunction(cb) ? cb :
(isFunction(undoCallback) ? undoCallback : function() { return orig; });
return cb(obj, prop, orig, newValue);
};
if (isFn) {
obj[prop].restore = undo;
}
return undo;
};