with Statement Considered Harmful

By YUI TeamApril 11, 2006

JavaScript's with statement was intended to provide a shorthand for writing recurring accesses to objects. So instead of writing

ooo.eee.oo.ah_ah.ting.tang.walla.walla.bing = true;
ooo.eee.oo.ah_ah.ting.tang.walla.walla.bang = true;

You can write

with (ooo.eee.oo.ah_ah.ting.tang.walla.walla) {
    bing = true;
    bang = true;
}

That looks a lot nicer. Except for one thing. There is no way that you can tell by looking at the code which bing and bang will get modifed. Will ooo.eee.oo.ah_ah.ting.tang.walla.walla be modified? Or will the global variables bing and bang get clobbered? It is impossible to know for sure.

The with statement adds the members of an object to the current scope. Only if there is a bing in ooo.eee.oo.ah_ah.ting.tang.walla.walla will ooo.eee.oo.ah_ah.ting.tang.walla.walla.bing be accessed.

If you can't read a program and be confident that you know what it is going to do, you can't have confidence that it is going to work correctly. For this reason, the with statement should be avoided.

Fortunately, JavaScript also provides a better alternative. We can simply define a var.

var o = ooo.eee.oo.ah_ah.ting.tang.walla.walla;
o.bing = true;
o.bang = true;

Now there is no ambiguity. We can have confidence that it is ooo.eee.oo.ah_ah.ting.tang.walla.walla.bing and ooo.eee.oo.ah_ah.ting.tang.walla.walla.bang that are being set, and not some hapless variables.