JavaScript has an operator called with. First, I'll demonstrate its usage, and then go over why you should stay away from it.

Imagine you have a few things that you are calling on the same object:

var obj = { person: { name: 'john', age: 25 } };
if (person.obj.age > 20) {
  console.log(person.obj.name);
}

You can rewrite this using with like so:

var obj = { person: { name: 'john', age: 25 } };
with(obj.person) {
  if (age > 20) {
    console.log(name);
  }
}

So you are essentially dropped into a place where the properties of the given object are treated like locals. This is appealing, until you see (1) the alternative which is much prettier, and (2) the problems with this approach.

I'd rather see this code written in the shorter form:

var obj = { person: { name: 'john', age: 25 } };
var person = obj.person;
if (person.age > 20) {
  console.log(person.name);
}

So, not only is the code without "with" shorter and more clean in its intent, it also prevents the ugly mind-mess involved with trying to figure out code when people write:

var name = 'kate';
var obj = { person: { name: 'john', age: 25 } };
with(obj.person) {
  name; // this value is 'john', but is it clear when reading this line?
}

Maybe don't use with. ;)