with.
It's rarely used, and frankly, rarely useful... But, in limited circumstances, it does have its uses.
For instance: object literals are quite handy for quickly setting up properties on a new object. But what if you need to change half of the properties on an existing object?
var user =
{
fname: 'Rocket',
mname: 'Aloysus',
lname: 'Squirrel',
city: 'Fresno',
state: 'California'
};
// ...
with (user)
{
mname = 'J';
city = 'Frostbite Falls';
state = 'Minnesota';
}
Alan Storm points out that this can be somewhat dangerous: if the object used as context doesn't have one of the properties being assigned to, it will be resolved in the outer scope, possibly creating or overwriting a global variable. This is especially dangerous if you're used to writing code to work with objects where properties with default or empty values are left undefined:
var user =
{
fname: "John",
// mname definition skipped - no middle name
lname: "Doe"
};
with (user)
{
mname = "Q"; // creates / modifies global variable "mname"
}
Therefore, it is probably a good idea to avoid the use of the with statement for such assignment.
See also: Are there legitimate uses for JavaScript’s “with” statement?
