What does “with” do in JavaScript?

Question

I saw JavaScript code which begins with with. That's a bit confusing. What does it do and how can it be used correctly?

with (sObj) return options[selectedIndex].value;

Answer 1

It brings pain and suffering among you and others

Answer 2

It isn't a function (as was indicated in the question title before it was edited) but a statement. It may make more sense if the code sample is formatted like so:

with (sObj){
    return options[selectedIndex].value;
}

Regarding what it does (Source)

The with statement establishes the default object for a set of statements. JavaScript looks up any unqualified names within the set of statements to determine if the names are properties of the default object. If an unqualified name matches a property, then the property is used in the statement; otherwise, a local or global variable is used.

Which means that in the code sample, it is first checked if options is a property of sObj. If it is then options refers to sObj.options, otherwise it checks other scopes for a variable defined by the name options

The downside of using a with statement is that it is impossible to know from just glancing at the code what gets accessed. There are other better alternatives as shown in this article