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 opti         


        

Answer 1

Its the equivalent of

return sObj.options[selectedIndex].value;

With lets you issue a block of statements in the context of a particular object. Therefore all of the statements in the with block are taken to be members of the object in parenthesis.

This can make code more readable at times, but it also can lead to ambiguity, since the variable references can either be with sObj or global.

legitimate uses for javascript's "with" statement :D

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