JavaScript: The Good Parts

with Statement

JavaScript has a with statement that was intended to provide a shorthand when accessing the properties of an object. Unfortunately, its results can sometimes be unpredictable, so it should be avoided.

The statement:

with (obj) {
    a = b;
}

does the same thing as:

if (obj.a === undefined) {
    a = obj.b === undefined ? b : obj.b;
} else {
    obj.a = obj.b === undefined ? b : obj.b;
}

So, it is the same as one of these statements:

a = b;
a = obj.b;
obj.a = b;
obj.a = obj.b;

It is not possible to tell from reading the program which of those statements you will get. It can vary from one running of the program to the next. It can even vary while the program is running. If you can't read a program and understand what it is going to do, it is impossible to have confidence that it will correctly do what you want.

Simply by being in the language, the with statement significantly slows down JavaScript processors because it frustrates the lexical binding of variable names. It was well intentioned, but the language would be better if it didn't have it.