adobeadobe-indesignextendscriptadobe-javascript

When is "var" parsed?

Sometimes var affects code before it, and sometimes not. Take this example:

base.jsx:

$.global.a = 1;
$.writeln("a: " + a);
var a = 2;

Running this prints a: 1. Running $.evalFile("base.jsx"); also prints a: 1. However, running

(function() {
    $.evalFile("base.jsx");
})();

prints a: undefined.

Why? What is the logic behind this?

Solution

  • After a bunch of testing, I figured it out.

    I knew that JavaScript's scopes are function-level, but I had assumed that files also have their own scope. I was wrong.

    Running

    $.global.a = 1;
var a = 2;
$.writeln("$.global.a: " + $.global.a);

    will print $.global.a: 2. This means that $.global.a and var a are exactly the same thing in this context, and the scope of the file is actually the global scope.

    Given that base.jsx is still

    $.global.a = 1;
$.writeln("a: " + a);
var a = 2;

    Running the code

    (function() {
    $.evalFile("base.jsx");
    $.writeln("a: " + a);
    $.writeln("$.global.a: " + $.global.a);
})();

    changes the scope of base.jsx to be this function instead of the global object. Suddenly $.global.a and var a are referring to two different objects. The result of this will be:

    a: undefined
a: 2
$.global.a: 1

    So the problem was never that var is sometimes parsed early and sometimes not. The problem is that files have no scope other than the global one.