Variable scope inside constructor

Question

In JavaScript, functions can always access global variables. I have a class that I am using, and it references global variables. Here's a similar class:

function Test(){
    this.abc = abc;
}

If I set the global abc then call this, it works.

var abc = 123,
    testA = new Test;

console.log(testA.abc); // 123

But what if I don't want abc to be global? I wrapped the code in a function call, but I get an error saying abc is not defined.

(function(){
    var abc = 123,
        testA = new Test;  // ERROR: abc is not defined

    console.log(testA.abc);
})();

How can I read local variables inside a JavaScript constructor without adding variables to the global scope?

@mbratch: Because I didn't write Test. I was just wondering if I could "fix" it without re-writing. :) — gen_Eric
– gen_Eric, Commented May 30, 2013 at 14:24

Alnitak · Accepted Answer · 2013-05-30 14:21:44Z

3

The problem is that local variables have lexical scope.

That means that to be resolved they must be within the same code block, or in enclosing code blocks.

Your code would only work if the definition of Test was also within the IIFE:

(function(){
    var abc = 123,
        testA = new Test;  // ERROR: abc is undefined

    function Test() {      // this will be hoisted
        this.abc = abc;
    }

    console.log(testA.abc);
})();

answered May 30, 2013 at 14:21

Alnitak

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8 Comments

gen_Eric Over a year ago

Yeah, I was afraid there was no way around this.

Alnitak Over a year ago

so you're trying to make Test work without rewriting it, but referring to a local variable instead of global?

gen_Eric Over a year ago

Yeah, pretty much. Test was written by someone else, and relies on global variables (for whatever reason). I hate global variables and was wondering if I avoid them.

Alnitak Over a year ago

You might be able to make it work using with, but that'll probably cause more problems than it fixes. It's also not allowed in ES5 "strict" mode.

gen_Eric Over a year ago

I thought of with, but I am using 'use strict'; on my code :)

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