Javascript: Two output when not using var keyword

Question

Google Chrome and Firebug give me two different outputs with this example.

if b gets global, then first should give me undefined and second one 14. right? but in firebug, it gives two 14s and Chrome gives reference error.

function a() {
      b = 14;
}
console.log(b);
a();
console.log(b);

Answer 1

Don't use the browser console for scope experiments. Different browser consoles run your code in different ways.

If you run that code exactly as quoted in a normal environment, the correct thing is that you'll get a ReferenceError from the first console.log(b) line:

function a() {
      b = 14;
}
console.log(b); // ReferenceError
a();
console.log(b);

Even in loose mode, trying to read the value of an undeclared identifier is a ReferenceError.

If we remove that initial console.log, we'd be in an area that varies depending on loose vs. strict mode:

// In loose mode
function a() {
      b = 14;
}
a();
console.log(b); // 14

That's The Horror of Implicit Globals;¹ in loose mode, assigning to an undeclared identifier creates a global variable.

Vs.

// In strict mode
"use strict";
function a() {
      b = 14;   // ReferenceError
}
a();
console.log(b);

...which is how it should be.

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¹That's a post on my anemic little blog.