how could `if (console == undefined)` break js execution?

Question

When I edit production code (little fixes) I want to add console.log for example, but not to break page for users who don't have firebug or don't use chrome I decide to redefine console object:

if (console == undefined) {
    console = {
        log     : function(){},
        info    : function(){},
        warn    : function(){},
        error   : function(){}
    };
};

After I inserted this code, JS-execution was broken in browsers that don't have console object (IE, firefox without firebug, etc). (By "broken" I mean that code after these lines doesn't execute at all) Why did it happen?

code that is after these lines just doesn't execute. Sorry, I edited my post (was in hurry) — Larry Foobar, Jul 20 '11 at 15:05
@Tomalak this is an obvouis problem. You should be able to understand it. I expected better :( — Raynos, Jul 20 '11 at 15:06
@Raynos: If nothing else, the OP needs to learn how to ask better questions. Yes, I could guess at what the OP means, but that doesn't make the question any better. — Lightness Races in Orbit, Jul 20 '11 at 15:14

score 10 · Accepted Answer · answered Jul 20 '11 at 15:06

10

(console == undefined)

will throw a ReferenceError if console is undefined.

Use typeof instead which does not throw ReferenceErrors for undeclared variables

(typeof console === "undefined")

answered Jul 20 '11 at 15:06

Raynos

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I created a console stub for these situations. Check it out at https://github.com/robertofrega/console-stub – Beto Frega Oct 26 '12 at 16:46

score 5 · Answer 2 · answered Jul 20 '11 at 15:04

5

Try this:

if ( window.console === undefined ) {

    window.console = {
        // Your methods
    }

}

answered Jul 20 '11 at 15:04

Seth

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Yeap, it works! But I posted in plain javascript (without function encapsulation) - so I was already in global scope (in browser it's window). Why doesn't `console` work, but exactly `window.console` ? – Larry Foobar Jul 20 '11 at 15:07
@Innuendo: as Raynos's post says, it will throw a ReferenceError if `console` hasn't been declared. If you use `window.console`, then it just returns `undefined`, like accessing a non-existent property of any object. – Reid Jul 20 '11 at 15:09

score 2 · Answer 3 · edited May 23 '17 at 12:30

2

It's not really good to compare a variable to undefined, since undefined is a simple undefined variable.

You can use something like this:

if (!window.console)
  console = {
    log   : function(){},
    info  : function(){},
    warn  : function(){},
    error : function(){}
  };

Edit: I just found this: How do I print debug messages in the Google Chrome JavaScript Console?

edited May 23 '17 at 12:30

Community

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answered Jul 20 '11 at 15:07

Floern

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score 1 · Answer 4 · answered May 03 '12 at 20:53

1

Any of the answer above should work fine.

One note: You might also add "dir" to your list of empty methods, since people sometimes use console.dir() to print out objects.

answered May 03 '12 at 20:53

Ryan

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8

score 1 · Answer 5 · answered Jul 20 '11 at 15:08

1

If console is not defined in the js then it will throw an error instead use typeof to check the variable's existense

if (typeof console == 'undefined') { }

answered Jul 20 '11 at 15:08

ShankarSangoli

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score 1 · Answer 6 · answered Jul 20 '11 at 15:10

1

If you change your if(console == undefined) to if(window.console == undefined) everything will work

answered Jul 20 '11 at 15:10

Akkuma

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how could `if (console == undefined)` break js execution?

6 Answers6