console.dir 和 console.log 有什么区别？

Question

In Chrome the console object defines two methods that seem to do the same thing:

console.log(...)
console.dir(...)

I read somewhere online that dir takes a copy of the object before logging it, whereas log just passes the reference to the console, meaning that by the time you go to inspect the object you logged, it may have changed. However some preliminary testing suggests that there's no difference and that they both suffer from potentially showing objects in different states than when they were logged.

Try this in the Chrome console ( Ctrl + Shift + J ) to see what I mean:

> o = { foo: 1 }
> console.log(o)
> o.foo = 2

Now, expand the [Object] beneath the log statement and notice that it shows foo with a value of 2. The same is true if you repeat the experiment using dir instead of log .

My question is, why do these two seemingly identical functions exist on console ?

Answer 1

In Firefox, these function behave quite differently: log only prints out a toString representation, whereas dir prints out a navigable tree.

In Chrome, log already prints out a tree -- most of the time . However, Chrome's log still stringifies certain classes of objects, even if they have properties. Perhaps the clearest example of a difference is a regular expression:

> console.log(/foo/);
/foo/

> console.dir(/foo/);
* /foo/
    global: false
    ignoreCase: false
    lastIndex: 0
    ...

You can also see a clear difference with arrays (eg, console.dir([1,2,3]) ) which are log ged differently from normal objects:

> console.log([1,2,3])
[1, 2, 3]

> console.dir([1,2,3])
* Array[3]
    0: 1
    1: 2
    2: 3
    length: 3
    * __proto__: Array[0]
        concat: function concat() { [native code] }
        constructor: function Array() { [native code] }
        entries: function entries() { [native code] }
        ...

DOM objects also exhibit differing behavior, as noted on another answer .

Answer 2

Another useful difference in Chrome exists when sending DOM elements to the console.

Notice:

• console.log prints the element in an HTML-like tree
• console.dir prints the element in a JSON-like tree

Specifically, console.log gives special treatment to DOM elements, whereas console.dir does not. This is often useful when trying to see the full representation of the DOM JS object.

There's more information in the Chrome Console API reference about this and other functions.

Answer 3

I think Firebug does it differently than Chrome's dev tools. It looks like Firebug gives you a stringified version of the object while console.dir gives you an expandable object. Both give you the expandable object in Chrome, and I think that's where the confusion might come from. Or it's just a bug in Chrome.

In Chrome, both do the same thing. Expanding on your test, I have noticed that Chrome gets the current value of the object when you expand it.

> o = { foo: 1 }
> console.log(o)
Expand now, o.foo = 1
> o.foo = 2
o.foo is still displayed as 1 from previous lines

> o = { foo: 1 }
> console.log(o)
> o.foo = 2
Expand now, o.foo = 2

You can use the following to get a stringified version of an object if that's what you want to see. This will show you what the object is at the time this line is called, not when you expand it.

console.log(JSON.stringify(o));

Answer 4

Use console.dir() to output a browse-able object you can click through instead of the .toString() version, like this:

console.dir(obj/this/anything)

How to show full object in Chrome console?

Answer 5

From the firebug site http://getfirebug.com/logging/

Calling console.dir(object) will log an interactive listing of an object's properties, like > a miniature version of the DOM tab.

Answer 6

None of the 7 prior answers mentioned that console.dir supports extra arguments : depth , showHidden , and whether to use colors .

Of particular interest is depth , which (in theory) allows travering objects into more than the default 2 levels that console.log supports.

I wrote "in theory" because in practice when I had a Mongoose object and ran console.log(mongoose) and console.dir(mongoose, { depth: null }) , the output was the same. What actually recursed deeply into the mongoose object was using util.inspect :

import * as util from 'util';
console.log(util.inspect(myObject, {showHidden: false, depth: null}));

Answer 7

Following Felix Klings advice I tried it out in my chrome browser.

console.dir([1,2]) gives the following output:

Array[2]
 0: 1
 1: 2
 length: 2
 __proto__: Array[0]

While console.log([1,2]) gives the following output:

[1, 2]

So I believe console.dir() should be used to get more information like prototype etc in arrays and objects.

Answer 8

Difference between console.log() and console.dir() :

Here is the difference in a nutshell:

• console.log(input) : The browser logs in a nicely formatted manner
• console.dir(input) : The browser logs just the object with all its properties

Example:

The following code:

let obj = {a: 1, b: 2};
let DOMel = document.getElementById('foo');
let arr = [1,2,3];

console.log(DOMel);
console.dir(DOMel);

console.log(obj);
console.dir(obj);

console.log(arr);
console.dir(arr);

Logs the following in google dev tools:

Answer 9

Well, the Console Standard (as of commit ef88ec7a39fdfe79481d7d8f2159e4a323e89648 ) currently calls for console.dir to apply generic JavaScript object formatting before passing it to Printer (a spec-level operation), but for a single-argument console.log call, the spec ends up passing the JavaScript object directly to Printer .

Since the spec actually leaves almost everything about the Printer operation to the implementation, it's left to their discretion what type of formatting to use for console.log().