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What is the complexity of JSON.parse() in JavaScript?

The title says it all. I'm going to be parsing a very large JSON string and was curious what the complexity of this built in method was.
I would hope that it's θ(n) where n is the number of characters in the string since it can determine whether there is a syntax error or not.
I tried searching but couldn't come up with anything.

JSON is very simple grammar that does not require even lookaheads. As soon as GC is not involved then it is purely O(n).

I do not know of the implementations in browsers, but your assumption is correct to a certain point. If the JSON includes mainly strings, it will be straight forward and very linear. If you have many floating points, it will take a bit of time to convert the numbers, but again quite linear (numbers with more digits take slightly longer, but in comparison to a long string... very similar).
Since in most cases arrays and objects are declared as maps, the memory allocation grows as required and will generally be linear. Many (if not most) implementations will make use of Java as a backend. This means garbage collection and thus a quite impossible way to know for sure how much time will be required to transform all the data as it will very much depend on things such as the size of the memory model used on the target computer and how often the garbage collection runs. However, it should generally just grow as items are added to the map and it will thus mostly look like it is linear as well. I would not expect an implementation to make use of a realloc() which would mean copying data and thus being slower and slower as an array/object grows bigger and bigger.

Was curious a little more so to add more info I believe this is the "high-level" implementation of JSON.parse. I tried finding if Chromium has their own source for it and not sure if this is it? This is going off of the source from Github.
Things to note:
worst case is probably the scenario of handling objects which requires O(N) time where N is the number of characters.
in the case a reviver function is passed, it has to rewalk the entire object after it's created but that only happens once so it's fairly negligible. Also depends what the reviver function is doing and you will have to account for it's own time complexity.

Is Javascript substring virtual?

If we have a huge string, named str1, say 5 million characters long, and then str2 = str1.substr(5555, 100) so that str2 is 100 characters long and is a substring of str1 starting at 5555 (or any other randomly selected position).
How JavaScript stores str2 internally? Is the string contents copied or the new string is sort of virtual and only a reference to the original string and values for position and size are stored?
I know this is implementation dependent, ECMAScript standard (probably) does not define what's under the hood of the string implementation. But I want to know from some expert who knows V8 or SpiderMonkey from inside well enough to clarify this.
Thank you

AFAIK V8 has four string representations:
ASCII
UTF-16
concatenation of multiple strings
slice of another string
Adventures in the land of substrings and RegExps has great explanations and illustrations.
Thus, it does not have to copy the string; it just has to beginning and ending markers to the other string.
SpiderMonkey does the same thing. (See Large substrings ~9000x faster in Firefox than Chrome: why? ... though the answer for Chrome is outdated.)
This can give real speed boosts, but sometimes this is undesirable, since it can cause small strings to hold onto the memory of the larger parent string (V8 bug report)

This old blog post of mine explains it, as well as some other string representation forms: https://web.archive.org/web/20170607033600/http://blog.cdleary.com:80/2012/01/string-representation-in-spidermonkey/
Search for "dependent string". I think I know what you might be getting at with the question: they can be problematic things, at times, because if there are no references to the original, you can keep a giant string around in order to keep a bitty little substring that's actually semantically reachable. There are things that an implementation could do to mitigate that problem, like record information on a GC-generation basis to see if such one-dependent-string entities exist and collapse them to their minimal size, but last I knew of that was not being done. (Essentially with that kind of approach you're recovering runtime_refcount == 1 style information at GC-sweep time.)

Javascript Performance: How come looping through an array and checking every value is faster than indexOf, search and match?

This came as a huge surprise for me, and I'd like to understand this result. I made a test in jsperf that is basically supposed to take a string (that is part of a URL that I'd like to check) and checks for the presence of 4 items (that are in fact, present in the string).
It checks in 5 ways:
plain indexOf;
Split the string, then indexOf;
regex search;
regex match;
Split the string, loop through the array of items, and then check if any of them matches the things it's supposed to match
To my huge surprise, number 5 is the fastest in Chrome 21. This is what I can't explain.
In Firefox 14, the plain indexOf is the fastest, that one I can believe.

I'm also surprised but Chrome uses v8, a highly optimized JavaScript engine which pulls all kinds of tricks. And the guys at Google probably have the largest set of JavaScript to run to test the performance of their implementation. So my guess is this happens:
The compiler notices that the array is a string array (type can be determine at compile time, no runtime checks necessary).
In the loop, since you use ===, builtin CPU op codes to compare strings (repe cmpsb) can be used. So no functions are being called (unlike in any other test case)
After the first loop, everything important (the array, the strings to compare against) is in CPU caches. Locality rulez them all.
All the other approaches need to invoke functions and locality might be an issue for the regexp versions because they build a parse tree.

I have added two more tests : http://jsperf.com/finding-components-of-a-url/2
The single regExp is fastest now (on Chrome). Also regExp literals are faster than string literals converted to RegExp.

String concatenation vs string buffers in Javascript

I was reading this book - Professional Javascript for Web Developers where the author mentions string concatenation is an expensive operation compared to using an array to store strings and then using the join method to create the final string. Curious, I did a couple test here to see how much time it would save and this is what I got -
http://jsbin.com/ivako
Somehow, the Firefox usually produces somewhat similar times to both ways, but in IE, string concatenation is much much faster. So, can this idea now be considered outdated (browsers probably have improved since?

Even if it were true and the join() was faster than concatenation it wouldn't matter. We are talking about tiny amounts of miliseconds here which are completely negligible.
I would always prefer well structured and easy to read code over microscopic performance boost and I think that using concatenation looks better and is easier to read.
Just my two cents.

On my system (IE 8 in Windows 7) the times of StringBuilder in that test very from about 70-100% in range -- that is, it is not stable -- although the mean is about 95% of that of the normal appending.
While it's easy now just to say "premature optimization" (and I suspect that in almost every case it is) there are things worth considering:
The problem with repeated string concatenation comes repeated memory allocations and repeated data copies (advanced string data-types can reduce/eliminate much of this, but let's keep assuming a simplistic model for now). From this lets raise some questions:
What memory allocation is used? In the naive case each str+=x requires str.length+x.length new memory to be allocated. The standard C malloc, for instance, is a rather poor memory allocator. JS implementations have undergone changes over the years including, among other things, better memory subsystems. Of course these changes don't stop there and touch really all aspects of modern JS code. Because now ancient implementations may have been incredibly slow in certain tasks does not necessarily imply that the same issues still exist, or to the same extents.
As with above the implementation of Array.join is very important. If it does NOT pre-allocate memory for the final string before building it then it only saves on data-copy costs -- how many GB/s is main memory these days? 10,000 x 50 is hardly pushing a limit. A smart Array.join operation with a POOR MEMORY ALLOCATOR would be expected to perform a good bit better simple because the amount of re-allocations is reduced. This difference would be expected to be minimized as allocation cost decreases.
The micro-benchmark code may be flawed depending on if the JS engine creates a new object per each UNIQUE string literal or not. (This would bias it towards the Array.join method but needs to be considered in general).
The benchmark is indeed a micro benchmark :)
Increase the growing size should have an impact of performance based on any or all (and then some) above conditions. It is generally easy to show extreme cases favoring some method or another -- the expected use case is generally of more importance.
Although, quite honestly, for any form of sane string building, I would just use normal string concatenation until such a time it was determined to be a bottleneck, if ever.
I would re-read the above statement from the book and see if there perhaps other implicit considerations the author was indeed meaning to invoke such as "for very large strings" or "insane amounts of string operations" or "in JScript/IE6", etc... If not, then such a statement is about as useful as "Insert sort is O(n*n)" [the realized costs depend upon the state of the data and the size of n of course].
And the disclaimer: the speed of the code depends upon the browser, operating system, the underlying hardware, moon gravitational forces and, of course, how your computer feels about you.

In principle the book is right. Joining an array should be much faster than repeatedly concatenating to the same string. As a simple algorithm on immutable strings it is demonstrably faster.
The trick is: JavaScript authors, being largely non-expert dabblers, have written a load of code out there in the wild that uses concatenating, and relatively little ‘good’ code that using methods like array-join. The upshot is that browser authors can get a better improvement in speed on the average web page by catering for and optimising the ‘bad’, more common option of concatenation.
So that's what happened. The newer browser versions have some fairly hairy optimisation stuff that detects when you're doing a load of concatenations, and hacks it about so that internally it is working more like an array-join, at more or less the same speed.

I actually have some experience in this area, since my primary product is a big, IE-only webapp that does a LOT of string concatenation in order to build up XML docs to send to the server. For example, in the worst case a page might have 5-10 iframes, each with a few hundred text boxes that each have 5-10 expando properties.
For something like our save function, we iterate through every tab (iframe) and every entity on that tab, pull out all the expando properties on each entity and stuff them all into a giant XML document.
When profiling and improving our save method, we found that using string concatention in IE7 was a lot slower than using the array of strings method. Some other points of interest were that accessing DOM object expando properties is really slow, so we put them all into javascript arrays instead. Finally, generating the javascript arrays themselves is actually best done on the server, then you write then onto the page as a literal control to be exectued when the page loads.

As we know, not all browsers are created equal. Because of this, performance in different areas is guaranteed to differ from browser to browser.
That aside, I noticed the same results as you did; however, after removing the unnecessary buffer class, and just using an array directly and a 10000 character string, the results were even tighter/consistent (in FF 3.0.12): http://jsbin.com/ehalu/
Unless you're doing a great deal of string concatenation, I would say that this type of optimization is a micro-optimization. Your time might be better spent limiting DOM reflows and queries (generally the use of document.getElementbyById/getElementByTagName), implementing caching of AJAX results (where applicable), and exploiting event bubbling (there's a link somewhere, I just can't find it now).

Okay, regarding this here is a related module:
http://www.openjsan.org/doc/s/sh/shogo4405/String/Buffer/0.0.1/lib/String/Buffer.html
This is an effective means of creating String buffers, by using
var buffer = new String.Buffer();
buffer.append("foo", "bar");
This is the fastest sort of implementation of String buffers I know of. First of all if you are implementing String Buffers, don't use push because that is a built-in method and it is slow, for one push iterates over the entire arguments array, rather then just adding one element.
It all really depends upon the implementation of the join method, some implementations of the join method are really slow and some are relatively large.

Performance question: String.split and then walk on the array, or RegExp?

I'll do some work on a line separated string. Which one will be faster, to split the text via String.split first and then walk on the resultant array or directly walk the whole text via a reg exp and construct the final array on the way?

Well, the best way to get your answer is to just take 2 minutes and write a loop that does it both ways a thousand times and check firebug to see which one is faster ;)
I've had to optimize a lot of string munging while working on MXHR and in my experience, plain String methods are significantly faster than RegExps in current browsers. Use RegExps on the shortest Strings possible and do everything you possibly can with String methods.
For example, I use this little number in my current code:
var mime = mimeAndPayload.shift().split('Content-Type:', 2)[1].split(";", 1)[0].replace(' ', '');
It's ugly as hell, but believe it or not it's significantly faster than the equivalent RegExp under high load.

While this is 2½ years late, hopefully this helps shed some light on the matter for any future viewers: http://jsperf.com/split-join-vs-regex-replace (Includes benchmarks results for multiple browsers, as well the functional benchmark code itself)

I expect that using split() will be much faster. It depends upon many specifics, number of lines vs. length, complexity of regex, etc.