Why Javascript ===/== string equality sometimes ha

问题:

After I found that the common/latest Javascript implementations are using String Interning for perfomance boost (Do common JavaScript implementations use string interning?), I thought === for strings would get the constant O(1) time. So I gave a wrong answer to this question:

JavaScript string equality performance comparison

Since according to the OP of that question it is O(N), doubling the string input doubles the time the equality needs. He didn't provide any jsPerf so more investigation is needed,

So my scenario using string interning would be:

var str1 = "stringwithmillionchars"; //stored in address 51242

var str2 = "stringwithmillionchars"; //stored in address 12313

The "stringwithmillionchars" would be stored once let's say in address 201012 of memory and both str1 and str2 would be "pointing" to this address 201012. This address could then be determined with some kind of hashing to map to specific locations in memory.

So when doing

"stringwithmillionchars" === "stringwithmillionchars"

would look like

getContentOfAddress(51242)===getContentOfAddress(12313)

or 201012 === 201012

which would take O(1)/constant time

JSPerfs/Performance updates:

JSPerf seems to show constant time even if the string is 16 times longer?? Please have a look:

http://jsperf.com/eqaulity-is-constant-time

Probably the strings are too small on the above: This probably show linear time (thanks to sergioFC) the strings are built with a loop. I tried without functions - still linear time / I changed it a bit http://jsfiddle.net/f8yf3c7d/3/ .

According to https://www.dropbox.com/s/8ty3hev1b109qjj/compare.html?dl=0 (12MB file that sergioFC made) when you have a string and you already have assigned the value in quotes no matter how big the t1 and t2 are (e.g 5930496 chars), it is taking it 0-1ms/instant time.

It seems that when you build a string using a for loop or a function then the string is not interned. So interning happens only when you directly assign a string with quotes like var str = "test";

回答1:

Based on all the Performance Tests (see original post) for strings a and b the operation a === b takes:

• constant time O(1) if the strings are interned. From the examples it seems that interning only happens with directly assigned strings like var str = "test"; and not if you build it with concatenation using for-loops or functions.

• linear time O(N) since in all the other cases the length of the two strings is compared first. If it is equal then we have character by character comparison. Else of course they are not equal. N is the length of the string.

回答2:

According to the ECMAScript 5.1 Specification's Strict Equal Comparison algorithm, even if the type of Objects being compared is String, all the characters are checked to see if they are equal.

5. If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length and same characters in corresponding positions); otherwise, return false.

Interning is strictly an implementation thingy, to boost performance. The language standard doesn't impose any rules in that regard. So, its up to the implementers of the specification to intern strings or not.

回答3:

First of all, it would be nice to see a JSPerf test which demonstrates the claim that doubling the string size doubles the execution time.

Next, let's take that as granted. Here's my (unproven, unchecked, and probably unrelated to reality) theory.

Compairing two memory addresses is fast, no matter how much data is references. But you have to INTERN this strings first. If you have in your code

var a = "1234";
var b = "1234";

Then the engine first has to understand that these two strings are the same and can point to the same address. So at least once these strings has to be compared fully. So basically here are the following options:

• The engine compares and interns strings directly when parsing the code. In this case equals strings should get the same address.
• The engine may say "these strings are two big, I don't want to intern them" and has two copies.
• The engine may intern these strings later.

In the two latter cases string comparison will influence the test results. In the last case - even if the strings are finally interned.

But as I wrote, a wild theory, for theory's sage. I'd first like to see some JSPerf.