Is the Javascript String length constant time?

Question

I'm fairly new to JS, and realize length is considered a property. But I received a comment to not use str.length in a loop:

for (i=0; i<str.length; i++){...}

vs

var len = str.length;
for (i=0; i<len; i++){...}

Now, I know str.length() is constant time operation in Java because length is stored as a field in String class. But then again, strings are immutable in Java. I am not sure about JS strings though. Is str.length guaranteed constant time in JS too? Couldn't find this discussed anywhere in the web.

Answer 1

Strings are also immutable in JavaScript. The length property does not need to be computed each time it is accessed.

I created a jsperf benchmark for you to view here.

You'll notice the speeds are the same.

Answer 2

Is str.length guaranteed constant time in JS too?

No, in fact there are no runtime performance or complexity guarantees in JavaScript whatsoever.

However, yes, it can be expected to be accessible in constant time, with no dynamic linear time length computation on access. The ECMAScript spec also describes the String .length property as immutable and that it is initialised when the string is constructed.