It used to be super slow. Like, you cannot imagine how slow it was.
The usual example I trot out is when I was writing a client-side pivot table creator in the mid-2000s, as far as I can remember, with just 100 items the js version took 20-30 seconds. I then tried it using XML/XSLT instead[1] and it was instant.
I haven't checked recently, but even a few years ago javascript is extremely slow with large datasets, I was mucking around with Mandelbrot generators and js compared to C# was like a bicycle vs a jet engine, they weren't even vaguely in the same league performance-wise. Just had a quick look at some js based versions looks like it's got a bit faster, but still slow.
[1] Awesome performance, super hard for others to understand the code. XSLT was great in some ways, but the learning curve was high.
In principle, Javascript on modern JIT can approach roughly 40-80% of the performance of C++, more or less across the board, not just in some toy benchmark. This benchmark is from 2014:
Granted, this benchmark is for generated Javascript, not "idiomatic Javascript" (if there is such a thing). Of course you can write arbitrarily slow Javascript as well. There's a lot of stuff that can throw a wrench into the JIT.
For the Firefox graph, none of them are below 35%, only three are below 40%. The average is well above 60%.
That said, there's a lot going on there. Javascript can not use SIMD or multiple threads. Some tests are heavier on C#, which is converted to C++ in the Javascript version and thus becomes "faster than native C#".
The point is not give an exact number on how fast Javascript is compared to any other language, there's wide variance across usage and implementations. The point is to show that Javascript to rival C# performance is feasible.
The usual example I trot out is when I was writing a client-side pivot table creator in the mid-2000s, as far as I can remember, with just 100 items the js version took 20-30 seconds. I then tried it using XML/XSLT instead[1] and it was instant.
I haven't checked recently, but even a few years ago javascript is extremely slow with large datasets, I was mucking around with Mandelbrot generators and js compared to C# was like a bicycle vs a jet engine, they weren't even vaguely in the same league performance-wise. Just had a quick look at some js based versions looks like it's got a bit faster, but still slow.
[1] Awesome performance, super hard for others to understand the code. XSLT was great in some ways, but the learning curve was high.
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