"With careful construction, an attacker can make the processor speculate based on some value of interest and use the cache changes to disclose what that speculated value actually was. This becomes particularly threatening in applications such as Web browsers: a malicious JavaScript can use data revealed in this way to learn about the memory layout of the process it's running in, then use this information to leverage other security flaws to execute arbitrary code. Browser developers have assumed that they can construct safe sandboxes within the browser process, such that scripts can't learn about the memory layout of their containing process. Architecturally, those assumptions are sound. But reality has Spectre, and it blows those assumptions out of the water."
So this is yet another reason to avoid Javascript.
So this is yet another reason to avoid Javascript.