Does coding language matter in interviews?

From most important to least important, here are the criteria I'd consider wrt to language choice:

• Do you know the language well?
• Is the language concise and easy to use?
• Can you find easy prep materials for that language?
• Does the company use the language?

The above criteria is true for almost all Big Tech jobs. Interviewers here are trained to be language agnostic since the company uses dozens of technologies. (If you interview at a consulting shop, they'll likely be much more strict about what they want. These companies also pay significantly less.)

If you know JavaScript the best, I'd recommend sticking with it. You want to be able to solve problems, but also be able to talk about the material with enough depth that you goes beyond the surface-level syntax of the language (this becomes especially important for more senior roles).

I definitely would not recommend C++ since it is more verbose and you may get tripped up on the language specifics as already happened to you. Python is a good reasonable choice, but I'd only pick it if you have ample time to practice with it, e.g. minimum 3 months. I'd still stick with JS unless you want to learn Python anyway, since translating Python code you find online into JavaScript shouldn't be that hard.

We touch on this in more detail here masterclass about DSA interviewing so I'd check that out! Also check out Daniel Tomko's advice about interview prep here.

At the end of the day, anything can potentially matter when it comes to interviews as they're inherently random - There's no standard among the industry for how they should be conducted. That being said, I don't think you should worry too much about it, especially given that you're working with JavaScript, one of the most concise and widely used languages. Python is actually also a great interview language for the same reasons.

Going into the issue further, here's the breakdown I've seen:

Situations That Care

• Early-stage startups that don't have the resources to thoroughly train people
• Specialist roles where you're going to be working on a very specific and niche tech stack (e.g. Android video infra)
• Interviewers that are lazy and don't want to spend mental bandwidth translating code they aren't familiar with (unfortunately there are a lot of these)

Situations That Don't Care

• Big Tech - I was explicitly trained at Meta to be language agnostic when it came to interviews. These companies literally have teams working on every possible language, so it's not reasonable to pigeonhole candidates. Also, many software engineers here add most of their value outside of the code, which I cover in this discussion here: "What are the Core skills a SDE should be consistently developing?"
• Companies that are more chill and give employees more time to ramp up
• Full-stack roles where it's less reasonable to expect someone to match both your front-end and back-end stack

If you're more interested in what the other side looks like, we actually gave a masterclass about it (heavily inspired by my experience back at Meta). You can watch it here: [Masterclass] How To Be An Amazing Interviewer In Tech

I'd say if you have a domain specific interviews like for Android/iOS or FrontEnd the choice of your language does play a role. For Android the interviewers would expect you to write code in Java and Kotlin. When I interviewed at Expedia I was asked to code in Kotlin as that's the official language for Android development.