Is there a built-in javascript function similar to os.path.join?

Use the path module. path.join is exactly what you’re looking for. From the docs:

path.join([path1][, path2][, ...])#
Join all arguments together and normalize the resulting path.

Arguments must be strings. In v0.8, non-string arguments were silently ignored. In v0.10 and up, an exception is thrown.

Example:

const path = require('node:path')
  
path.join('/foo', 'bar', 'baz/asdf', 'quux', '..')
// returns
'/foo/bar/baz/asdf'

path.join('foo', {}, 'bar')
// throws exception
TypeError: Arguments to path.join must be strings

You can also use import path from 'path' instead of const path = require('node:path') if you’re loading modules with that style.

Edit:

I assumed here that you’re using server-side Javascript like node.js. If you want to use it in the browser, you can use path-browserify.