CSS has a @font-face rule which allows us to "use" custom fonts.
Does JavaScript have this functionality?
More specifically, is it possible to "use" custom fonts without using the CSS @font-face rule?
There is no way to cause the browser to load a font from the network other than with a @font-face rule. You could theoretically create that @font-face rule from JavaScript without having to use document.write (or document.createElement("style") etc), if instead you used the CSS Object Model; but I wouldn't count on that to be implemented in any browser at the moment.