About Android Fonts

Android does not offer functions to enumerate the pre-installed fonts and the official names are aliased to real font names, so the information provided by the rnTextSize fontFamilyNames function is hardcoded and limited to a subset of fonts that you will (probably) find already installed.

For example, in devices with API 21 or above, the system fonts are:

Generic Name	Font File (.ttf)	Weight	Italic
sans-serif-thin	Roboto-Regular (aliased) Roboto-ThinItalic	100	yes
sans-serif-light	Roboto-Light Roboto-LightItalic	300	yes
sans-serif	Roboto-Regular Roboto-RegularItalic	400	yes
	Roboto-Bold Roboto-BoldItalic	700*	yes
sans-serif-medium	Roboto-Medium Roboto-MediumItalic	500	yes
sans-serif-black	Roboto-Black Roboto-BlackItalic	900	yes
sans-serif-condensed-light	RobotoCondensed-Light RobotoCondensed-LightItalic	300	yes
sans-serif-condensed	RobotoCondensed-Regular RobotoCondensed-Italic	400	yes
	RobotoCondensed-Bold RobotoCondensed-BoldItalic	700*	yes
sans-serif-smallcaps	CarroisGothicSC-Regular	400	--
serif	NotoSerif-Regular NotoSerif-RegularItalic	400	yes
	NotoSerif-Bold NotoSerif-BoldItalic	700*	yes
monospace	DroidSansMono	400	--
serif-monospace	CutiveMono	400	--
casual	ComingSoon	400	--
cursive	DancingScript-Regular	400	--
	DancingScript-Bold	700*	--

* There is not *-bold generic names of this fonts, to obtain the weight 700 of them, you must use fontWeight: 'bold'.

Up to SDK 27, Android did not support setting different weights other than 'normal' and 'bold' by code. RN 0.57, the last version released on this date, does not support it either and converts weights '500'-'900' to 'bold'.

So, for example, if you want "Roboto Medium Italic" (weight 500), you have only two choices:

// recommended
{
  fontFamily: 'sans-serif-medium',
  fontStyle: 'italic'
}
// by filename ...looks more like an iOS font face :)
{
  fontFamily: 'Roboto-MediumItalic'
}

But for "Roboto Bold Italic", you can use any of this forms:

// recommended
{
  fontWeight: 'bold',
  fontStyle: 'italic'
}
// generic name, specific weight & style
{
  fontFamily: 'sans-serif',
  fontWeight: 'bold',
  fontStyle: 'italic'
}
// by filename, with embedded weight & style
{
  fontFamily: 'Roboto-BoldItalic'
}

Also, be careful not to overwrite the desired weight with fontWeight, as here:

{
  fontFamily: 'sans-serif-medium',
  fontStyle: 'bold'     // bye bye weight 500
}

Some of the predefined, alternative names:

• sans-serif: arial, helvetica, tahoma, verdana.
• serif: times, times new roman, palatino, georgia, baskerville, goudy, fantasy, ITC Stone Serif
• monospace: sans-serif-monospace, monaco
• serif-monospace: courier, courier new

If you are curious, se the fonts.xml or system_fonts.xml file in the ~/Android/Sdk/platforms/android-[sdk-num]/data/fonts directory.