JavaScript has three very commonly used primitives: string, number, and boolean. Each has a corresponding type in TypeScript. As you might expect, these are the same names you'd see if you used the JavaScript typeof operator on a value of those types:
string
represents string values like"Hello, world"
number
is for numbers like42
. JavaScript does not have a special runtime value for integers, so there's no equivalent toint
orfloat
- everything is simplynumber
boolean
is for the two valuestrue
andfalse
The type names
String
,Number
, andBoolean
(starting with capital letters) are legal, but refer to some special built-in types that will very rarely appear in your code. Always usestring
,number
, orboolean
for types.
— The TypeScript Handbook: Everyday Types