Type conversion
Also called: Type coercion, Casting
Turning a value of one type into another, such as an Int into a Double or the text "42" into a number.
Definition
Some conversions are written explicitly (Double(x) in Swift, x.toDouble() in Kotlin, a cast in Java), and some happen automatically (Java widens int to double; JavaScript turns values into strings or numbers when operators need them). Converting text to a number can fail, which each language reports differently: nil, null, an exception or NaN.
In practice
Int("42") is 42 in Swift, but Int("42 years") is nil, and Number("") is 0 in TypeScript.