The in operator now acts as a narrowing expression for types.
For a n in x expression, where n is a string literal or string literal type and x is a union type, the “true” branch narrows to types which have an optional or required property n, and the “false” branch narrows to types which have an optional or missing property n.
function move(pet: Fish | Bird) {
if ("swim" in pet) {
return pet.swim();
}
return pet.fly();
}
Hi, thanks for this awesome new handbook, I like the new way that you're presenting TypeScript here!
Type guards with the
inoperator are not mentioned in the Narrowing section.They are mentioned in the current handbook on the Advanced Types page under the Using the in Operator section:
Using the in operator
The in operator now acts as a narrowing expression for types.
For a
n in xexpression, wherenis a string literal or string literal type andxis a union type, the “true” branch narrows to types which have an optional or required propertyn, and the “false” branch narrows to types which have an optional or missing propertyn.Potentially related to #9 by @gcrev93