Description
Use literal types to narrow down the object type in a conditional block
Syntax
type TriState = 'yes' | 'no' | 'unknown';
function logOutState(state:TriState) {
if (state == 'yes') {
console.log('User selected yes');
} else if (state == 'no') {
console.log('User selected no');
} else {
console.log('User has not made a selection yet');
}
}type Foo = {
kind: 'foo', // Literal type
foo: number
}
type Bar = {
kind: 'bar', // Literal type
bar: number
}
function doStuff(arg: Foo | Bar) {
if (arg.kind === 'foo') {
console.log(arg.foo); // OK
console.log(arg.bar); // Error!
}
else { // MUST BE Bar!
console.log(arg.foo); // Error!
console.log(arg.bar); // OK
}
}