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Typeof

In vanilla JavaScript, the typeof operator returns a string specifying the data type of an expression. Here is a simple example:

/* @flow */
var index: number = 10;
var result: string = typeof index;
// result: 'number'

However, in Flow, typeof can also be used to as a mechanism to capture types in type annotating positions as well.

Use of typeof

Take the following code example:

/* @flow */
class X {}
var a = X; // a infers its type from X
var b: typeof X; // b has the same type as X. It is the same as a

There is no real advantage of using typeof for variable typing in the above case.

However, imagine X has a static function:

/* @flow */
class X {
  static bar(): string {
    return 'Hi';
  }
}
var a: X = new X();
a.bar(); // Type error
var b: typeof X = X;
b.bar(); // Good

typeof allows the capturing of the class X itself, rather than just an instance of X. So constructs like static functions can be called using a variable that captured a class via typeof.

Here is the error that would occur if Flow was used to check the status of the code above:

/tmp/flow/f.js:8:1,7: call of method bar
Property not found in
  /tmp/flow/f.js:2:7,7: X

typeof is very useful for being able to work with the actual object itself, whether that is a class, module or some other construct.

Another Example

/* @flow */
class Foo { }
class Bar { }
// b ends up being a Foo type, since f evaluates to Foo
var b: { f : typeof Foo } = { f : Foo };
// Since the type of b.f is typeof Foo (i.e. Class<Foo>), the following
// assignment is valid because the type of the new instance is Foo:
var inst1: Foo = new b.f();
// However, this fails because the type of the new instance is not Bar:
var inst2: Bar = new b.f();
tmp/flow/f.js:10
 10: var inst2: Bar = new b.f();
                      ^^^^^^^^^ Foo. This type is incompatible with
 10: var inst2: Bar = new b.f();
                ^^^ Bar

Found 1 error

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https://flowtype.org/docs/typeof.html