Typeof Types

Getting the internal type of a value

JavaScript has a typeof operator which returns a string describing a value.

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typeof 1 === 'number'
typeof true === 'boolean'
typeof 'three' === 'string'

However it is limited in that this string only describes so much about the type.

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typeof { foo: true } === 'object'
typeof { bar: true } === 'object'
typeof [true, false] === 'object'

In Flow, there is a similar typeof operator, but it’s much more powerful.

typeof type syntax

The typeof operator returns the Flow type of a given value to be used as a type.

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// @flow
let num1 = 42;
let num2: typeof num1 = 3.14;     // Works!
// $ExpectError
let num3: typeof num1 = 'world';  // Error!

let bool1 = true;
let bool2: typeof bool1 = false;  // Works!
// $ExpectError
let bool3: typeof bool1 = 42;     // Error!

let str1 = 'hello';
let str2: typeof str1 = 'world'; // Works!
// $ExpectError
let str3: typeof str1 = false;   // Error!
Cannot assign `'world'` to `num3` because string [1] is incompatible with number [2]. Cannot assign `42` to `bool3` because number [1] is incompatible with boolean [2]. Cannot assign `false` to `str3` because boolean [1] is incompatible with string [2].

You can use any value with typeof:

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// @flow
let obj1 = { foo: 1, bar: true, baz: 'three' };
let obj2: typeof obj1 = { foo: 42, bar: false, baz: 'hello' };

let arr1 = [1, 2, 3];
let arr2: typeof arr1 = [3, 2, 1];

typeof inherits behaviors of inference

Flow does all sorts of type inference on your code so that you don’t have to type annotate anything. Generally, inference avoids getting in your way while still preventing you from introducing bugs.

But when you use typeof, you’re taking the results of Flow’s inference and asserting it as a type. While this can be very useful, it can also lead to some unexpected results.

For example, when you use literal values in Flow, their inferred type is the primitive that it belongs to. Thus, the number 42 has the inferred type of number. You can see this when you use typeof.

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// @flow
let num1 = 42;
let num2: typeof num1 = 3.14;    // Works!

let bool1 = true;
let bool2: typeof bool1 = false; // Works!

let str1 = 'hello';
let str2: typeof str1 = 'world'; // Works!

However, this only happens with the inferred type. If you specify the literal type, it will be used in typeof.

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// @flow
let num1: 42 = 42;
// $ExpectError
let num2: typeof num1 = 3.14;    // Error!

let bool1: true = true;
// $ExpectError
let bool2: typeof bool1 = false; // Error!

let str1: 'hello' = 'hello';
// $ExpectError
let str2: typeof str1 = 'world'; // Error!
Cannot assign `3.14` to `num2` because number [1] is incompatible with number literal `42` [2]. Cannot assign `false` to `bool2` because boolean [1] is incompatible with boolean literal `true` [2]. Cannot assign `'world'` to `str2` because string [1] is incompatible with string literal `hello` [2].

typeof inherits behaviors of other types

There are many different types in Flow, some of these types behave differently than others. These differences make sense for that particular type but not for others.

When you use typeof, you’re inserting another type with all of its behaviors. This can make typeof seem inconsistent where it is not.

For example, if you use typeof with a class you need to remember that classes are nominally typed instead of structurally typed. So that two classes with the same exact shape are not considered equivalent.

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// @flow
class MyClass {
  method(val: number) { /* ... */ }
}

class YourClass {
  method(val: number) { /* ... */ }
}

// $ExpectError
let test1: typeof MyClass = YourClass; // Error!
let test2: typeof MyClass = MyClass;   // Works!
Cannot assign `YourClass` to `test1` because `YourClass` [1] is incompatible with `MyClass` [2].
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