# User-defined Type Guards

# Type guarding functions

You can declare functions that serve as type guards using any logic you'd like.

They take the form:

function functionName(variableName: any): variableName is DesiredType {
    // body that returns boolean
}

If the function returns true, TypeScript will narrow the type to DesiredType in any block guarded by a call to the function.

For example (try it):

function isString(test: any): test is string {
    return typeof test === "string";
}

function example(foo: any) {
    if (isString(foo)) {
        // foo is type as a string in this block
        console.log("it's a string: " + foo);
    } else {
        // foo is type any in this block
        console.log("don't know what this is! [" + foo + "]");
    }
}

example("hello world");          // prints "it's a string: hello world"
example({ something: "else" });  // prints "don't know what this is! [[object Object]]"

A guard's function type predicate (the foo is Bar in the function return type position) is used at compile time to narrow types, the function body is used at runtime. The type predicate and function must agree, or your code won't work.

Type guard functions don't have to use typeof or instanceof, they can use more complicated logic.

For example, this code determines if you've got a jQuery object by checking for it's version string.

function isJQuery(foo): foo is JQuery {
    // test for jQuery's version string
    return foo.jquery !== undefined;
}

function example(foo) {
    if (isJQuery(foo)) {
        // foo is typed JQuery here
        foo.eq(0);
    }
}

# Using instanceof

instanceof requires that the variable is of type any.

This code (try it):

class Pet { }
class Dog extends Pet {
    bark() {
        console.log("woof");
    }
}
class Cat extends Pet {
    purr() {
        console.log("meow");
    }
}

function example(foo: any) {
    if (foo instanceof Dog) {
        // foo is type Dog in this block
        foo.bark();
    }

    if (foo instanceof Cat) {
        // foo is type Cat in this block
        foo.purr();
    }
}

example(new Dog());
example(new Cat());

prints

woof
meom

to the console.

# Using typeof

typeof is used when you need to distinguish between types number, string, boolean, and symbol. Other string constants will not error, but won't be used to narrow types either.

Unlike instanceof, typeof will work with a variable of any type. In the example below, foo could be typed as number | string without issue.

This code (try it):

function example(foo: any) {
    if (typeof foo === "number") {
        // foo is type number in this block
        console.log(foo + 100);
    }

    if (typeof foo === "string") {
        // fooi is type string in this block
        console.log("not a number: " + foo);
    }
}

example(23);
example("foo");

prints

123
not a number: foo

# Syntax

  • typeof x === "type name"
  • x instanceof TypeName
  • function(foo: any): foo is TypeName { /* code returning boolean */ }

# Remarks

Using type annotations in TypeScript constrains the possible types your code will need to deal with, but it is still common to need to take different code paths based on the runtime type of a variable.

Type guards let you write code that discriminates based on the runtime type of a variable, while remaining strongly typed and avoiding casts (also known as type assertions).