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Learning TypeScript 2.x - Second Edition
book

Learning TypeScript 2.x - Second Edition

by Remo H. Jansen
April 2018
Beginner content levelBeginner
536 pages
13h 21m
English
Packt Publishing
Content preview from Learning TypeScript 2.x - Second Edition

Custom type guards

We can define custom type guards using by declaring a function with a special return type:

interface Supplier {    orderItems(): void;    getAddress(): void;}interface Customer {    sellItems(): void;    getAddress(): void;}function isSupplier(person: Supplier | Customer): person is Supplier {    return (<Supplier> person).orderItems !== undefined;}function handleItems(person: Supplier | Customer) {    if (isSupplier(person)) {        person.orderItems(); // OK    } else {        person.sellItems(); // OK    }}

The preceding code snippet declares two types (Supplier and Customer); it then declares a custom type guard function. The custom type guard returns a Boolean value. The function returns true when the provided value person has a property named orderItems ...

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Publisher Resources

ISBN: 9781788391474Supplemental Content