Typescript type assertion

Author: Dawid Adach


During development of the application, it might happen that you will have to deal with a situation where you will know more about the value than TypeScrip does. Most often in a situation, where you know that type of certain entity could be more specific than it is now.

Type assertion is a way to explicitly inform the compiler that you want to treat the entity as it would have a different type. This allows you to treat any as a number, or number as a string. Let's have a look at following example:

let someValue: any = "I am sure I am a string";

Although, we are sure that someValue contains a string, for the compiler it still has a type of any. As result, you won't be able to use methods like length() which is a default method for a string type variable. What you can do is to inform compile to treat as a string. You can do it in two ways:

let strLength: number = (someValue).length;

Or using the as syntax:

let strLength: number = (someValue as string).length;

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About author

Dawid Adach
For more than 5 years Dawid was working as an IT Consultant specializing in SOA/EAI/ESB for banking domain. He was gaining experience working in countries like Netherlands, Belgium, Poland and India developing enterprise-class systems for the biggest companies within a domain. Since 2 years as a co-founder of mdbootstrap.com & brandflow.net is using and teaching others technologies like Angular, TypeScript, PHP, AJAX, Mongo, SQL, Hadoop Stack, Virtualization, Automation and many others...