Answer

Type Assertion is a way for developers to inform the TypeScript compiler how to interpret the type of a value. Syntax:

let someValue: any = "example";
let strLen: number = (someValue as string).length;

Type Assertion does not convert data at runtime; it merely hints the compiler about the type. If the assertion is incorrect, the program will not stop, which can lead to runtime errors. Unlike "casting" in C# or Java, TypeScript does not perform any conversions.

Direct assertions between unrelated types (e.g., as number as User) are not allowed, but there are workarounds via as unknown.

Trick question

Is it safe to use double assertion value as unknown as Type to cast a value to another type?

Incorrect answer: Yes, it is safe because most TypeScript developers write this way.

Correct answer: No, it is dangerous. This technique bypasses compiler checks and allows any type to be converted to any other, circumventing typing, which can lead to runtime errors. Use it only if you are sure that the value is actually compatible with the target type.

Examples of real errors due to lack of knowledge on the subject

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Story

A developer made a Type Assertion for an object from a third-party API to their own interface (as MyType), without checking the fields. The API changed its structure, and the application started crashing on access to nonexistent keys.

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Story

In a DOM manipulation library, someone made a double assertion el as unknown as HTMLElement, without verifying the type, and later encountered an error when trying to call HTMLElement methods on another object.

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Story

In an attempt to resolve type errors when integrating with JS code, one developer applied as any to the configuration. This masked the problem, and a bug with incorrect data handling surfaced only in production.