Type assertions

The as keyword is used to make a type assertion that a value is of a specified type. For example, consider the example below which asserts that y has a number type.

Clearly, y is not actually holding a number value. It is important to not misuse the as keyword for type casting, such as converting a string to an integer.

TypeScript can detect several situations where an obviously incorrect type assertion is taking place, but it is very easy to misuse this keyword when attempting to resolve compiler errors.

One common use for the as keyword is with the as const syntax, which indicates that a value should be inferred as a literal type rather than a general type.

Another common use of the as keyword is to explicitly tell the TypeScript compiler that an array value is actually a tuple.

You can also use as to represent a value as a less specific type.