TypeScript Decorators

With the introduction of Classes in TypeScript and ES6, there now exist certain scenarios that require additional features to support annotating or modifying classes and class members. Decorators provide a way to add both annotations and a meta-programming syntax for class declarations and members. Decorators are a stage 2 proposal for JavaScript and are available as an experimental feature of TypeScript.

A Decorator is a special kind of declaration that can be attached to a class declaration, method, accessor, property, or parameter. Decorators use the form @expression, where expression must evaluate to a function that will be called at runtime with information about the decorated declaration.

For example, given the decorator @sealed we might write the sealed function as follows:

function sealed(target) {
// do something with ‘target’ …
}

Decorator Factories

If we want to customize how a decorator is applied to a declaration, we can write a decorator factory. A Decorator Factory is simply a function that returns the expression that will be called by the decorator at runtime.

We can write a decorator factory in the following fashion:

function color(value: string) { // this is the decorator factory
return function (target) { // this is the decorator
// do something with ‘target’ and ‘value’…
}
}

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