Understanding what is created with each declaration will help you understand what is merged when you perform a declaration merge. Lastly, value-creating declarations create values that are visible in the output JavaScript. Type-creating declarations do just that: they create a type that is visible with the declared shape and bound to the given name. Namespace-creating declarations create a namespace, which contains names that are accessed using a dotted notation. In TypeScript, a declaration creates entities in at least one of three groups: namespace, type, or value. This merged definition has the features of both of the original declarations.Īny number of declarations can be merged it’s not limited to just two declarations. It also opens the door to more advanced abstraction concepts.įor the purposes of this article, “declaration merging” means that the compiler merges two separate declarations declared with the same name into a single definition. Understanding this concept will give you an advantage when working with existing JavaScript. One example that is especially unique to TypeScript is the concept of ‘declaration merging’. Some of the unique concepts in TypeScript describe the shape of JavaScript objects at the type level.
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