TypeScript has two corresponding types by the same names. JavaScript has two primitive values used to signal absent or uninitialized value: null and undefined. The as const suffix acts like const but for the type system, ensuring that all properties are assigned the literal type instead of a more general version like string or number. In this situation, you can use a type assertion to specify a more specific type: Sometimes you will have information about the type of a value that TypeScript can’t know about.įor example, if you’re using document.getElementById, TypeScript only knows that this will return some kind of HTMLElement, but you might know that your page will always have an HTMLCanvasElement with a given ID. If you would like a heuristic, use interface until you need to use features from type. Interface names will always appear in their original form in error messages, but only when they are used by name.įor the most part, you can choose based on personal preference, and TypeScript will tell you if it needs something to be the other kind of declaration.Interfaces may only be used to declare the shapes of objects, not rename primitives.Type aliases may not participate in declaration merging, but interfaces can.Interfaces will always be named in error messages. Prior to TypeScript version 4.2, type alias names may appear in error messages, sometimes in place of the equivalent anonymous type (which may or may not be desirable).You’ll learn more about these concepts in later chapters, so don’t worry if you don’t understand all of these right away. } // Error: Duplicate identifier 'Window'. When a value is of type any, you can access any properties of it (which will in turn be of type any), call it like a function, assign it to (or from) a value of any type, or pretty much anything else that’s syntactically legal:Īdding new fields to an existing interfaceĪ type cannot be changed after being created TypeScript also has a special type, any, that you can use whenever you don’t want a particular value to cause typechecking errors. Note that is a different thing refer to the section on Tuples. We’ll learn more about the syntax T when we cover generics. You may also see this written as Array, which means the same thing. string is an array of strings, and so on). To specify the type of an array like, you can use the syntax number this syntax works for any type (e.g. Always use string, number, or boolean for types. The type names String, Number, and Boolean (starting with capital letters) are legal, but refer to some special built-in types that will very rarely appear in your code.
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