> For the complete documentation index, see [llms.txt](https://docs.superstructjs.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.superstructjs.org/api-reference/core.md). # Core ### `assert` `assert(value: unknown, struct: Struct, message?: string) => asserts value is T` ```ts assert(value, User, 'The user is invalid!') ``` Assert that `value` is valid according to a `struct`. If the value is invalid a [`StructError`](/api-reference/errors.md#structerror) will be thrown (the optional `message` parameter allows you to override error's message). > 🤖 When using TypeScript `assert` acts as an [assertion function](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#assertion-functions), so you can ensure that after calling it the type of the `value` matches the shape of the struct. ### `create` `create(value: unknown, struct: Struct, message?: string) => T` ```ts const user = create(value, User, 'Unable to create a user!') ``` Create a `value` using the coercion logic that is built-in to the struct, returning the newly coerced value. If the value is invalid a [`StructError`](/api-reference/errors.md#structerror) will be thrown (the optional `message` parameter allows you to override error's message). > 🤖 If you want coercion logic like defaulted values, you **must** call this helper before running validation. ### `is` `is(value: unknown, struct: Struct) => value is T` ```ts if (is(value, User)) { // ... } ``` Test that `value` is valid, returning a boolean representing whether it is valid or not. > 🤖 When using TypeScript `is` acts as a type guard, so you can use it in an `if` statement to ensure that inside the statement the `value` matches the shape of the struct. ### `mask` `mask(value: unknown, struct: Struct, message?: string) => T` ```ts const user = mask(value, User, 'The value is incompatible with type User!') ``` Mask a `value`, returning a new value containing only properties defined by a `struct`. Conceptually this is similar to `create`, except that extra properties are omitted from the newly created value instead of throwing a [`StructError`](/api-reference/errors.md#structerror). If an error is thrown anyway, the optional `message` parameter allows you to override its message. Note that when `mask` is used with `type` — given that `type` signals to the core that an object might have arbitrary additional properties — unknown properties will be retained in the returned value. > 🤖 Just like `create`, `mask` includes coercion logic and works recursively. ### `validate` `validate(value: unknown, struct: Struct, options: Object) => [StructError, T]` ```ts const [err, user] = validate(value, User) ``` Validate `value`, returning a result tuple. If the value is invalid the first element will be a [`StructError`](/api-reference/errors.md#structerror). Otherwise, the first element will be `undefined` and the second element will be a value that is guaranteed to match the struct. You can pass `{ coerce: true }` as the third argument to enable coercion of the input value. As well as pass `{ message: 'Your custom error message' }` to override the message of the [`StructError`](/api-reference/errors.md#structerror).