arg

Overview

arg allows referring to other arguments in the header of a producer or view. It makes it possible to treat other keys of the header object as local variables. All of the following are valid uses of arg:

        const result: producer = ({
          a1 = '123',
          a2 = arg.a1,            // Access previously defined argument
          a3 = arg.b1.b2.b3.b4    // Access nested properties of another arg
          a4 = arg.a2[arg.a1],    // Dynamically access argument properties based on other arg
          a5 = arg.a3[prop.foo],  // Dynamically access argument properties based on other Engine operators
        }) => { }

Example

If we have a TodoItem component which accepts a single arg id: string, and global state which looks like:

{
  todosById: {
    todo1: { title: "My first todo" }
  },
  tagsByTitle: {
    "My first todo": ["tag1", "tag2"]
  }
}

It is possible to access tags for the Todo with id, by composing path using arg:

const TodoItem: view = ({
  title: observe.todosById[prop.id],
  tags: observe.tagsByTitle[arg.title]
}) => { ... }

In this example, TodoItem will have access to tags = ["tag1", "tag2"].

arg is also very useful with combined with wildcard