A stable, composable query mechnism for JavaScript iterators and generators.
Generation lets you describe relationships between any kind of iterable data, without actually doing any interation until you need it.
All query functions can take any object that implements the JavaScript
iteration protocol, or any no-arg function that returns an object
that implements the iteration protocol such as a
GeneratorFunction.
Quick example:
import { map, filter, union, query } from 'generation';
let todos = [
{ title: 'get milk', isCompleted: false, isInProgress: true },
{ title: 'get cookies', isCompleted: false, isInProgress: false },
{ title: 'eat snack', isCompleted: false, isInprogress: false }
]
let activeTodos = filter(todos, todo => !todo.isCompleted);
let inProgressTodos = filter(todos, todo => todo.isInProgress);
let completedTodos = filter(todos, todo => todo.isCompleted)All we've done here is define the queries, but no filtering actually happens at this point until we actually enumerate the result:
for (let todo of inProgressTodos) {
console.log(todo);
}
console.log(inProgressTodos.length)
//=> { title: 'get milk', isCompleted: false, isInProgress: true }Queries can be combined freely, for example if an interface is selecting just the completed and in-progress items:
let visible = union(inProgressTodos, completedTodos);
let visibleComponents = map(visible, todo => <Todo todo={todo}/>)All queries are immutable and stable. What this means is that no matter how many times you iterate over the same query object, you will always get the same result, even if the generator function from which it was originally derived would yield a different set of objects given a second go-round. This is especcially desirable behavior in the context of a continually re-rendering user interface because it equates object identity with query content. In other words, if a query object has not changed, then there is no need to re-render it.
In order to determine if a query has in fact changed, every query has
a recompute method which will return a new query if it has, but
return the old query if it hasn't.
activeTodos.recompute() === activeTodos //=> true
todos[0].isCompleted = true;
activeTodos.recompute() === activeTodos //=> false