A simple filebased eventsourced store
With Eventstore, a model is created using javascript. This document lists some “how-to” patterns to help create a model from events.
Arrays in Javascript are un-keyed, indexed collections. They are dynamically sized, so you can add or remove elements.
Other un-keyed collection types in Javascript include Set and WeakSet.
Set model to be an empty array like this:
let model = [];
When you want to keep all occurrences of items, and store them in an unkeyed collection (ie. array), just push or shift them onto the array.
model.push(eventdata.item); // adds it at the end of the array
model.shift(eventdata.item); // adds it at the beginning of the array
You have a few choices to find an item or an index in an array.
For finding the position (or index), the easiest ways are to use let index = model.indexOf(item) or let index = model.findIndex(compareCallback).
If you need to retrieve the actual element in an array, use let itemFromArray = model.find(compareCallback).
You’ll see these same approaches used in the patterns below.
When you want to keep only the first occurrence of an item, (ie. ignoring repeats), you’ll need to find if the item is an element of the the array. If it doesn’t, you can add the item.
let index = model.indexOf(eventdata.item); // when item is a primitive type you can use .indexOf to find the index of an element
if(index === -1) {
model.push(eventdata.item);
}
let index = model.findIndex(item => item.serialnumber = model.serialnumber); // when item is a complex type, you're in charge of writing the comparer to use with .findIndex.
if(index === -1) {
model.push(item);
}
An alternative approach would be to use a Set instead of an array, since sets won’t allow holding duplicate values.
When you want to remove an occurrence of an item from the array, you’ll need to find the items position (index) in the array. Use the position (index) and remove the element at that position.
let index = model.indexOf(item); // when item is a primitive type you can use .indexOf to find the index of an element
if(index !== -1) {
model.splice(index, 1, ...model.slice(ix + 1));
}
let index = array.findIndex(item => item.serialnumber = model.serialnumber); // when item is a complex, you're in charge of writing the comparer to use with .findIndex.
if(index !== -1) {
model.splice(index, 1, ...model.slice(ix + 1));
}
Objects in Javascript are keyed collections or dictionaries. That is a collections of elements with an associated string key. You add elements at a given key, and retrieve them with the same key.
Other dictionary types in Javascript are Map and WeakMap.
Set model to be an empty object like this:
let model = {};
When you want to keep all occurrences of items, and store them in a keyed collection (ie. object), just set the property with the key as name.
model.keyname and model["keyname"] are equivalent, but the latter allows to use a variable or an expression to return the keyname.
// create the key with a name from the item
model[eventdata.keycomponent] = eventdata.item;
// run a function to create the key name
const fullname = item => `${item.firstname} ${item.lastname}`;
model[fullname(eventdata)] = eventdata.item;
When you want to keep only the first occurrence of an keyed item, (ie. ignoring repeats), you’ll need to find if the item’s key is a property of the the object. If it isn’t, you can add the item as the desired property.
if(model[eventdata.keycomponent] === undefined){
model[eventdata.keycomponent] = eventdata.item;
}
When you want to keep the latest occurrence of a keyed item, you should set the property to the item every time the item occurs. (In the same way as when first adding the key. See “add or replace a property on an object” above.)
When you want to remove an keyed item from the object, you can just delete that property.
delete model[eventdata.keycomponent];
//TODO: ???