Map Watch

A javascript map object that can be watched for changes.

NB: Changes are only watched via the map set()/delete() methods. Deep watching of object properties is not good practice but can be achieved through other modules.

Install

npm install map-watch

To save to your package.json

npm install --save map-watch

Creating a Map

WatchMap objects are extensions of the native Map class in Javascript. You can use them in exactly the same way.

const WatchMap = require('map-watch');
 
let map = new WatchMap();
 
map.set('mykey', 'myvalue');
 
console.log(map.get('mykey')) // logs 'myvalue' to the console.

Watching items

To watch an item, simply use the watch() method.

map.watch('mykey', (value, oldValue)={
    console.log(value, oldValue);
});
 
map.set('mykey', 1); // The console will log: 1 undefined
map.set('mykey', 2); // The console will log: 1 2

Watching once

To watch only the next change (like once() on events).

map.once('mykey', (value, oldValue)={
    console.log(value, oldValue);
});
 
map.set('mykey', 1); // The console will log: 1 undefined
map.set('mykey', 2); // Nothing happens

Always fire

If you need to fire a watch when the value is set or changes and fire straight away if already set then you can set the always property of the options object.

map.set('mykey', 1); // Nothing happens yet as no watchers set
 
map.once('mykey', (value, oldValue)={
    console.log(value, oldValue); // Never fired as value set before watcher.
});
 
map.once('mykey', (value, oldValue)={
    console.log(value, oldValue); // Console logs: 1 undefined.
}, {always: true});
 

Options Object

You can supply and options object to watch() and once() as defined above.

• always boolean - Always fire watcher if valuealready set.
• context Object - Context for callbacks (the this applied).

Count Watchers

You can also count the watchers set on a key by calling countWatchers().

map.watch('mykey', (value, oldValue)={
    // Do something
});
 
map.watch('mykey', (value, oldValue)={
    // Do something
});
 
map.watch('mykey', (value, oldValue)={
    // Do something
});
 
console.log(map.countWatchers('myKey')) // Logs 3.