Posted on Jul 23, 2019 • Originally published at blog.neverendingqs.com on Jul 23, 2019

Using Proxy’s get() Trap for Easy Defaults and Even method_missing!

Proxy’shandler.get() trap is very powerful. Here are some examples of what it can do.

Counter

Need to keep a counter of something? The following example shows how many of each animal appears in a list.

const animals = [ 'dog', 'cat', 'mouse', 'cat', 'dog', 'dog', 'mouse', 'cat', 'cat', 'dog', 'mouse', 'cat', 'mouse', 'dog', 'dog' ]; const counter = new Proxy({}, { get: (obj, property) => obj[property] || 0 }); animals.forEach(animal => counter[animal] += 1); console.log(JSON.stringify(counter, null, 2));

{ "dog": 6, "cat": 5, "mouse": 4 }

Arrays

Or how about grouping things together in an object of arrays?

const data = { posts: [ { name: 'post1', tags: ['tag1', 'tag2', 'tag3'] }, { name: 'post2', tags: ['tag2', 'tag5', 'tag7'] }, { name: 'post3', tags: ['tag3', 'tag6', 'tag7'] }, { name: 'post3', tags: ['tag2', 'tag3', 'tag6'] }, { name: 'post3', tags: ['tag1', 'tag4', 'tag5'] } ] }; const postsByTag = new Proxy({}, { get: (obj, property) => obj[property] || [] }); data.posts.forEach(post => post.tags.forEach(tag => postsByTag[tag] = postsByTag[tag].concat(post.name) ) ); console.log(JSON.stringify(postsByTag, null, 2));

{ "tag1": [ "post1", "post3" ], "tag2": [ "post1", "post2", "post3" ], "tag3": [ "post1", "post3", "post3" ], "tag5": [ "post2", "post3" ], "tag7": [ "post2", "post3" ], "tag6": [ "post3", "post3" ], "tag4": [ "post3" ] }

Ruby’s `method_missing`

You can even imitate Ruby’s method_missing by making decisions on the fly!

const greeter = new Proxy({}, { get: (obj, property) => { if(property.startsWith('print')) { const toPrint = property.split('_').slice(1); return () => toPrint.forEach(p => console.log(obj[p])); } else { return obj[property]; } } }); greeter.firstName = 'Jane'; greeter.lastName = 'Doe'; greeter.helloWorld = () => console.log('Hello World'); greeter.helloWorld(); greeter.print_firstName_lastName();