Proxy’s
handler.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();
Hello World
Jane
Doe
These are only a small set of examples of the handler.get() trap. What are
some things you have done with the Proxy object?