JS โฉ object โฉ built-in โฉ Proxy โฉ example โฉ identity (proxy)
an (proxy) object that appears to have an infinite number of read-only properties, where each property value is its key, that is, identity[key] === key
Copy // identity (Proxy)
let identity = new Proxy ({} , {
// ------------------------------------
// handler methods, triggered when ...
// ------------------------------------
// accessing `identity[key]`
get (o , key
,
receiver) {
return
key; }
,
// identity[key] === key
// testing if `key in identity`
has (o , key) { return true ; } , // always true
// iterating through own keys of `identity`
ownKeys (o) {
throw new RangeError ( "Infinite number of properties" );
} ,
// accessing property descriptor of `identity[key]`
getOwnPropertyDescriptor (o , key) {
return {
value : key ,
enumerable : false , // non-writable/non-configurable/non-enumerable
writable : false ,
configurable : false
};
} ,
// setting `identity[key]`
set (o , key , value , receiver) { return false } , // cannot set
// deleting `identity[key]`
deleteProperty (o , key) { return false } , // cannot delete
// (re)defining properties of `identity`
defineProperty (o , key , desc) { return false } , // cannot (re)define
// testing if `identity` is extensible
isExtensible (o) { return false } , // always false
// getting prototype of `identity`
getPrototypeOf (o) { return null } , // no prototype
// setting prototype of `identity`
setPrototypeOf (o , proto) { return false } , // cannot set
});
// test identity
identity .x , // 'x'
identity .toString , // 'toString'
identity[ 0 ] , // '0'
delete identity .x , // โ [TypeError] 'deleteProperty' on proxy: trap returned falsish for property 'x'
identity .x = 1 , // โ [TypeError] 'set' on proxy: trap returned falsish for property 'x'
Object .keys (identity) , // โ [RangeError] Infinite number of properties
for ( let p of identity); , // โ [TypeError] identity is not iterable