使用装饰器函数，休息参数，调用和应用

Question

 let worker = { slow(min, max) { alert(`Called with ${min},${max}`); return min + max; } }; function cachingDecorator(func, hash) { let cache = new Map(); return function() { let key = hash(arguments); //...arguments also works, but only with this name, another no, why? if (cache.has(key)) { return cache.get(key); } let result = func.call(this, ...arguments); cache.set(key, result); return result; }; } function hash(args) { return args[0] + ',' + args[1]; } worker.slow = cachingDecorator(worker.slow, hash); alert( worker.slow(3, 5) ); // works alert( "Again " + worker.slow(3, 5) ); // same (cached)

It's about using decorator function. First call is calculated, and then it's cashed and is taken from cash. I've read, that arguments object it's old way to use rest parameter, and it can be replaced. Then why when I try to replace arguments object in let key = hash(arguments)

return function() {
    let key = hash(arguments); 
    if (cache.has(key)) {
    return cache.get(key);
}

to rest parameter, it doesn't work...

Actually it works, but only if add ...(...arguments) , but it doesn't if change on smth else(I mean arguments), eg arr , ars etc. Why?

Answer 1

For all non-arrow function there is a local variable arguments see here

Without arguments you can get the values by using rest parameters

return function(...myArgs) {
    let key = hash(myArgs)
    ...

Or, if you are passing arguments to hash function after destructuring like

let key = hash(...arguments)

change your hash function as

function hash(hMin, hMax) {
  return hMin + ',' + hMax;
}

Answer 2

'arguments object it's old way to use rest parameter, and it can be replaced'

Actually, they are different. Any function declared with keyword 'function' has an object 'arguments' which can be used inside of function. Even if you have use rest operator inside, it still available as well as your 'rest' array.

function foo (...args) {
console.log(args) // returns an array when foo below is called [1,23,5]
console.log(arguments) // returns an object when foo below is called {"0":1,"1":23,"2":5}
}

foo(1,23,5)

The 'arguments' object is not available when arrow function is used.

Answer 3

You can use your own name instead of the built-in arguments , by using ... in the function definition and the call.

 let worker = { slow(min, max) { alert(`Called with ${min},${max}`); return min + max; } }; function cachingDecorator(func, hash) { let cache = new Map(); return function(...myArgs) { let key = hash(myArgs); if (cache.has(key)) { return cache.get(key); } let result = func.call(this, ...myArgs); cache.set(key, result); return result; }; } function hash(args) { return args[0] + ',' + args[1]; } worker.slow = cachingDecorator(worker.slow, hash); alert(worker.slow(3, 5)); // works alert("Again " + worker.slow(3, 5)); // same (cached)