2-4. 컬렉션 중심 프로그래밍 - 명령형으로 map, filter의 Promise 다형성 구현, Promise의 규칙

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1function map(f, coll) {
2  const res = [];
3  const iter = coll[Symbol.iterator]();
4  return function recur() {
5    for(const a of iter) {
6      const b = f(a);
7      if (b instanceof Promise) {
8        return b.then(function(b) {
9          res.push(b);
10          return recur();
11        })
12      } else {
13        res.push(b);
14      }
15    }
16    return res;
17  } ();
18}
19
20function filter(f, coll) {
21  const res = [];
22  const iter = coll[Symbol.iterator]();
23  return function recur() {
24    for(const a of iter) {
25      const b = f(a);
26      if (b instanceof Promise) {
27        return b.then(function(b) {
28          if(b) res.push(a);
29          return recur();
30        })
31      } else {
32        if(b) res.push(a);
33      }
34    }
35    return res;
36  } ();
37}
38
39map(a => Promise.resolve(a + 100), [1,2,3]).then(console.log) // [101,102,103]
40filter(a => Promise.resolve(a > 2), [1,2,3]).then(console.log) // [3]

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1const then2 = (f, a) => a instanceof Promise ? a.then(f) : f(a);
2
3Promise.resolve(10)
4  .then(a => a + 10)
5  .then(a => a + 10)
6  .then(a => a + 10)
7  .then(console.log) // 40
8  
9then2(console.log, Promise.resolve(10)
10  .then(a => a + 10)
11  .then(a => a + 10)
12  .then(a => a + 10)
13  .then(console.log) // 40 
14)
15
16new Promise(function(resolve) {
17  resolve(Promise.resolve(Promise.resolve(Promise.resolve(10))))
18}).then(a => a + 100)
19  .then(console.log) // 110
20
21
22new Promise(function(resolve) {
23  resolve(new Promise(function(resolve) {
24    setTimeout(function() {
25      resolve(10)
26    }, 1000)
27  }))
28}).then(a => a + 1000)
29  .then(console.log) // 1010
30
31
32then2(console.log, new Promise(function(resolve) {
33  resolve(new Promise(function(resolve) {
34    setTimeout(function() {
35      resolve(10)
36    }, 1000)
37  }))
38}).then(a => a + 1000)) // 1010