2nd/10_Wiki/Topic_JavaScript/JavaScript_Promise.md

---
id: javascript-promises
title: "JavaScript Promises"
category: "Frontend"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["Promise", "Promises", "Promise.resolve", "Promise.reject", "Promise chaining", "then catch"]
duplicate_of: ""
source_trust_level: "B"
confidence_score: 0.89
created_at: 2026-06-23
updated_at: 2026-06-23
review_reason: ""
merge_history: []
tags: ["javascript", "js", "web", "frontend", "w3schools", "promises", "asynchronous"]
raw_sources: ["https://www.w3schools.com/js/js_promise.asp"]
applied_in: []
github_commit: ""
---

# [[JavaScript Promises]]

## 🎯 한 줄 통찰 (One-line insight)
A Promise is an object representing the eventual completion or failure of an asynchronous operation, letting you chain steps with `then()`/`catch()` instead of nesting callbacks. [S1]

## 🧠 핵심 개념 (Core concepts)
- **Promises replace callback nesting** — they were created to make asynchronous JavaScript easier than deeply nested callbacks ("callback hell"). [S1]
- **Three states** — a Promise is pending, fulfilled (resolved), or rejected. [S1]
- **Construct with resolve/reject** — `new Promise(function(resolve, reject) { ... })`; call `resolve(value)` on success or `reject(value)` on failure. [S1]
- **Consume with then/catch** — `then()` takes success (and optional failure) handlers; `catch()` handles rejection across a chain. [S1]
- **Chaining requires return** — each `then()` should `return` the next promise so the chain links correctly; forgetting to return breaks the chain. [S1]
- **Shortcuts** — `Promise.resolve(value)` and `Promise.reject(value)` create already-settled promises. [S1]

## 🧩 추출된 패턴 (Extracted patterns)
- **Resolve/reject wrapper** — wrap slow work in `new Promise`, calling `resolve`/`reject` when done. [S1]
- **then-chain with return** — `step1().then(v => step2(v)).then(v => step3(v))...` runs steps in sequence. [S1]
- **Single catch for the chain** — one `.catch()` at the end handles any failure along the chain. [S1]
- **Promisify a timer or XHR** — wrap `setTimeout` or `XMLHttpRequest` so its completion resolves a promise. [S1]

## 📖 세부 내용 (Details)
**The problem promises solve (callback hell)**
Deeply nested callbacks are hard to read: [S1]
```javascript
step1(function(r1) {
  step2(r1, function(r2) {
    step3(r2, function(r3) {
      console.log(r3);
    });
  });
});
```

**Promise syntax — resolve case**
[S1]
```javascript
let myPromise = new Promise(function(resolve, reject) {
  ok = true;

// Code that may take some time

  if (ok) {
    resolve("OK");
  } else {
    reject("Error");
  }
});

// Using then() to display the result
myPromise.then(
  function(value) {myDisplayer(value);},
  function(value) {myDisplayer(value);}
);
```

**Promise syntax — reject case**
[S1]
```javascript
let myPromise = new Promise(function(resolve, reject) {
  ok = false;

// Code that may take some time

  if (ok) {
    resolve("OK");
  } else {
    reject("Error");
  }
});

// Using then() to display the result
myPromise.then(
  function(value) {myDisplayer(value);},
  function(value) {myDisplayer(value);}
);
```

**Promise.resolve()**
[S1]
```javascript
let promise = Promise.resolve("OK");

promise
.then(function(value) {
  console.log(value);
})
.catch(function(value) {
  myDisplayer(value);
});
```

**Promise.reject()**
[S1]
```javascript
let promise = Promise.reject("Error");

promise
.then(function(value) {
  console.log(value);
})
.catch(function(value) {
  myDisplayer(value);
});
```

**Promise chaining**
Run three functions in steps, each returning a promise: [S1]
```javascript
// Three functions to run in steps
function step1() {
  return Promise.resolve("A");
}
function step2(value) {
  return Promise.resolve(value + "B");
}
function step3(value) {
  return Promise.resolve(value + "C");
}

// Run the three functions in steps
step1()
.then(function(value) {
  return step2(value);
})
.then(function(value) {
  return step3(value);
})
.then(function(value) {
  myDisplayer(value);
});
```

**Error handling with catch()**
A single `catch()` handles failure anywhere in the chain: [S1]
```javascript
step1()
.then(function(value) {
  return step2(value);
})
.then(function(value) {
  return step3(value);
})
.catch(function(error) {
  console.log(error);
});
```

**Common mistake — forgetting to return**
Without `return`, the next `then()` does not receive the resolved value: [S1]
```javascript
step1()
.then(function(value) {
  step2(value);
})
.then(function(value) {
  console.log(value);
});
```

**Fetch API with promises**
[S1]
```javascript
fetch("data.json")
.then(function(response) {
  return response.json();
})
.then(function(data) {
  console.log(data);
})
.catch(function(error) {
  console.log(error);
});
```

**Timeout with a callback vs a promise**
Callback version: [S1]
```javascript
setTimeout(function() { myFunction("I love You !!!"); }, 3000);

function myFunction(value) {
  document.getElementById("demo").innerHTML = value;
}
```
Promise version (promisified timer): [S1]
```javascript
let myPromise = new Promise(function(myResolve, myReject) {
  setTimeout(function() { myResolve("I love You !!"); }, 3000);
});

myPromise.then(function(value) {
  document.getElementById("demo").innerHTML = value;
});
```

**File loading — callback vs promise**
Callback version using XMLHttpRequest: [S1]
```javascript
function getFile(myCallback) {
  let req = new XMLHttpRequest();
  req.open('GET', "mycar.html");
  req.onload = function() {
    if (req.status == 200) {
      myCallback(req.responseText);
    } else {
      myCallback("Error: " + req.status);
    }
  }
  req.send();
}

getFile(myDisplayer);
```
Promise version: [S1]
```javascript
let myPromise = new Promise(function(myResolve, myReject) {
  let req = new XMLHttpRequest();
  req.open('GET', "mycar.html");
  req.onload = function() {
    if (req.status == 200) {
      myResolve(req.response);
    } else {
      myReject("File not Found");
    }
  };
  req.send();
});

myPromise.then(
  function(value) {myDisplayer(value);},
  function(error) {myDisplayer(error);}
);
```

## 🛠️ 적용 사례 (Applied in summary)
The page's snippets — the `step1/step2/step3` chain, the promisified `setTimeout`, and the promisified `XMLHttpRequest` file loader — are the canonical applied examples. No external project/commit applications found in the source.

## 💻 코드 패턴 (Code patterns)
Construct and consume a promise (language: JavaScript):
```javascript
let myPromise = new Promise(function(resolve, reject) {
  if (ok) { resolve("OK"); } else { reject("Error"); }
});

myPromise.then(
  function(value) {myDisplayer(value);},
  function(value) {myDisplayer(value);}
);
```
Chain steps with a single catch:
```javascript
step1()
.then(function(value) { return step2(value); })
.then(function(value) { return step3(value); })
.catch(function(error) { console.log(error); });
```

## ⚖️ 비교 및 선택 기준 (Comparison & decision criteria)
- **Callbacks** — the original approach; deeply nested callbacks become "callback hell," hard to read. [S1]
- **Promises** — created to make async JavaScript easier; chainable with `then()` and a single `catch()`, but require remembering to `return` each step. [S1]

## ⚖️ 모순 및 업데이트 (Contradictions & updates)
No contradictions found in the source.

## ✅ 검증 상태 및 신뢰도
- **상태:** draft
- **검증 단계:** conceptual (실제 적용 사례 발견 시 applied/validated로 승격 가능)
- **출처 신뢰도:** B (W3Schools — widely used educational reference, not a primary standards body)
- **신뢰 점수:** 0.89
- **중복 검사 결과:** 신규 생성 (New discovery)

## 🔗 지식 그래프 (Knowledge Graph)
- **상위/루트:** [[JavaScript Tutorial]]
- **관련 개념:** [[JavaScript Async Callbacks]], [[JavaScript Async Await]], [[JavaScript Async Fetch]], [[JavaScript Async Timeouts]]
- **참조 맥락:** The cleaner successor to callbacks and the foundation that async/await is built on.

## 📚 출처 (Sources)
- [S1] W3Schools — JavaScript Promises — https://www.w3schools.com/js/js_promise.asp

## 📝 변경 이력 (Change history)
- 2026-06-23: Initial draft synthesized from the W3Schools "JavaScript Promises" page (Astra wiki-curation, P-Reinforce v3.1 format).