2nd/10_Wiki/Topic_JavaScript/JavaScript_Promise.md

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

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]

step1(function(r1) {
  step2(r1, function(r2) {
    step3(r2, function(r3) {
      console.log(r3);
    });
  });
});

Promise syntax — resolve case [S1]

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]

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]

let promise = Promise.resolve("OK");

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

Promise.reject() [S1]

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]

// 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]

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]

step1()
.then(function(value) {
  step2(value);
})
.then(function(value) {
  console.log(value);
});

Fetch API with promises [S1]

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]

setTimeout(function() { myFunction("I love You !!!"); }, 3000);

function myFunction(value) {
  document.getElementById("demo").innerHTML = value;
}

Promise version (promisified timer): [S1]

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]

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]

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):

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:

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).