docs(10_Wiki): W3Schools 위키화 — HTML/CSS/JavaScript(core)
W3Schools 튜토리얼을 P-Reinforce v3.1 포맷으로 위키화(영어 본문, 한/영 섹션 헤더). - Topic_HTML: 59문서 (튜토리얼+예제, 레퍼런스/메타 제외) - Topic_CSS: 190문서 (메인 + Advanced/Flexbox/Grid/RWD 전체) - Topic_JavaScript: 120문서 (코어 언어; Temporal/DOM상세/BOM/WebAPI/AJAX/jQuery/Graphics 등은 후속) 각 폴더 00_INDEX.md(MOC) 포함. 코드 verbatim, 미확인분은 "Not found in source" 표기. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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---
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id: javascript-promises
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title: "JavaScript Promises"
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category: "Frontend"
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status: "draft"
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verification_status: "conceptual"
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canonical_id: ""
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aliases: ["Promise", "Promises", "Promise.resolve", "Promise.reject", "Promise chaining", "then catch"]
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duplicate_of: ""
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source_trust_level: "B"
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confidence_score: 0.89
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created_at: 2026-06-23
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updated_at: 2026-06-23
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review_reason: ""
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merge_history: []
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tags: ["javascript", "js", "web", "frontend", "w3schools", "promises", "asynchronous"]
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raw_sources: ["https://www.w3schools.com/js/js_promise.asp"]
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applied_in: []
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github_commit: ""
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---
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# [[JavaScript Promises]]
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## 🎯 한 줄 통찰 (One-line insight)
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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]
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## 🧠 핵심 개념 (Core concepts)
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- **Promises replace callback nesting** — they were created to make asynchronous JavaScript easier than deeply nested callbacks ("callback hell"). [S1]
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- **Three states** — a Promise is pending, fulfilled (resolved), or rejected. [S1]
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- **Construct with resolve/reject** — `new Promise(function(resolve, reject) { ... })`; call `resolve(value)` on success or `reject(value)` on failure. [S1]
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- **Consume with then/catch** — `then()` takes success (and optional failure) handlers; `catch()` handles rejection across a chain. [S1]
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- **Chaining requires return** — each `then()` should `return` the next promise so the chain links correctly; forgetting to return breaks the chain. [S1]
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- **Shortcuts** — `Promise.resolve(value)` and `Promise.reject(value)` create already-settled promises. [S1]
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## 🧩 추출된 패턴 (Extracted patterns)
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- **Resolve/reject wrapper** — wrap slow work in `new Promise`, calling `resolve`/`reject` when done. [S1]
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- **then-chain with return** — `step1().then(v => step2(v)).then(v => step3(v))...` runs steps in sequence. [S1]
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- **Single catch for the chain** — one `.catch()` at the end handles any failure along the chain. [S1]
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- **Promisify a timer or XHR** — wrap `setTimeout` or `XMLHttpRequest` so its completion resolves a promise. [S1]
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## 📖 세부 내용 (Details)
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**The problem promises solve (callback hell)**
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Deeply nested callbacks are hard to read: [S1]
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```javascript
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step1(function(r1) {
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step2(r1, function(r2) {
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step3(r2, function(r3) {
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console.log(r3);
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});
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});
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});
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```
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**Promise syntax — resolve case**
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[S1]
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```javascript
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let myPromise = new Promise(function(resolve, reject) {
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ok = true;
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// Code that may take some time
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if (ok) {
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resolve("OK");
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} else {
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reject("Error");
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}
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});
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// Using then() to display the result
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myPromise.then(
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function(value) {myDisplayer(value);},
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function(value) {myDisplayer(value);}
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);
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```
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**Promise syntax — reject case**
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[S1]
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```javascript
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let myPromise = new Promise(function(resolve, reject) {
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ok = false;
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// Code that may take some time
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if (ok) {
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resolve("OK");
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} else {
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reject("Error");
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}
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});
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// Using then() to display the result
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myPromise.then(
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function(value) {myDisplayer(value);},
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function(value) {myDisplayer(value);}
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);
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```
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**Promise.resolve()**
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[S1]
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```javascript
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let promise = Promise.resolve("OK");
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promise
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.then(function(value) {
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console.log(value);
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})
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.catch(function(value) {
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myDisplayer(value);
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});
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```
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**Promise.reject()**
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[S1]
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```javascript
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let promise = Promise.reject("Error");
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promise
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.then(function(value) {
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console.log(value);
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})
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.catch(function(value) {
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myDisplayer(value);
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});
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```
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**Promise chaining**
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Run three functions in steps, each returning a promise: [S1]
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```javascript
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// Three functions to run in steps
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function step1() {
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return Promise.resolve("A");
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}
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function step2(value) {
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return Promise.resolve(value + "B");
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}
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function step3(value) {
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return Promise.resolve(value + "C");
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}
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// Run the three functions in steps
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step1()
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.then(function(value) {
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return step2(value);
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})
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.then(function(value) {
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return step3(value);
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})
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.then(function(value) {
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myDisplayer(value);
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});
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```
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**Error handling with catch()**
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A single `catch()` handles failure anywhere in the chain: [S1]
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```javascript
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step1()
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.then(function(value) {
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return step2(value);
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})
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.then(function(value) {
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return step3(value);
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})
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.catch(function(error) {
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console.log(error);
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});
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```
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**Common mistake — forgetting to return**
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Without `return`, the next `then()` does not receive the resolved value: [S1]
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```javascript
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step1()
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.then(function(value) {
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step2(value);
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})
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.then(function(value) {
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console.log(value);
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});
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```
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**Fetch API with promises**
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[S1]
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```javascript
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fetch("data.json")
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.then(function(response) {
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return response.json();
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})
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.then(function(data) {
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console.log(data);
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})
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.catch(function(error) {
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console.log(error);
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});
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```
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**Timeout with a callback vs a promise**
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Callback version: [S1]
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```javascript
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setTimeout(function() { myFunction("I love You !!!"); }, 3000);
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function myFunction(value) {
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document.getElementById("demo").innerHTML = value;
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}
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```
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Promise version (promisified timer): [S1]
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```javascript
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let myPromise = new Promise(function(myResolve, myReject) {
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setTimeout(function() { myResolve("I love You !!"); }, 3000);
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});
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myPromise.then(function(value) {
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document.getElementById("demo").innerHTML = value;
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});
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```
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**File loading — callback vs promise**
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Callback version using XMLHttpRequest: [S1]
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```javascript
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function getFile(myCallback) {
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let req = new XMLHttpRequest();
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req.open('GET', "mycar.html");
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req.onload = function() {
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if (req.status == 200) {
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myCallback(req.responseText);
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} else {
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myCallback("Error: " + req.status);
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}
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}
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req.send();
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}
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getFile(myDisplayer);
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```
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Promise version: [S1]
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```javascript
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let myPromise = new Promise(function(myResolve, myReject) {
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let req = new XMLHttpRequest();
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req.open('GET', "mycar.html");
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req.onload = function() {
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if (req.status == 200) {
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myResolve(req.response);
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} else {
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myReject("File not Found");
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}
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};
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req.send();
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});
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myPromise.then(
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function(value) {myDisplayer(value);},
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function(error) {myDisplayer(error);}
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);
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```
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## 🛠️ 적용 사례 (Applied in summary)
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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.
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## 💻 코드 패턴 (Code patterns)
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Construct and consume a promise (language: JavaScript):
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```javascript
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let myPromise = new Promise(function(resolve, reject) {
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if (ok) { resolve("OK"); } else { reject("Error"); }
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});
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myPromise.then(
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function(value) {myDisplayer(value);},
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function(value) {myDisplayer(value);}
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);
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```
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Chain steps with a single catch:
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```javascript
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step1()
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.then(function(value) { return step2(value); })
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.then(function(value) { return step3(value); })
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.catch(function(error) { console.log(error); });
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```
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## ⚖️ 비교 및 선택 기준 (Comparison & decision criteria)
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- **Callbacks** — the original approach; deeply nested callbacks become "callback hell," hard to read. [S1]
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- **Promises** — created to make async JavaScript easier; chainable with `then()` and a single `catch()`, but require remembering to `return` each step. [S1]
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## ⚖️ 모순 및 업데이트 (Contradictions & updates)
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No contradictions found in the source.
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## ✅ 검증 상태 및 신뢰도
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- **상태:** draft
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- **검증 단계:** conceptual (실제 적용 사례 발견 시 applied/validated로 승격 가능)
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- **출처 신뢰도:** B (W3Schools — widely used educational reference, not a primary standards body)
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- **신뢰 점수:** 0.89
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- **중복 검사 결과:** 신규 생성 (New discovery)
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## 🔗 지식 그래프 (Knowledge Graph)
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- **상위/루트:** [[JavaScript Tutorial]]
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- **관련 개념:** [[JavaScript Async Callbacks]], [[JavaScript Async Await]], [[JavaScript Async Fetch]], [[JavaScript Async Timeouts]]
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- **참조 맥락:** The cleaner successor to callbacks and the foundation that async/await is built on.
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## 📚 출처 (Sources)
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- [S1] W3Schools — JavaScript Promises — https://www.w3schools.com/js/js_promise.asp
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## 📝 변경 이력 (Change history)
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- 2026-06-23: Initial draft synthesized from the W3Schools "JavaScript Promises" page (Astra wiki-curation, P-Reinforce v3.1 format).
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