[{"data":1,"prerenderedAt":-1},["ShallowReactive",2],{"playground-sendbeacon-reliable-unload-data":3,"verticals":19},{"slug":4,"title":5,"summary":6,"html":7,"css":8,"js":9,"config":10,"doc":13},"sendbeacon-reliable-unload-data","navigator.sendBeacon() — reliable unload data",null,"\u003Cdiv class=\"pg\">\n    \u003Ch1 class=\"pg-title\">🎮 navigator.sendBeacon()\u003C\u002Fh1>\n    \u003Cp class=\"pg-sub\">\n      Simulate a page navigating away. See how \u003Ccode>fetch()\u003C\u002Fcode> gets silently\n      cancelled and \u003Ccode>sendBeacon()\u003C\u002Fcode> delivers through the unload.\n    \u003C\u002Fp>\n\n    \u003Cdiv class=\"panel\">\n      \u003Cdiv class=\"panel-label\">\u003Cspan class=\"n\">1\u003C\u002Fspan> Pick a method\u003C\u002Fdiv>\n      \u003Cdiv class=\"method-toggle\">\n        \u003Cbutton class=\"method-btn active-bad\" id=\"btnFetch\" onclick=\"setMethod('fetch')\">\n          ❌ fetch() in beforeunload\n        \u003C\u002Fbutton>\n        \u003Cbutton class=\"method-btn\" id=\"btnBeacon\" onclick=\"setMethod('beacon')\">\n          ✅ navigator.sendBeacon()\n        \u003C\u002Fbutton>\n      \u003C\u002Fdiv>\n      \u003Cp class=\"hint\" id=\"methodHint\">\n        \u003Cstrong>fetch() in beforeunload:\u003C\u002Fstrong> The call starts, but the browser cancels\n        in-flight async requests the moment it navigates away. Your data is silently dropped.\n      \u003C\u002Fp>\n    \u003C\u002Fdiv>\n\n    \u003Cdiv class=\"panel\">\n      \u003Cdiv class=\"panel-label\">\u003Cspan class=\"n\">2\u003C\u002Fspan> Simulate a page exit\u003C\u002Fdiv>\n      \u003Cdiv class=\"scene\">\n        \u003Cdiv class=\"page-box\">\n          \u003Cdiv class=\"box-label\">Your Page\u003C\u002Fdiv>\n          \u003Cdiv class=\"page-status alive\" id=\"pageStatus\">🟢 alive\u003C\u002Fdiv>\n          \u003Cdiv style=\"font-size:0.75rem;color:var(--muted);margin-top:8px;\" id=\"pageDetail\">\n            Waiting for navigation…\n          \u003C\u002Fdiv>\n        \u003C\u002Fdiv>\n        \u003Cdiv class=\"wire\">\n          \u003Cdiv class=\"packet\" id=\"packet\">📦\u003C\u002Fdiv>\n        \u003C\u002Fdiv>\n        \u003Cdiv class=\"server-box\">\n          \u003Cdiv class=\"box-label\">Server \u002F API endpoint\u003C\u002Fdiv>\n          \u003Cdiv class=\"server-log\" id=\"serverLog\">\n            \u003Cdiv style=\"color:var(--muted);font-size:0.78rem;\">No requests received yet.\u003C\u002Fdiv>\n          \u003C\u002Fdiv>\n        \u003C\u002Fdiv>\n      \u003C\u002Fdiv>\n      \u003Cdiv class=\"result-banner\" id=\"resultBanner\">\u003C\u002Fdiv>\n      \u003Cdiv class=\"btn-row\" style=\"margin-top:10px;\">\n        \u003Cbutton class=\"btn primary\" id=\"btnSimulate\" onclick=\"runSim()\">\n          ▶ Navigate Away (send data)\n        \u003C\u002Fbutton>\n        \u003Cbutton class=\"btn\" onclick=\"reset()\">↺ Reset\u003C\u002Fbutton>\n      \u003C\u002Fdiv>\n    \u003C\u002Fdiv>\n\n    \u003Cdiv class=\"panel\">\n      \u003Cdiv class=\"panel-label\">\u003Cspan class=\"n\">3\u003C\u002Fspan> Code comparison\u003C\u002Fdiv>\n      \u003Cdiv class=\"code-compare\">\n        \u003Cdiv class=\"code-pane\">\n          \u003Cdiv class=\"code-pane-label bad-label\">❌ fetch() — silently dropped\u003C\u002Fdiv>\n          \u003Cdiv class=\"code\">\n\u003Cspan class=\"tok-kw\">window\u003C\u002Fspan>.\u003Cspan class=\"tok-fn\">addEventListener\u003C\u002Fspan>(\u003Cspan class=\"tok-str\">'beforeunload'\u003C\u002Fspan>, () => {\n  \u003Cspan class=\"tok-fn\">fetch\u003C\u002Fspan>(\u003Cspan class=\"tok-str\">'\u002Fapi\u002Flog'\u003C\u002Fspan>, {\n    method: \u003Cspan class=\"tok-str\">'POST'\u003C\u002Fspan>,\n    body: \u003Cspan class=\"tok-fn\">JSON.stringify\u003C\u002Fspan>(payload),\n  });\n  \u003Cspan class=\"tok-cmt\">\u002F\u002F ⚠ browser navigates away\u003C\u002Fspan>\n  \u003Cspan class=\"tok-cmt\">\u002F\u002F request is cancelled mid-flight\u003C\u002Fspan>\n});\u003C\u002Fdiv>\n        \u003C\u002Fdiv>\n        \u003Cdiv class=\"code-pane\">\n          \u003Cdiv class=\"code-pane-label good-label\">✅ sendBeacon() — always delivered\u003C\u002Fdiv>\n          \u003Cdiv class=\"code\">\n\u003Cspan class=\"tok-kw\">window\u003C\u002Fspan>.\u003Cspan class=\"tok-fn\">addEventListener\u003C\u002Fspan>(\n  \u003Cspan class=\"tok-str\">'visibilitychange'\u003C\u002Fspan>, () => {\n  \u003Cspan class=\"tok-kw\">if\u003C\u002Fspan> (document.visibilityState === \u003Cspan class=\"tok-str\">'hidden'\u003C\u002Fspan>) {\n    \u003Cspan class=\"tok-kw\">navigator\u003C\u002Fspan>.\u003Cspan class=\"tok-fn\">sendBeacon\u003C\u002Fspan>(\n      \u003Cspan class=\"tok-str\">'\u002Fapi\u002Flog'\u003C\u002Fspan>,\n      blob  \u003Cspan class=\"tok-cmt\">\u002F\u002F Blob, FormData, string…\u003C\u002Fspan>\n    );\n    \u003Cspan class=\"tok-cmt\">\u002F\u002F ✓ browser queues & delivers\u003C\u002Fspan>\n  }\n});\u003C\u002Fdiv>\n        \u003C\u002Fdiv>\n      \u003C\u002Fdiv>\n    \u003C\u002Fdiv>\n\n    \u003Cdiv class=\"panel\">\n      \u003Cdiv class=\"panel-label\">\u003Cspan class=\"n\">4\u003C\u002Fspan> What you can send\u003C\u002Fdiv>\n      \u003Cdiv class=\"code\">\n\u003Cspan class=\"tok-cmt\">\u002F\u002F Plain string (Content-Type: text\u002Fplain)\u003C\u002Fspan>\n\u003Cspan class=\"tok-kw\">navigator\u003C\u002Fspan>.\u003Cspan class=\"tok-fn\">sendBeacon\u003C\u002Fspan>(\u003Cspan class=\"tok-str\">'\u002Fapi\u002Flog'\u003C\u002Fspan>, \u003Cspan class=\"tok-str\">'user-exited'\u003C\u002Fspan>);\n\n\u003Cspan class=\"tok-cmt\">\u002F\u002F JSON (wrap in Blob to set Content-Type correctly)\u003C\u002Fspan>\n\u003Cspan class=\"tok-kw\">const\u003C\u002Fspan> blob = \u003Cspan class=\"tok-kw\">new\u003C\u002Fspan> \u003Cspan class=\"tok-fn\">Blob\u003C\u002Fspan>([\u003Cspan class=\"tok-fn\">JSON.stringify\u003C\u002Fspan>({ event: \u003Cspan class=\"tok-str\">'exit'\u003C\u002Fspan> })], {\n  type: \u003Cspan class=\"tok-str\">'application\u002Fjson'\u003C\u002Fspan>,\n});\n\u003Cspan class=\"tok-kw\">navigator\u003C\u002Fspan>.\u003Cspan class=\"tok-fn\">sendBeacon\u003C\u002Fspan>(\u003Cspan class=\"tok-str\">'\u002Fapi\u002Flog'\u003C\u002Fspan>, blob);\n\n\u003Cspan class=\"tok-cmt\">\u002F\u002F FormData (Content-Type: multipart\u002Fform-data)\u003C\u002Fspan>\n\u003Cspan class=\"tok-kw\">const\u003C\u002Fspan> form = \u003Cspan class=\"tok-kw\">new\u003C\u002Fspan> \u003Cspan class=\"tok-fn\">FormData\u003C\u002Fspan>();\nform.\u003Cspan class=\"tok-fn\">append\u003C\u002Fspan>(\u003Cspan class=\"tok-str\">'event'\u003C\u002Fspan>, \u003Cspan class=\"tok-str\">'exit'\u003C\u002Fspan>);\n\u003Cspan class=\"tok-kw\">navigator\u003C\u002Fspan>.\u003Cspan class=\"tok-fn\">sendBeacon\u003C\u002Fspan>(\u003Cspan class=\"tok-str\">'\u002Fapi\u002Flog'\u003C\u002Fspan>, form);\n\n\u003Cspan class=\"tok-cmt\">\u002F\u002F Returns true if queued, false if payload > 64 KB\u003C\u002Fspan>\n\u003Cspan class=\"tok-kw\">const\u003C\u002Fspan> queued = \u003Cspan class=\"tok-kw\">navigator\u003C\u002Fspan>.\u003Cspan class=\"tok-fn\">sendBeacon\u003C\u002Fspan>(\u003Cspan class=\"tok-str\">'\u002Fapi\u002Flog'\u003C\u002Fspan>, blob);\n      \u003C\u002Fdiv>\n    \u003C\u002Fdiv>\n  \u003C\u002Fdiv>","\u002F*\n * Shared playground design system — daily-post\n * ============================================\n * Link it from any post's playground\u002Findex.html (no per-post boilerplate):\n *\n *   \u003Clink rel=\"stylesheet\" href=\"..\u002F..\u002F..\u002Fplayground\u002Fassets\u002Fbase.css\" \u002F>\n *\n * The relative path resolves the same locally (posts\u002F\u003Cbase>\u002Fplayground\u002F) and on\n * the deployed host (public\u002Fposts\u002F\u003Cbase>\u002Fplayground\u002F), because build-site.js\n * mirrors this file to public\u002Fplayground\u002Fassets\u002Fbase.css. For a CodeSandbox\n * --embed, playground.js inlines this file so the single-file sandbox still styles.\n *\n * Then write ONLY the demo-specific CSS in your inline \u003Cstyle>. Everything below\n * — tokens, layout, panels, buttons, toggles, sliders, code blocks, status\u002Flog,\n * banners, presets — comes for free and looks consistent across every post.\n *\n * Theme-aware: dark by default, light via prefers-color-scheme AND a\n * [data-theme] override on \u003Chtml> (so a toggle can force either). Accessible:\n * real focus rings, adequate contrast, reduced-motion honored.\n *\u002F\n\n\u002F* ---------- design tokens ---------- *\u002F\n:root {\n  --bg: #0d1117;\n  --bg-grid: #ffffff0a;\n  --panel: #161b22;\n  --panel-2: #0b0e14;\n  --card: #0b0e14;\n  --ink: #e8ebf2;\n  --muted: #9aa4b2;\n  --faint: #6b7688;\n  --line: #232a35;\n  --line-soft: #ffffff12;\n  --accent: #7c9cff;\n  --accent-ink: #0b0e14;\n  --good: #3fb950;\n  --warn: #f0883e;\n  --bad: #f85149;\n  --info: #58a6ff;\n  --radius: 14px;\n  --radius-sm: 9px;\n  --shadow: 0 8px 30px #0006;\n  --shadow-sm: 0 4px 14px #0005;\n  --mono: ui-monospace, SFMono-Regular, \"SF Mono\", Menlo, Consolas, \"Liberation Mono\", monospace;\n  --sans: ui-sans-serif, system-ui, -apple-system, \"Segoe UI\", Roboto, \"Helvetica Neue\", sans-serif;\n  color-scheme: dark;\n}\n\n\u002F* Light theme — system preference, overridable by [data-theme] on \u003Chtml>. *\u002F\n@media (prefers-color-scheme: light) {\n  :root:not([data-theme=\"dark\"]) {\n    --bg: #f4f6fb;\n    --bg-grid: #00000008;\n    --panel: #ffffff;\n    --panel-2: #f0f3f9;\n    --card: #f7f9fc;\n    --ink: #131722;\n    --muted: #5b6472;\n    --faint: #8a93a3;\n    --line: #e2e7f0;\n    --line-soft: #00000010;\n    --accent: #2f6df0;\n    --accent-ink: #ffffff;\n    --good: #1a7f37;\n    --warn: #bc4c00;\n    --bad: #cf222e;\n    --info: #0969da;\n    --shadow: 0 8px 30px #0000001f;\n    --shadow-sm: 0 4px 14px #00000014;\n    color-scheme: light;\n  }\n}\n:root[data-theme=\"light\"] {\n  --bg: #f4f6fb;\n  --bg-grid: #00000008;\n  --panel: #ffffff;\n  --panel-2: #f0f3f9;\n  --card: #f7f9fc;\n  --ink: #131722;\n  --muted: #5b6472;\n  --faint: #8a93a3;\n  --line: #e2e7f0;\n  --line-soft: #00000010;\n  --accent: #2f6df0;\n  --accent-ink: #ffffff;\n  --good: #1a7f37;\n  --warn: #bc4c00;\n  --bad: #cf222e;\n  --info: #0969da;\n  --shadow: 0 8px 30px #0000001f;\n  --shadow-sm: 0 4px 14px #00000014;\n  color-scheme: light;\n}\n\n\u002F* ---------- reset & base ---------- *\u002F\n*, *::before, *::after { box-sizing: border-box; }\n\nbody {\n  margin: 0;\n  min-height: 100vh;\n  font-family: var(--sans);\n  color: var(--ink);\n  background:\n    radial-gradient(circle at 1px 1px, var(--bg-grid) 1px, transparent 0) 0 0 \u002F 24px 24px,\n    var(--bg);\n  display: flex;\n  justify-content: center;\n  align-items: flex-start;\n  padding: clamp(14px, 3vw, 26px);\n  line-height: 1.55;\n  -webkit-font-smoothing: antialiased;\n  text-rendering: optimizeLegibility;\n}\n\n\u002F* The single content column. *\u002F\n.pg {\n  width: 100%;\n  max-width: 860px;\n}\n\n\u002F* ---------- header ---------- *\u002F\n.pg-title {\n  font-size: clamp(1.15rem, 1rem + 1vw, 1.4rem);\n  font-weight: 800;\n  letter-spacing: -0.02em;\n  margin: 0 0 4px;\n  display: flex;\n  align-items: center;\n  gap: 9px;\n}\n.pg-sub {\n  color: var(--muted);\n  font-size: 0.92rem;\n  line-height: 1.5;\n  margin: 0 0 16px;\n  max-width: 66ch;\n}\n.pg-sub strong, .pg-sub b { color: var(--ink); font-weight: 700; }\n.pg-sub code { font-family: var(--mono); background: var(--line-soft); color: var(--accent);\n  padding: 1px 6px; border-radius: 6px; font-size: 0.85em; }\n\n\u002F* ---------- panels ---------- *\u002F\n.panel {\n  background: var(--panel);\n  border: 1px solid var(--line);\n  border-radius: var(--radius);\n  padding: 16px 18px;\n  margin-bottom: 14px;\n  box-shadow: var(--shadow-sm);\n}\n.panel-label {\n  font-size: 0.7rem;\n  text-transform: uppercase;\n  letter-spacing: 0.08em;\n  color: var(--muted);\n  font-weight: 800;\n  margin: 0 0 13px;\n  display: flex;\n  align-items: center;\n  gap: 8px;\n}\n.panel-label .n {\n  display: inline-grid; place-items: center;\n  width: 18px; height: 18px; border-radius: 50%;\n  background: var(--accent); color: var(--accent-ink);\n  font-size: 0.66rem; font-weight: 800;\n}\n\n\u002F* ---------- layout helpers ---------- *\u002F\n.row { display: flex; flex-wrap: wrap; gap: 12px; align-items: center; }\n.row.between { justify-content: space-between; }\n.col { display: flex; flex-direction: column; gap: 10px; }\n.grow { flex: 1 1 0; min-width: 0; }\n.wrap-controls { display: flex; flex-wrap: wrap; gap: 16px 24px; align-items: flex-end; }\n.field { display: flex; flex-direction: column; gap: 7px; min-width: 0; }\n.field > .label, .ctl-label {\n  font-size: 0.7rem; text-transform: uppercase; letter-spacing: 0.04em;\n  color: var(--muted); font-weight: 800;\n}\n\n\u002F* ---------- buttons ---------- *\u002F\n.btn {\n  font: inherit; font-weight: 650; font-size: 0.88rem;\n  cursor: pointer; user-select: none;\n  border: 1px solid var(--line);\n  background: var(--card); color: var(--ink);\n  padding: 9px 15px; border-radius: var(--radius-sm);\n  display: inline-flex; align-items: center; gap: 8px;\n  transition: background .15s ease, border-color .15s ease, transform .05s ease, color .15s ease;\n}\n.btn:hover { background: var(--line-soft); border-color: var(--muted); }\n.btn:active { transform: translateY(1px); }\n.btn:focus-visible { outline: 2px solid var(--accent); outline-offset: 2px; }\n.btn[disabled] { opacity: 0.45; cursor: not-allowed; }\n.btn.primary { background: var(--accent); border-color: var(--accent); color: var(--accent-ink); }\n.btn.primary:hover { filter: brightness(1.08); background: var(--accent); }\n.btn.good { background: var(--good); border-color: var(--good); color: #04140a; }\n.btn.bad  { background: var(--bad);  border-color: var(--bad);  color: #fff; }\n.btn.ghost { background: transparent; }\n.btn.sm { padding: 6px 11px; font-size: 0.82rem; }\n\n.btn-row { display: flex; flex-wrap: wrap; gap: 9px; }\n\n\u002F* Segmented control — a row of buttons where one is active (aria-pressed). *\u002F\n.seg { display: inline-flex; background: var(--card); border: 1px solid var(--line);\n  border-radius: var(--radius-sm); padding: 3px; gap: 3px; }\n.seg button {\n  font: inherit; font-weight: 650; font-size: 0.84rem; cursor: pointer;\n  border: none; background: transparent; color: var(--muted);\n  padding: 7px 13px; border-radius: 7px;\n  transition: background .15s ease, color .15s ease;\n}\n.seg button:hover { color: var(--ink); }\n.seg button[aria-pressed=\"true\"] { background: var(--accent); color: var(--accent-ink); }\n.seg button:focus-visible { outline: 2px solid var(--accent); outline-offset: 2px; }\n\n\u002F* Preset chips — worked-example shortcuts. *\u002F\n.presets { display: flex; flex-wrap: wrap; gap: 8px; }\n.presets .btn { background: var(--panel-2); }\n\n\u002F* ---------- toggle switch ---------- *\u002F\n.toggle { display: inline-flex; align-items: center; gap: 10px; cursor: pointer;\n  font-size: 0.88rem; font-weight: 600; user-select: none; }\n.toggle input { position: absolute; opacity: 0; width: 0; height: 0; }\n.switch { position: relative; width: 44px; height: 25px; border-radius: 999px; flex: none;\n  background: var(--line); border: 1px solid var(--line); transition: background .18s ease; }\n.switch::after { content: \"\"; position: absolute; top: 2px; left: 2px; width: 19px; height: 19px;\n  border-radius: 50%; background: #cfd6e4; transition: transform .18s ease; box-shadow: 0 1px 3px #0006; }\n.toggle input:checked + .switch { background: var(--good); border-color: var(--good); }\n.toggle input:checked + .switch::after { transform: translateX(19px); background: #fff; }\n.toggle input:focus-visible + .switch { outline: 2px solid var(--accent); outline-offset: 2px; }\n\n\u002F* ---------- range slider ---------- *\u002F\ninput[type=\"range\"] {\n  -webkit-appearance: none; appearance: none; width: 100%; height: 6px;\n  background: var(--line); border-radius: 999px; outline: none; cursor: pointer; margin: 8px 0;\n}\ninput[type=\"range\"]::-webkit-slider-thumb {\n  -webkit-appearance: none; appearance: none; width: 19px; height: 19px; border-radius: 50%;\n  background: var(--accent); border: 2px solid var(--panel); box-shadow: var(--shadow-sm); cursor: pointer;\n}\ninput[type=\"range\"]::-moz-range-thumb {\n  width: 19px; height: 19px; border-radius: 50%; background: var(--accent);\n  border: 2px solid var(--panel); box-shadow: var(--shadow-sm); cursor: pointer;\n}\ninput[type=\"range\"]:focus-visible { outline: 2px solid var(--accent); outline-offset: 3px; }\n\n\u002F* ---------- text inputs \u002F selects ---------- *\u002F\ninput[type=\"text\"], input[type=\"number\"], input[type=\"search\"], select, textarea {\n  font: inherit; color: var(--ink); background: var(--card);\n  border: 1px solid var(--line); border-radius: var(--radius-sm); padding: 8px 11px; width: 100%;\n}\ntextarea { font-family: var(--mono); font-size: 0.85rem; line-height: 1.6; resize: vertical; }\ninput:focus-visible, select:focus-visible, textarea:focus-visible {\n  outline: 2px solid var(--accent); outline-offset: 1px; border-color: var(--accent);\n}\n\n\u002F* ---------- status \u002F output \u002F log ---------- *\u002F\n.status {\n  display: flex; align-items: center; gap: 9px; flex-wrap: wrap;\n  font-size: 0.9rem; font-weight: 600;\n  padding: 11px 14px; border-radius: var(--radius-sm);\n  background: var(--panel-2); border: 1px solid var(--line); color: var(--ink);\n}\n.status.good { border-color: color-mix(in srgb, var(--good) 55%, var(--line)); }\n.status.warn { border-color: color-mix(in srgb, var(--warn) 55%, var(--line)); }\n.status.bad  { border-color: color-mix(in srgb, var(--bad) 55%, var(--line)); }\n.dot { width: 9px; height: 9px; border-radius: 50%; background: var(--muted); flex: none; }\n.dot.good { background: var(--good); } .dot.warn { background: var(--warn); }\n.dot.bad { background: var(--bad); } .dot.info { background: var(--info); }\n\n.output { background: var(--panel-2); border: 1px solid var(--line); border-radius: var(--radius-sm);\n  padding: 13px 15px; }\n.log { font-family: var(--mono); font-size: 0.8rem; line-height: 1.7; background: var(--card);\n  border: 1px solid var(--line); border-radius: var(--radius-sm); padding: 11px 14px;\n  max-height: 220px; overflow-y: auto; white-space: pre-wrap; }\n\n\u002F* ---------- code block + syntax tokens ---------- *\u002F\n.code {\n  font-family: var(--mono); font-size: 0.82rem; line-height: 1.7;\n  background: var(--panel-2); border: 1px solid var(--line); border-radius: var(--radius-sm);\n  padding: 13px 15px; overflow-x: auto; white-space: pre; tab-size: 2;\n}\n.code .tok-sel  { color: var(--accent); }\n.code .tok-prop { color: #86e29b; }\n.code .tok-kw   { color: #ff9bcb; }\n.code .tok-str  { color: #f0c674; }\n.code .tok-num  { color: #f0883e; }\n.code .tok-cmt  { color: var(--faint); font-style: italic; }\n.code .hl { background: color-mix(in srgb, var(--good) 22%, transparent); border-radius: 4px; padding: 0 3px; }\n\n\u002F* ---------- hint \u002F caption ---------- *\u002F\n.hint { font-size: 0.83rem; color: var(--muted); line-height: 1.55; margin: 10px 0 0; }\n.hint code { font-family: var(--mono); color: var(--ink); background: var(--line-soft);\n  padding: 1px 5px; border-radius: 5px; font-size: 0.86em; }\n.hint strong { color: var(--ink); }\n\n\u002F* ---------- banner (feature-support \u002F notices) ---------- *\u002F\n.banner {\n  display: none; margin-bottom: 14px; padding: 11px 14px; border-radius: 11px;\n  font-size: 0.85rem; line-height: 1.5;\n  color: var(--ink); background: color-mix(in srgb, var(--warn) 12%, var(--panel));\n  border: 1px solid color-mix(in srgb, var(--warn) 45%, var(--line));\n}\n.banner.show { display: block; }\n.banner.info { background: color-mix(in srgb, var(--info) 12%, var(--panel));\n  border-color: color-mix(in srgb, var(--info) 45%, var(--line)); }\n.banner code { font-family: var(--mono); background: var(--line-soft); padding: 1px 5px; border-radius: 5px; }\n\n\u002F* ---------- misc atoms ---------- *\u002F\n.pill { display: inline-flex; align-items: center; gap: 6px; font-size: 0.75rem; font-weight: 700;\n  padding: 3px 9px; border-radius: 999px; background: var(--line-soft); color: var(--muted); }\n.pill.good { color: var(--good); background: color-mix(in srgb, var(--good) 16%, transparent); }\n.pill.bad  { color: var(--bad);  background: color-mix(in srgb, var(--bad) 16%, transparent); }\n.mono { font-family: var(--mono); }\n.muted { color: var(--muted); }\n.big-num { font-family: var(--mono); font-size: 1.6rem; font-weight: 800; letter-spacing: -0.02em; }\n\n\u002F* Optional theme toggle button (top-right). Wire it in JS if you want it. *\u002F\n.theme-btn { position: fixed; top: 12px; right: 12px; z-index: 20; }\n\n\u002F* ---------- responsive ---------- *\u002F\n@media (max-width: 560px) {\n  .panel { padding: 14px; }\n  .wrap-controls { gap: 14px; }\n  .btn { padding: 9px 13px; }\n}\n\n\u002F* ---------- reduced motion ---------- *\u002F\n@media (prefers-reduced-motion: reduce) {\n  *, *::before, *::after { transition-duration: 0.01ms !important; animation-duration: 0.01ms !important; }\n}\n\n\n.method-toggle {\n    display: grid;\n    grid-template-columns: 1fr 1fr;\n    gap: 8px;\n    margin-bottom: 12px;\n  }\n  .method-btn {\n    padding: 10px 12px;\n    border-radius: var(--radius-sm);\n    border: 2px solid var(--line);\n    background: var(--panel-2);\n    color: var(--ink);\n    font-family: var(--mono);\n    font-size: 0.8rem;\n    font-weight: 700;\n    cursor: pointer;\n    text-align: left;\n    transition: border-color 0.15s, background 0.15s;\n  }\n  .method-btn.active-bad {\n    border-color: var(--bad);\n    background: color-mix(in srgb, var(--bad) 10%, var(--panel-2));\n    color: var(--bad);\n  }\n  .method-btn.active-good {\n    border-color: var(--good);\n    background: color-mix(in srgb, var(--good) 10%, var(--panel-2));\n    color: var(--good);\n  }\n\n  .scene {\n    position: relative;\n    display: flex;\n    gap: 10px;\n    align-items: stretch;\n    margin-bottom: 10px;\n  }\n  @media (max-width: 520px) { .scene { flex-direction: column; } }\n\n  .page-box, .server-box {\n    flex: 1;\n    border: 1.5px solid var(--line);\n    border-radius: var(--radius-sm);\n    padding: 12px 14px;\n    background: var(--card);\n    min-height: 90px;\n  }\n  .box-label {\n    font-size: 0.68rem;\n    font-weight: 700;\n    text-transform: uppercase;\n    letter-spacing: 0.07em;\n    color: var(--muted);\n    margin-bottom: 8px;\n  }\n  .page-status {\n    font-family: var(--mono);\n    font-size: 0.85rem;\n    font-weight: 700;\n    padding: 4px 8px;\n    border-radius: 4px;\n    display: inline-block;\n  }\n  .page-status.alive { background: color-mix(in srgb, var(--good) 15%, transparent); color: var(--good); }\n  .page-status.unloading { background: color-mix(in srgb, var(--warn) 15%, transparent); color: var(--warn); }\n  .page-status.gone { background: color-mix(in srgb, var(--bad) 15%, transparent); color: var(--bad); }\n\n  .server-log {\n    font-family: var(--mono);\n    font-size: 0.78rem;\n    min-height: 42px;\n  }\n  .server-entry {\n    padding: 3px 0;\n    border-bottom: 1px solid var(--line-soft);\n    animation: fadeIn 0.25s ease;\n  }\n  .server-entry:last-child { border-bottom: none; }\n  .server-entry.received { color: var(--good); }\n  .server-entry.dropped { color: var(--bad); }\n  .server-entry.pending { color: var(--muted); }\n  @keyframes fadeIn { from { opacity: 0; transform: translateY(4px); } to { opacity: 1; transform: none; } }\n\n  .wire {\n    display: flex;\n    align-items: center;\n    justify-content: center;\n    padding: 0 4px;\n    flex-shrink: 0;\n  }\n  @media (max-width: 520px) { .wire { transform: rotate(90deg); } }\n\n  .packet {\n    width: 28px; height: 28px;\n    border-radius: 50%;\n    border: 2px solid var(--accent);\n    background: color-mix(in srgb, var(--accent) 20%, var(--card));\n    font-size: 0.65rem;\n    display: flex; align-items: center; justify-content: center;\n    font-weight: 700;\n    color: var(--accent);\n    transition: all 0.3s;\n    position: relative;\n  }\n  .packet.gone-cancelled {\n    border-color: var(--bad);\n    background: color-mix(in srgb, var(--bad) 15%, var(--card));\n    color: var(--bad);\n  }\n  .packet.gone-delivered {\n    border-color: var(--good);\n    background: color-mix(in srgb, var(--good) 15%, var(--card));\n    color: var(--good);\n  }\n\n  .result-banner {\n    padding: 10px 14px;\n    border-radius: var(--radius-sm);\n    font-size: 0.85rem;\n    font-weight: 700;\n    text-align: center;\n    display: none;\n  }\n  .result-banner.show { display: block; animation: fadeIn 0.3s ease; }\n  .result-banner.fail { background: color-mix(in srgb, var(--bad) 12%, var(--panel-2)); color: var(--bad); border: 1px solid color-mix(in srgb, var(--bad) 30%, var(--line)); }\n  .result-banner.pass { background: color-mix(in srgb, var(--good) 12%, var(--panel-2)); color: var(--good); border: 1px solid color-mix(in srgb, var(--good) 30%, var(--line)); }\n\n  .code-compare {\n    display: grid;\n    grid-template-columns: 1fr 1fr;\n    gap: 8px;\n  }\n  @media (max-width: 520px) { .code-compare { grid-template-columns: 1fr; } }\n  .code-pane {\n    border: 1.5px solid var(--line);\n    border-radius: var(--radius-sm);\n    overflow: hidden;\n  }\n  .code-pane-label {\n    font-size: 0.68rem;\n    font-weight: 700;\n    text-transform: uppercase;\n    letter-spacing: 0.07em;\n    padding: 6px 10px;\n    border-bottom: 1px solid var(--line);\n  }\n  .code-pane-label.bad-label { background: color-mix(in srgb, var(--bad) 10%, var(--panel-2)); color: var(--bad); }\n  .code-pane-label.good-label { background: color-mix(in srgb, var(--good) 10%, var(--panel-2)); color: var(--good); }\n  .code-pane .code { border-radius: 0; border: none; font-size: 0.73rem; }\n\n  .warn { color: var(--warn); }\n  .good { color: var(--good); }","let method = 'fetch';\n  let simRunning = false;\n\n  function setMethod(m) {\n    method = m;\n    document.getElementById('btnFetch').className =\n      'method-btn ' + (m === 'fetch' ? 'active-bad' : '');\n    document.getElementById('btnBeacon').className =\n      'method-btn ' + (m === 'beacon' ? 'active-good' : '');\n    document.getElementById('methodHint').innerHTML = m === 'fetch'\n      ? '\u003Cstrong>fetch() in beforeunload:\u003C\u002Fstrong> The call starts, but the browser cancels in-flight async requests the moment it navigates away. Your data is silently dropped.'\n      : '\u003Cstrong>navigator.sendBeacon():\u003C\u002Fstrong> The browser queues the request and guarantees delivery even as the page is discarded. It returns true\u002Ffalse to indicate whether it was queued.';\n    reset();\n  }\n\n  function reset() {\n    if (simRunning) return;\n    document.getElementById('pageStatus').className = 'page-status alive';\n    document.getElementById('pageStatus').textContent = '🟢 alive';\n    document.getElementById('pageDetail').textContent = 'Waiting for navigation…';\n    document.getElementById('packet').className = 'packet';\n    document.getElementById('packet').textContent = '📦';\n    document.getElementById('serverLog').innerHTML =\n      '\u003Cdiv style=\"color:var(--muted);font-size:0.78rem;\">No requests received yet.\u003C\u002Fdiv>';\n    document.getElementById('resultBanner').className = 'result-banner';\n    document.getElementById('btnSimulate').disabled = false;\n  }\n\n  function serverEntry(text, type) {\n    const log = document.getElementById('serverLog');\n    if (log.querySelector('[data-placeholder]')) log.innerHTML = '';\n    const div = document.createElement('div');\n    div.className = 'server-entry ' + type;\n    div.textContent = text;\n    log.appendChild(div);\n  }\n\n  function delay(ms) { return new Promise(r => setTimeout(r, ms)); }\n\n  async function runSim() {\n    if (simRunning) return;\n    simRunning = true;\n    document.getElementById('btnSimulate').disabled = true;\n\n    const status = document.getElementById('pageStatus');\n    const detail = document.getElementById('pageDetail');\n    const packet = document.getElementById('packet');\n    const banner = document.getElementById('resultBanner');\n\n    \u002F\u002F Step 1: page is alive, send starts\n    status.className = 'page-status alive';\n    status.textContent = '🟢 alive';\n    detail.textContent = 'User clicked a link — navigation triggered';\n    packet.textContent = '📦';\n    packet.className = 'packet';\n\n    await delay(600);\n\n    \u002F\u002F Step 2: beforeunload \u002F visibilitychange fires — request sent\n    detail.textContent = method === 'fetch'\n      ? 'beforeunload fires → fetch() called, request in-flight…'\n      : 'visibilitychange fires → sendBeacon() called, queued by browser…';\n    packet.textContent = '→';\n\n    serverEntry('⏳ request in flight…', 'pending');\n    await delay(800);\n\n    \u002F\u002F Step 3: page unloads\n    status.className = 'page-status unloading';\n    status.textContent = '🟡 unloading';\n    detail.textContent = 'Browser navigating away…';\n    await delay(600);\n\n    status.className = 'page-status gone';\n    status.textContent = '🔴 gone';\n    await delay(400);\n\n    \u002F\u002F Step 4: result\n    if (method === 'fetch') {\n      packet.className = 'packet gone-cancelled';\n      packet.textContent = '✕';\n      const log = document.getElementById('serverLog');\n      log.innerHTML = '';\n      serverEntry('✕ request cancelled — never received', 'dropped');\n      banner.className = 'result-banner show fail';\n      banner.textContent = '❌ Data lost. The fetch() was cancelled when the page unloaded.';\n      detail.textContent = 'fetch() was in-flight when the page was discarded — browser cancelled it.';\n    } else {\n      packet.className = 'packet gone-delivered';\n      packet.textContent = '✓';\n      const log = document.getElementById('serverLog');\n      log.innerHTML = '';\n      serverEntry('✓ POST \u002Fapi\u002Flog received — 200 OK', 'received');\n      serverEntry('  body: { event: \"exit\", duration: 43200 }', 'received');\n      banner.className = 'result-banner show pass';\n      banner.textContent = '✅ Data delivered. sendBeacon() survived the page unload.';\n      detail.textContent = 'Browser delivered the beacon in the background, even after tab closed.';\n    }\n\n    simRunning = false;\n  }",{"height":11,"autorun":12},520,true,{"slug":4,"title":14,"excerpt":15,"coverUrl":16,"vertical":17},"Your `fetch()` in `beforeunload` is being silently dropped. Use `navigator.sendBeacon()`.","Browsers cancel in-flight network requests when a page unloads. The common fix — synchronous XHR — is deprecated. `navigator.sendBeacon()` is the correct, fire-and-forget API designed exactly for this case.","\u002Fmedia\u002Fcovers\u002Fsendbeacon-reliable-unload-data.png",{"slug":18},"dev",[20,34],{"id":21,"slug":18,"name":22,"tagline":23,"description":24,"accentFrom":25,"accentTo":26,"icon":27,"defaultLocale":28,"locales":29,"features":31,"position":33},"019fe637-3d33-714b-b57f-23e163ffca0c","Web Development","Build. Learn. Ship.","Practical courses, engineering-grade articles and open-source tools for people who ship.","violet-500","cyan-400","◇","en",[28,30],"fa",{"courses":12,"paths":12,"articles":12,"exams":32,"flashcards":32,"packages":12,"community":12,"certificates":12,"teams":12,"commerce":12},false,0,{"id":35,"slug":36,"name":37,"tagline":38,"description":39,"accentFrom":40,"accentTo":25,"icon":41,"defaultLocale":28,"locales":42,"features":44,"position":45},"019fe637-3dc2-754c-8657-0f175bfee7c6","lang","Languages","Learn a language the way you learn a codebase.","Structured paths, listening drills and spaced repetition that actually sticks.","amber-400","⌘",[28,30,43],"es",{"courses":32,"paths":32,"articles":12,"exams":32,"flashcards":12,"packages":32,"community":12,"certificates":12,"teams":32,"commerce":32},2]