[{"data":1,"prerenderedAt":-1},["ShallowReactive",2],{"playground-error-cause-rethrow-context":3,"verticals":17},{"slug":4,"title":5,"summary":6,"html":7,"css":8,"js":9,"config":10,"doc":13},"error-cause-rethrow-context","Error.cause — interactive playground",null,"\u003Cdiv class=\"pg\">\n    \u003Ch1 class=\"pg-title\">🔗 \u003Ccode>Error.cause\u003C\u002Fcode> — don't lose the root cause when you rethrow\u003C\u002Fh1>\n    \u003Cp class=\"pg-sub\">\n      \u003Ccode>loadUser()\u003C\u002Fcode> calls \u003Ccode>fetchJSON()\u003C\u002Fcode> calls \u003Ccode>parse()\u003C\u002Fcode> — a real\n      \u003Ccode>SyntaxError\u003C\u002Fcode> is thrown three layers down. Toggle whether each \u003Ccode>catch\u003C\u002Fcode> block\n      rethrows \u003Cstrong>with\u003C\u002Fstrong> or \u003Cstrong>without\u003C\u002Fstrong> \u003Ccode>{ cause: err }\u003C\u002Fcode>, then trigger\n      the failure and watch how much of the original error survives to the top.\n    \u003C\u002Fp>\n\n    \u003Cdiv class=\"panel\">\n      \u003Cdiv class=\"panel-label\">\u003Cspan class=\"n\">1\u003C\u002Fspan> Call stack + rethrow style\u003C\u002Fdiv>\n\n      \u003Cdiv class=\"stack-row\" id=\"stackRow\" aria-hidden=\"true\">\n        \u003Cspan class=\"stack-pill\" data-stage=\"loadUser\">loadUser()\u003C\u002Fspan>\n        \u003Cspan class=\"stack-arrow\">→\u003C\u002Fspan>\n        \u003Cspan class=\"stack-pill\" data-stage=\"fetchJSON\">fetchJSON()\u003C\u002Fspan>\n        \u003Cspan class=\"stack-arrow\">→\u003C\u002Fspan>\n        \u003Cspan class=\"stack-pill\" data-stage=\"parse\">parse()\u003C\u002Fspan>\n        \u003Cspan class=\"stack-arrow\">→ 💥\u003C\u002Fspan>\n      \u003C\u002Fdiv>\n\n      \u003Cdiv class=\"field\" style=\"margin-bottom:14px\">\n        \u003Cspan class=\"label\">Each catch block will\u003C\u002Fspan>\n        \u003Cdiv class=\"seg\" role=\"group\" aria-label=\"Rethrow style\">\n          \u003Cbutton type=\"button\" data-style=\"without\" aria-pressed=\"true\">Rethrow WITHOUT cause\u003C\u002Fbutton>\n          \u003Cbutton type=\"button\" data-style=\"with\" aria-pressed=\"false\">Rethrow WITH cause\u003C\u002Fbutton>\n        \u003C\u002Fdiv>\n      \u003C\u002Fdiv>\n\n      \u003Cdiv class=\"presets\" style=\"margin-bottom:14px\">\n        \u003Cbutton class=\"btn sm\" data-preset=\"without\">▶ Worked example: show the loss\u003C\u002Fbutton>\n        \u003Cbutton class=\"btn sm\" data-preset=\"with\">▶ Worked example: show the fix\u003C\u002Fbutton>\n      \u003C\u002Fdiv>\n\n      \u003Cdiv class=\"btn-row\">\n        \u003Cbutton class=\"btn primary\" id=\"run\">Trigger the failure\u003C\u002Fbutton>\n        \u003Cbutton class=\"btn ghost\" id=\"reset\">Reset\u003C\u002Fbutton>\n      \u003C\u002Fdiv>\n    \u003C\u002Fdiv>\n\n    \u003Cdiv class=\"panel\">\n      \u003Cdiv class=\"panel-label\">\u003Cspan class=\"n\">2\u003C\u002Fspan> What each \u003Ccode>catch\u003C\u002Fcode> block actually runs\u003C\u002Fdiv>\n      \u003Cdiv class=\"code\" id=\"codeView\">\u003C\u002Fdiv>\n      \u003Cp class=\"hint\">This is the same pattern at both \u003Ccode>fetchJSON\u003C\u002Fcode>'s and \u003Ccode>loadUser\u003C\u002Fcode>'s catch blocks — only the \u003Ccode>{ cause: err }\u003C\u002Fcode> option changes.\u003C\u002Fp>\n    \u003C\u002Fdiv>\n\n    \u003Cdiv class=\"panel\">\n      \u003Cdiv class=\"panel-label\">\u003Cspan class=\"n\">3\u003C\u002Fspan> Error chain received by the caller\u003C\u002Fdiv>\n      \u003Cdiv class=\"status\" id=\"status\" style=\"margin-bottom:14px\">\n        \u003Cspan class=\"dot info\" id=\"dot\">\u003C\u002Fspan>\u003Cspan id=\"statusText\">Ready — click \"Trigger the failure\" above.\u003C\u002Fspan>\n      \u003C\u002Fdiv>\n      \u003Cdiv class=\"output\">\n        \u003Cdiv class=\"chain\" id=\"chain\">\n          \u003Cp class=\"muted\" style=\"margin:0;font-size:0.88rem\">No error yet. Trigger the failure to see what the top-level catch receives.\u003C\u002Fp>\n        \u003C\u002Fdiv>\n      \u003C\u002Fdiv>\n      \u003Cp class=\"hint\" id=\"hint\">The chain shows every error the top-level \u003Ccode>catch\u003C\u002Fcode> can reach by walking \u003Ccode>.cause\u003C\u002Fcode>.\u003C\u002Fp>\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\u002F* Demo-specific CSS ONLY — base.css owns tokens, layout, panels, buttons, seg, code, status. *\u002F\n  .stack-row {\n    display: flex; flex-wrap: wrap; align-items: center; gap: 6px;\n    margin-bottom: 14px; font-family: var(--mono); font-size: 0.84rem;\n  }\n  .stack-pill {\n    display: inline-flex; align-items: center; padding: 5px 11px;\n    border-radius: 999px; background: var(--panel-2); border: 1px solid var(--line);\n    color: var(--ink); font-weight: 700;\n  }\n  .stack-pill.active { border-color: var(--accent); color: var(--accent); background: color-mix(in srgb, var(--accent) 14%, var(--panel-2)); }\n  .stack-arrow { color: var(--faint); }\n\n  .chain {\n    display: flex; flex-direction: column; gap: 0; align-items: stretch;\n  }\n  .chain-node {\n    border: 1px solid var(--line); background: var(--card); border-radius: var(--radius-sm);\n    padding: 11px 13px; font-family: var(--mono); font-size: 0.84rem; line-height: 1.5;\n  }\n  .chain-node .nm { font-weight: 800; color: var(--bad); }\n  .chain-node .msg { color: var(--ink); }\n  .chain-node .lbl { display: block; font-family: var(--sans); font-size: 0.68rem; text-transform: uppercase;\n    letter-spacing: 0.06em; color: var(--muted); font-weight: 800; margin-bottom: 4px; }\n  .chain-node.root { border-color: color-mix(in srgb, var(--good) 55%, var(--line)); }\n  .chain-node.root .nm { color: var(--good); }\n  .chain-arrow {\n    align-self: center; color: var(--faint); font-family: var(--mono); font-size: 0.78rem;\n    padding: 4px 0; display: flex; align-items: center; gap: 6px;\n  }\n  .chain-stop {\n    border: 1px dashed color-mix(in srgb, var(--bad) 55%, var(--line));\n    border-radius: var(--radius-sm); padding: 9px 13px; margin-top: 2px;\n    font-size: 0.82rem; color: var(--bad); font-weight: 700; background: color-mix(in srgb, var(--bad) 8%, transparent);\n  }\n  .depth-wrap { display: flex; align-items: center; gap: 14px; margin-bottom: 14px; flex-wrap: wrap; }","\"use strict\";\nconst $ = (id) => document.getElementById(id);\nlet withCause = false; \u002F\u002F current toggle state\n\n\u002F\u002F ---- the simulated call stack ------------------------------------------\n\n\u002F\u002F The deepest layer: a REAL JSON.parse failure, not a fabricated message.\nfunction parseResponse(raw) {\n  return JSON.parse(raw); \u002F\u002F \"not-json\" throws a genuine SyntaxError\n}\n\nfunction fetchJSON(useCause) {\n  try {\n    return parseResponse(\"not-json\");\n  } catch (err) {\n    if (useCause) {\n      throw new Error(\"fetchJSON: response parsing failed\", { cause: err });\n    }\n    throw new Error(\"fetchJSON: response parsing failed\");\n  }\n}\n\nfunction loadUser(id, useCause) {\n  try {\n    return fetchJSON(useCause);\n  } catch (err) {\n    if (useCause) {\n      throw new Error(`Failed to load user ${id}`, { cause: err });\n    }\n    throw new Error(`Failed to load user ${id}`);\n  }\n}\n\n\u002F\u002F ---- code readout --------------------------------------------------------\n\nfunction renderCode(useCause) {\n  const withBlock =\n`\u003Cspan class=\"tok-kw\">catch\u003C\u002Fspan> (err) {\n  \u003Cspan class=\"tok-kw\">throw new\u003C\u002Fspan> \u003Cspan class=\"tok-sel\">Error\u003C\u002Fspan>(\u003Cspan class=\"tok-str\">'…failed'\u003C\u002Fspan>, \u003Cspan class=\"hl\">{ cause: err }\u003C\u002Fspan>);\n}`;\n  const withoutBlock =\n`\u003Cspan class=\"tok-kw\">catch\u003C\u002Fspan> (err) {\n  \u003Cspan class=\"tok-kw\">throw new\u003C\u002Fspan> \u003Cspan class=\"tok-sel\">Error\u003C\u002Fspan>(\u003Cspan class=\"tok-str\">'…failed'\u003C\u002Fspan>); \u003Cspan class=\"tok-cmt\">\u002F\u002F err is dropped\u003C\u002Fspan>\n}`;\n  $(\"codeView\").innerHTML = useCause ? withBlock : withoutBlock;\n}\n\n\u002F\u002F ---- rendering the result --------------------------------------------------\n\nfunction setStatus(text, kind) {\n  $(\"statusText\").textContent = text;\n  $(\"dot\").className = \"dot \" + kind;\n}\n\nfunction nameOf(e) {\n  if (e instanceof Error) return e.name;\n  return typeof e;\n}\n\nfunction renderChain(topErr) {\n  const nodes = [];\n  let cur = topErr;\n  while (cur !== undefined && cur !== null) {\n    nodes.push(cur);\n    cur = cur instanceof Error ? cur.cause : undefined;\n  }\n\n  const chainEl = $(\"chain\");\n  chainEl.innerHTML = \"\";\n\n  nodes.forEach((e, i) => {\n    if (i > 0) {\n      const arrow = document.createElement(\"div\");\n      arrow.className = \"chain-arrow\";\n      arrow.textContent = \"↓ .cause\";\n      chainEl.appendChild(arrow);\n    }\n    const isRoot = i === nodes.length - 1 && !(e instanceof Error && e.cause !== undefined);\n    const node = document.createElement(\"div\");\n    node.className = \"chain-node\" + (isRoot && nodes.length > 1 ? \" root\" : \"\");\n    const label = i === 0 ? \"top-level catch received\" : `layer ${i + 1} down`;\n    node.innerHTML =\n      `\u003Cspan class=\"lbl\">${label}${isRoot && nodes.length > 1 ? \" — original root cause\" : \"\"}\u003C\u002Fspan>` +\n      `\u003Cspan class=\"nm\">${nameOf(e)}\u003C\u002Fspan>: \u003Cspan class=\"msg\">${String(e.message ?? e)}\u003C\u002Fspan>`;\n    chainEl.appendChild(node);\n  });\n\n  if (nodes.length === 1) {\n    const stop = document.createElement(\"div\");\n    stop.className = \"chain-stop\";\n    stop.textContent = \"err.cause is undefined here — the SyntaxError from parse() never made it out. Gone for good.\";\n    chainEl.appendChild(stop);\n  }\n\n  return nodes.length;\n}\n\nfunction runFailure() {\n  try {\n    loadUser(42, withCause);\n  } catch (err) {\n    const depth = renderChain(err);\n    if (withCause) {\n      setStatus(`Chain has ${depth} links — the original SyntaxError survived all the way to the top.`, \"good\");\n    } else {\n      setStatus(\"Only 1 link reached the top — the root SyntaxError was lost at the first catch.\", \"bad\");\n    }\n    $(\"stackRow\").querySelectorAll(\".stack-pill\").forEach((p) => p.classList.add(\"active\"));\n  }\n}\n\nfunction setStyle(style) {\n  withCause = style === \"with\";\n  document.querySelectorAll('.seg button[data-style]').forEach((b) => {\n    b.setAttribute(\"aria-pressed\", String(b.dataset.style === style));\n  });\n  renderCode(withCause);\n}\n\nfunction resetAll() {\n  setStyle(\"without\");\n  $(\"chain\").innerHTML = '\u003Cp class=\"muted\" style=\"margin:0;font-size:0.88rem\">No error yet. Trigger the failure to see what the top-level catch receives.\u003C\u002Fp>';\n  setStatus('Ready — click \"Trigger the failure\" above.', \"info\");\n  $(\"stackRow\").querySelectorAll(\".stack-pill\").forEach((p) => p.classList.remove(\"active\"));\n}\n\ndocument.querySelectorAll('.seg button[data-style]').forEach((btn) => {\n  btn.addEventListener(\"click\", () => setStyle(btn.dataset.style));\n});\n\ndocument.querySelectorAll('[data-preset]').forEach((btn) => {\n  btn.addEventListener(\"click\", () => {\n    setStyle(btn.dataset.preset);\n    runFailure();\n  });\n});\n\n$(\"run\").addEventListener(\"click\", runFailure);\n$(\"reset\").addEventListener(\"click\", resetAll);\n\nresetAll();",{"height":11,"autorun":12},520,true,{"slug":4,"title":14,"excerpt":15,"coverUrl":16},"You're rethrowing errors and losing context. `Error.cause` fixes that.","Every time you catch an error and rethrow a new one without forwarding the original, you lose the stack trace, the error type, and everything useful about what actually went wrong. ES2022 added `Error.cause` to fix exactly this.","\u002Fmedia\u002Fcovers\u002Ferror-cause-rethrow-context.png",[18,33],{"id":19,"slug":20,"name":21,"tagline":22,"description":23,"accentFrom":24,"accentTo":25,"icon":26,"defaultLocale":27,"locales":28,"features":30,"position":32},"019fe637-3d33-714b-b57f-23e163ffca0c","dev","Web Development","Build. Learn. Ship.","Practical courses, engineering-grade articles and open-source tools for people who ship.","violet-500","cyan-400","◇","en",[27,29],"fa",{"courses":12,"paths":12,"articles":12,"exams":31,"flashcards":31,"packages":12,"community":12,"certificates":12,"teams":12,"commerce":12},false,0,{"id":34,"slug":35,"name":36,"tagline":37,"description":38,"accentFrom":39,"accentTo":24,"icon":40,"defaultLocale":27,"locales":41,"features":43,"position":44},"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","⌘",[27,29,42],"es",{"courses":12,"paths":12,"articles":31,"exams":31,"flashcards":31,"packages":31,"community":12,"certificates":31,"teams":31,"commerce":12},2]