[{"data":1,"prerenderedAt":-1},["ShallowReactive",2],{"verticals":3,"playground-iterator-helpers-lazy-filter-map":32},[4,20],{"id":5,"slug":6,"name":7,"tagline":8,"description":9,"accentFrom":10,"accentTo":11,"icon":12,"defaultLocale":13,"locales":14,"features":16,"position":19},"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",[13,15],"fa",{"courses":17,"paths":17,"articles":17,"exams":18,"flashcards":18,"packages":17,"community":17,"certificates":17,"teams":17,"commerce":17},true,false,0,{"id":21,"slug":22,"name":23,"tagline":24,"description":25,"accentFrom":26,"accentTo":10,"icon":27,"defaultLocale":13,"locales":28,"features":30,"position":31},"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","⌘",[13,15,29],"es",{"courses":17,"paths":17,"articles":18,"exams":18,"flashcards":18,"packages":18,"community":17,"certificates":18,"teams":18,"commerce":17},2,{"slug":33,"title":34,"summary":35,"html":36,"css":37,"js":38,"config":39,"doc":41},"iterator-helpers-lazy-filter-map","Eager arrays vs lazy iterator helpers",null,"\u003Cdiv class=\"pg\">\n    \u003Ch1 class=\"pg-title\">⚡ Eager arrays vs. lazy iterator helpers\u003C\u002Fh1>\n    \u003Cp class=\"pg-sub\">\n      The same pipeline — \u003Ccode>filter(even) → map(square) → take(n)\u003C\u002Fcode> — run two\n      ways over the same source. Array methods are \u003Cstrong>eager\u003C\u002Fstrong>: they walk\n      everything and allocate an array at each step. Iterator helpers are\n      \u003Cstrong>lazy\u003C\u002Fstrong>: they pull one element at a time and stop the moment\n      \u003Ccode>take(n)\u003C\u002Fcode> is full. The counters below are \u003Cstrong>real\u003C\u002Fstrong> — the\n      filter\u002Fmap callbacks are instrumented, so you see exactly how many elements each\n      approach actually touched.\n    \u003C\u002Fp>\n\n    \u003Cdiv class=\"banner info\" id=\"nativeBanner\">\n      ℹ️ Your browser has \u003Cstrong>native\u003C\u002Fstrong> \u003Ccode>Iterator.from()\u003C\u002Fcode> helpers\n      (Baseline 2024) — the lazy side below runs the real thing.\n    \u003C\u002Fdiv>\n\n    \u003Cdiv class=\"panel\">\n      \u003Cdiv class=\"panel-label\">\u003Cspan class=\"n\">1\u003C\u002Fspan> Set up the pipeline\u003C\u002Fdiv>\n      \u003Cdiv class=\"wrap-controls\">\n        \u003Cdiv class=\"field grow\">\n          \u003Clabel class=\"label\" for=\"sizeRange\">Source size — \u003Cspan class=\"mono\" id=\"sizeVal\">10,000\u003C\u002Fspan> numbers\u003C\u002Flabel>\n          \u003Cinput id=\"sizeRange\" type=\"range\" min=\"100\" max=\"100000\" step=\"100\" value=\"10000\" aria-label=\"Source size\" \u002F>\n        \u003C\u002Fdiv>\n        \u003Cdiv class=\"field grow\">\n          \u003Clabel class=\"label\" for=\"takeRange\">Take the first \u003Cspan class=\"mono\" id=\"takeVal\">5\u003C\u002Fspan> results\u003C\u002Flabel>\n          \u003Cinput id=\"takeRange\" type=\"range\" min=\"1\" max=\"50\" step=\"1\" value=\"5\" aria-label=\"How many to take\" \u002F>\n        \u003C\u002Fdiv>\n      \u003C\u002Fdiv>\n      \u003Cdiv class=\"code\" id=\"pipeCode\" style=\"margin-top:14px\">\u003C\u002Fdiv>\n      \u003Cdiv class=\"btn-row\" style=\"margin-top:14px\">\n        \u003Cbutton class=\"btn primary\" id=\"runBtn\">▶ Run both\u003C\u002Fbutton>\n        \u003Cbutton class=\"btn ghost\" id=\"infiniteBtn\">♾️ Infinite source\u003C\u002Fbutton>\n      \u003C\u002Fdiv>\n      \u003Cdiv class=\"presets\" style=\"margin-top:12px\">\n        \u003Cbutton class=\"btn sm\" data-take=\"3\" data-size=\"10000\">first 3 of 10k\u003C\u002Fbutton>\n        \u003Cbutton class=\"btn sm\" data-take=\"5\" data-size=\"100000\">first 5 of 100k\u003C\u002Fbutton>\n        \u003Cbutton class=\"btn sm\" data-take=\"50\" data-size=\"200\">first 50 of 200\u003C\u002Fbutton>\n      \u003C\u002Fdiv>\n      \u003Cp class=\"hint\" id=\"scenarioHint\" style=\"margin-top:12px\">\u003C\u002Fp>\n    \u003C\u002Fdiv>\n\n    \u003Cdiv class=\"cards\">\n      \u003Cdiv class=\"card2 eager\">\n        \u003Ch3>❌ Array methods · eager\u003C\u002Fh3>\n        \u003Cdiv class=\"metric\">\n          \u003Cspan class=\"big-num\" id=\"eagerVisits\">—\u003C\u002Fspan>\n          \u003Cspan class=\"unit\">elements visited\u003C\u002Fspan>\n        \u003C\u002Fdiv>\n        \u003Cdiv class=\"sub-metrics\" id=\"eagerSubs\">\u003C\u002Fdiv>\n        \u003Cdiv class=\"trace\" id=\"eagerTrace\">\u003C\u002Fdiv>\n        \u003Cdiv class=\"result-arr\" id=\"eagerResult\">\u003C\u002Fdiv>\n      \u003C\u002Fdiv>\n      \u003Cdiv class=\"card2 lazy\">\n        \u003Ch3>✅ Iterator helpers · lazy\u003C\u002Fh3>\n        \u003Cdiv class=\"metric\">\n          \u003Cspan class=\"big-num\" id=\"lazyVisits\">—\u003C\u002Fspan>\n          \u003Cspan class=\"unit\">elements visited\u003C\u002Fspan>\n        \u003C\u002Fdiv>\n        \u003Cdiv class=\"sub-metrics\" id=\"lazySubs\">\u003C\u002Fdiv>\n        \u003Cdiv class=\"trace\" id=\"lazyTrace\">\u003C\u002Fdiv>\n        \u003Cdiv class=\"result-arr\" id=\"lazyResult\">\u003C\u002Fdiv>\n      \u003C\u002Fdiv>\n    \u003C\u002Fdiv>\n\n    \u003Cdiv class=\"status\" style=\"margin-top:14px\" role=\"status\" aria-live=\"polite\">\n      \u003Cspan class=\"dot\" id=\"dot\">\u003C\u002Fspan>\u003Cspan id=\"statusText\">Press \u003Cstrong>Run both\u003C\u002Fstrong> to compare.\u003C\u002Fspan>\n    \u003C\u002Fdiv>\n\n    \u003Cdiv class=\"panel\" style=\"margin-top:14px\">\n      \u003Cdiv class=\"panel-label\">\u003Cspan class=\"n\">2\u003C\u002Fspan> Same output, different work\u003C\u002Fdiv>\n      \u003Cp class=\"hint\" style=\"margin-top:0\">\n        Both pipelines return the identical array. The difference is everything you\n        \u003Cem>didn't\u003C\u002Fem> see: the eager version visited the whole source and allocated a\n        fresh intermediate array after \u003Ccode>filter\u003C\u002Fcode> and again after \u003Ccode>map\u003C\u002Fcode>.\n        The lazy version threaded each element straight through and stopped early —\n        \u003Cspan id=\"ratioLine\">no wasted allocations, no wasted passes\u003C\u002Fspan>.\n      \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 provides tokens, layout, panels, buttons,\n     segmented control, sliders, code, status, pills, log, big-num, etc. *\u002F\n\n  \u002F* Two result cards side by side: eager vs lazy. *\u002F\n  .cards { display: grid; grid-template-columns: 1fr 1fr; gap: 14px; }\n  @media (max-width: 560px) { .cards { grid-template-columns: 1fr; } }\n  .card2 { border: 1px solid var(--line); border-radius: var(--radius-sm); background: var(--panel-2);\n    padding: 14px; display: flex; flex-direction: column; gap: 12px; }\n  .card2.eager { border-color: color-mix(in srgb, var(--bad) 40%, var(--line)); }\n  .card2.lazy  { border-color: color-mix(in srgb, var(--good) 40%, var(--line)); }\n  .card2 h3 { margin: 0; font-size: 0.82rem; text-transform: uppercase; letter-spacing: .05em;\n    display: flex; align-items: center; gap: 8px; }\n  .card2.eager h3 { color: var(--bad); }\n  .card2.lazy  h3 { color: var(--good); }\n\n  \u002F* The headline metric: elements the pipeline actually visited. *\u002F\n  .metric { display: flex; align-items: baseline; gap: 8px; }\n  .metric .big-num { color: var(--ink); }\n  .metric .unit { font-size: 0.72rem; color: var(--muted); font-weight: 700; }\n  .sub-metrics { display: flex; flex-wrap: wrap; gap: 6px; }\n\n  \u002F* Per-element visit trace: a strip of dots, filled as each element is touched. *\u002F\n  .trace { display: flex; flex-wrap: wrap; gap: 3px; }\n  .trace .d { width: 9px; height: 9px; border-radius: 2px; background: var(--line); }\n  .trace .d.seen { background: var(--warn); }\n  .trace .d.kept { background: var(--good); }\n  .trace .more { font-size: 0.66rem; color: var(--faint); align-self: center; margin-left: 4px; }\n\n  .result-arr { font-family: var(--mono); font-size: 0.8rem; color: var(--accent); word-break: break-all; }","\"use strict\";\nconst $ = (id) => document.getElementById(id);\n\nconst HAS_NATIVE =\n  typeof Iterator !== \"undefined\" && typeof Iterator.from === \"function\" &&\n  typeof Iterator.from([]).take === \"function\";\n\nconst state = { size: 10000, take: 5, infinite: false };\nconst MAX_DOTS = 60; \u002F\u002F cap the visual trace; the counter is exact regardless\n\n\u002F\u002F The two callbacks, instrumented so every invocation is counted for real.\nfunction makePipeline() {\n  const counts = { filterCalls: 0, mapCalls: 0 };\n  const isEven = (n) => { counts.filterCalls++; return n % 2 === 0; };\n  const square = (n) => { counts.mapCalls++; return n * n; };\n  return { counts, isEven, square };\n}\n\n\u002F\u002F Eager: plain array methods. filter allocates, map allocates, slice allocates.\n\u002F\u002F We also count how many elements the FIRST pass had to visit (the whole source).\nfunction runEager(source, take) {\n  const { counts, isEven, square } = makePipeline();\n  const filtered = source.filter(isEven);        \u002F\u002F allocates array #1, visits ALL\n  const mapped = filtered.map(square);            \u002F\u002F allocates array #2\n  const result = mapped.slice(0, take);           \u002F\u002F allocates array #3\n  return {\n    visited: source.length,                       \u002F\u002F filter walked the entire source\n    filterCalls: counts.filterCalls,\n    mapCalls: counts.mapCalls,\n    allocs: 3,\n    result,\n  };\n}\n\n\u002F\u002F Lazy: native iterator helpers when present, else a faithful generator pipeline.\n\u002F\u002F Either way the SAME instrumented callbacks are used, so filter\u002Fmap call counts\n\u002F\u002F are the genuine number of elements pulled through the chain.\nfunction* lazyFilter(src, pred) { for (const v of src) if (pred(v)) yield v; }\nfunction* lazyMap(src, fn) { for (const v of src) yield fn(v); }\nfunction* lazyTake(src, n) { let i = 0; if (n \u003C= 0) return; for (const v of src) { yield v; if (++i >= n) return; } }\n\nfunction runLazy(source, take) {\n  const { counts, isEven, square } = makePipeline();\n  const out = [];\n  if (HAS_NATIVE) {\n    for (const v of Iterator.from(source).filter(isEven).map(square).take(take)) out.push(v);\n  } else {\n    for (const v of lazyTake(lazyMap(lazyFilter(source, isEven), square), take)) out.push(v);\n  }\n  \u002F\u002F Elements visited = how many source items the filter had to inspect before\n  \u002F\u002F take was satisfied. That's exactly filterCalls (one filter test per pulled item).\n  return {\n    visited: counts.filterCalls,\n    filterCalls: counts.filterCalls,\n    mapCalls: counts.mapCalls,\n    allocs: 0,\n    result: out,\n  };\n}\n\nfunction pill(text, cls) { return '\u003Cspan class=\"pill ' + (cls || \"\") + '\">' + text + \"\u003C\u002Fspan>\"; }\n\n\u002F\u002F A dot strip: warn = element was inspected, good = element survived into the result.\nfunction renderTrace(el, visited, keptCount, sourceLen) {\n  el.innerHTML = \"\";\n  const shown = Math.min(visited, MAX_DOTS);\n  for (let i = 0; i \u003C shown; i++) {\n    const d = document.createElement(\"span\");\n    d.className = \"d \" + (i \u003C keptCount ? \"kept\" : \"seen\");\n    el.appendChild(d);\n  }\n  if (visited > MAX_DOTS) {\n    const more = document.createElement(\"span\");\n    more.className = \"more\";\n    more.textContent = \"+\" + (visited - MAX_DOTS).toLocaleString() + \" more visited\";\n    el.appendChild(more);\n  } else if (visited \u003C sourceLen) {\n    const more = document.createElement(\"span\");\n    more.className = \"more\";\n    more.textContent = \"· \" + (sourceLen - visited).toLocaleString() + \" never touched\";\n    el.appendChild(more);\n  }\n}\n\nfunction fmt(n) { return n.toLocaleString(); }\n\nfunction renderCode() {\n  $(\"pipeCode\").innerHTML =\n    '\u003Cspan class=\"tok-cmt\">\u002F\u002F eager — walks all ' + fmt(state.size) + ', allocates 3 arrays\u003C\u002Fspan>\\n' +\n    'source\u003Cspan class=\"tok-prop\">.filter\u003C\u002Fspan>(isEven)\u003Cspan class=\"tok-prop\">.map\u003C\u002Fspan>(square)\u003Cspan class=\"tok-prop\">.slice\u003C\u002Fspan>(0, ' + state.take + ')\\n\\n' +\n    '\u003Cspan class=\"tok-cmt\">\u002F\u002F lazy — pulls one at a time, stops at ' + state.take + '\u003C\u002Fspan>\\n' +\n    '\u003Cspan class=\"tok-kw\">Iterator\u003C\u002Fspan>.from(source)\u003Cspan class=\"tok-prop\">.filter\u003C\u002Fspan>(isEven)\u003Cspan class=\"tok-prop\">.map\u003C\u002Fspan>(square)\u003Cspan class=\"tok-prop\">.take\u003C\u002Fspan>(' + state.take + ')\u003Cspan class=\"tok-prop\">.toArray\u003C\u002Fspan>()';\n}\n\nfunction run() {\n  renderCode();\n\n  \u002F\u002F Build the source. For the \"infinite\" scenario the array version genuinely\n  \u002F\u002F cannot run, so we show that failure honestly and only run the lazy side.\n  if (state.infinite) {\n    runInfinite();\n    return;\n  }\n\n  const source = Array.from({ length: state.size }, (_, i) => i + 1);\n  const e = runEager(source, state.take);\n  const l = runLazy(source, state.take);\n\n  $(\"eagerVisits\").textContent = fmt(e.visited);\n  $(\"lazyVisits\").textContent = fmt(l.visited);\n\n  $(\"eagerSubs\").innerHTML =\n    pill(\"filter ×\" + fmt(e.filterCalls), \"bad\") + pill(\"map ×\" + fmt(e.mapCalls), \"bad\") +\n    pill(e.allocs + \" arrays allocated\", \"bad\");\n  $(\"lazySubs\").innerHTML =\n    pill(\"filter ×\" + fmt(l.filterCalls), \"good\") + pill(\"map ×\" + fmt(l.mapCalls), \"good\") +\n    pill(\"0 arrays allocated\", \"good\");\n\n  renderTrace($(\"eagerTrace\"), e.visited, e.result.length, source.length);\n  renderTrace($(\"lazyTrace\"), l.visited, l.result.length, source.length);\n\n  const preview = (arr) => \"[\" + arr.slice(0, 8).join(\", \") + (arr.length > 8 ? \", …\" : \"\") + \"]\";\n  $(\"eagerResult\").textContent = preview(e.result);\n  $(\"lazyResult\").textContent = preview(l.result);\n\n  const saved = e.visited - l.visited;\n  const times = l.visited > 0 ? (e.visited \u002F l.visited) : e.visited;\n  $(\"dot\").className = \"dot good\";\n  $(\"statusText\").innerHTML =\n    \"Same result \u003Ccode>\" + preview(l.result) + \"\u003C\u002Fcode>. Eager visited \u003Cstrong>\" + fmt(e.visited) +\n    \"\u003C\u002Fstrong>, lazy visited \u003Cstrong>\" + fmt(l.visited) + \"\u003C\u002Fstrong> — \u003Cstrong>\" + fmt(saved) +\n    \"\u003C\u002Fstrong> elements the lazy pipeline never touched.\";\n  $(\"ratioLine\").innerHTML = l.visited > 0\n    ? \"the lazy pipeline did \u003Cstrong>\" + times.toFixed(times >= 10 ? 0 : 1) + \"×\u003C\u002Fstrong> less work here\"\n    : \"the lazy pipeline touched nothing extra\";\n}\n\nfunction runInfinite() {\n  \u002F\u002F An endless generator: 1, 2, 3, … forever.\n  function* naturals() { let n = 1; while (true) yield n++; }\n\n  const { counts, isEven, square } = makePipeline();\n  const out = [];\n  if (HAS_NATIVE) {\n    for (const v of Iterator.from(naturals()).filter(isEven).map(square).take(state.take)) out.push(v);\n  } else {\n    for (const v of lazyTake(lazyMap(lazyFilter(naturals(), isEven), square), state.take)) out.push(v);\n  }\n\n  $(\"eagerVisits\").textContent = \"∞\";\n  $(\"lazyVisits\").textContent = fmt(counts.filterCalls);\n  $(\"eagerSubs\").innerHTML = pill(\"never terminates\", \"bad\");\n  $(\"lazySubs\").innerHTML =\n    pill(\"filter ×\" + fmt(counts.filterCalls), \"good\") + pill(\"map ×\" + fmt(counts.mapCalls), \"good\") +\n    pill(\"0 arrays allocated\", \"good\");\n  $(\"eagerTrace\").innerHTML = '\u003Cspan class=\"more\">source.filter() would loop forever before it could return\u003C\u002Fspan>';\n  renderTrace($(\"lazyTrace\"), counts.filterCalls, out.length, counts.filterCalls);\n\n  const preview = (arr) => \"[\" + arr.join(\", \") + \"]\";\n  $(\"eagerResult\").textContent = \"\u002F\u002F 💥 hangs — array never finishes\";\n  $(\"lazyResult\").textContent = preview(out);\n\n  $(\"dot\").className = \"dot warn\";\n  $(\"statusText\").innerHTML =\n    \"On an \u003Cstrong>infinite\u003C\u002Fstrong> source, \u003Ccode>source.filter(...)\u003C\u002Fcode> can never return — it has to reach the end first. \" +\n    \"The lazy pipeline just pulled \u003Cstrong>\" + fmt(counts.filterCalls) + \"\u003C\u002Fstrong> elements and stopped: \u003Ccode>\" + preview(out) + \"\u003C\u002Fcode>.\";\n  $(\"ratioLine\").innerHTML = \"lazy is the \u003Cstrong>only\u003C\u002Fstrong> option that terminates\";\n}\n\nfunction setScenarioHint() {\n  $(\"scenarioHint\").innerHTML = state.infinite\n    ? \"Infinite mode: the source yields \u003Ccode>1, 2, 3, …\u003C\u002Fcode> forever. Watch the eager side give up.\"\n    : \"Only the first \u003Cstrong>\" + state.take + \"\u003C\u002Fstrong> results are wanted. The eager side still processes all \u003Cstrong>\" + fmt(state.size) + \"\u003C\u002Fstrong>.\";\n}\n\n\u002F\u002F Controls.\n$(\"sizeRange\").addEventListener(\"input\", () => {\n  state.size = Number($(\"sizeRange\").value);\n  $(\"sizeVal\").textContent = fmt(state.size);\n  state.infinite = false;\n  setScenarioHint(); renderCode();\n});\n$(\"takeRange\").addEventListener(\"input\", () => {\n  state.take = Number($(\"takeRange\").value);\n  $(\"takeVal\").textContent = fmt(state.take);\n  setScenarioHint(); renderCode();\n});\n$(\"runBtn\").addEventListener(\"click\", () => { state.infinite = false; setScenarioHint(); run(); });\n$(\"infiniteBtn\").addEventListener(\"click\", () => { state.infinite = true; setScenarioHint(); run(); });\ndocument.querySelectorAll(\"[data-take]\").forEach((btn) =>\n  btn.addEventListener(\"click\", () => {\n    state.infinite = false;\n    state.take = Number(btn.dataset.take);\n    state.size = Number(btn.dataset.size);\n    $(\"takeRange\").value = String(state.take); $(\"takeVal\").textContent = fmt(state.take);\n    $(\"sizeRange\").value = String(state.size); $(\"sizeVal\").textContent = fmt(state.size);\n    setScenarioHint(); run();\n  }));\n\nif (!HAS_NATIVE) {\n  const b = $(\"nativeBanner\");\n  b.classList.remove(\"info\");\n  b.innerHTML = \"ℹ️ Your browser doesn't expose native \u003Ccode>Iterator.from()\u003C\u002Fcode> yet — the lazy side runs a faithful generator-based equivalent, so the counters are still exact.\";\n}\n$(\"nativeBanner\").classList.add(\"show\");\n\nsetScenarioHint();\nrenderCode();\nrun();",{"height":40,"autorun":17},520,{"slug":33,"title":42,"excerpt":43,"coverUrl":44},"Array methods are eager. Iterator helpers are lazy. Here's why that matters.","Every `.map()` and `.filter()` on an array creates a new intermediate array. Iterator helpers — built into JavaScript since 2024 — give you the same pipeline but evaluate one element at a time, with no wasted allocations.","\u002Fmedia\u002Fcovers\u002Fiterator-helpers-lazy-filter-map.png"]