31:[] 2f:{"title":"Five-Minute Introduction","cells":[{"id":410,"value":"# Five-Minute Introduction","pinned":false,"hidden":false,"mode":"md"},{"id":417,"value":"md `\n

***Update, March 2022:*** [A Taste of Observable](https://observablehq.com/@observablehq/a-taste-of-observable?collection=@observablehq/overview) is the most recently updated and best introduction to the Observable platform. This notebook will remain for history, but please check out A Taste of Observable!

\n`","pinned":false,"hidden":false,"mode":"ojs"},{"id":0,"value":"md `\nWelcome! This notebook gives a quick overview of Observable. For a more technical introduction, see [Observable’s not JavaScript](/@observablehq/observables-not-javascript). For hands-on, see our [introductory tutorial series](https://observablehq.com/tutorials). To watch rather than read, see our [short introductory video](https://www.youtube.com/watch?v=igsyGfQ5yZ4)!\n\nAn Observable notebook consists of cells. Each cell is a snippet of JavaScript. You can see (and edit!) the code for any cell by clicking the menu in the left margin.`","pinned":false,"hidden":false,"mode":"ojs"},{"id":5,"value":"2 * 3 * 7","pinned":true,"hidden":false,"mode":"ojs"},{"id":15,"value":"{\n let sum = 0;\n for (let i = 0; i <= 100; ++i) {\n sum += i;\n }\n return sum;\n}","pinned":true,"hidden":false,"mode":"ojs"},{"id":31,"value":"md`Cells can have names. This allows a cell’s value to be referenced by other cells.`","pinned":false,"hidden":false,"mode":"ojs"},{"id":44,"value":"color = \"red\"","pinned":true,"hidden":false,"mode":"ojs"},{"id":47,"value":"`My favorite color is ${color}.`","pinned":true,"hidden":false,"mode":"ojs"},{"id":54,"value":"md`A cell referencing another cell is re-evaluated automatically when the referenced value changes. Try editing the definition of *color* above and shift-return to re-evaluate.\n\nCells can generate DOM (HTML, SVG, Canvas, WebGL, *etc.*). You can use the standard DOM API like *document*.createElement, or use the built-in *html* tagged template literal:`","pinned":false,"hidden":false,"mode":"ojs"},{"id":67,"value":"html`\n My favorite language is HTML.\n`","pinned":true,"hidden":false,"mode":"ojs"},{"id":73,"value":"md`There’s a Markdown tagged template literal, too. (This notebook is written in Markdown.)`","pinned":false,"hidden":false,"mode":"ojs"},{"id":77,"value":"md`My favorite language is *Markdown*.`","pinned":true,"hidden":false,"mode":"ojs"},{"id":81,"value":"md`DOM can be made reactive simply by referring to other cells. The next cell refers to *color*. (Try editing the definition of *color* above.)`","pinned":false,"hidden":false,"mode":"ojs"},{"id":83,"value":"html`My favorite color is ${color}.`","pinned":true,"hidden":false,"mode":"ojs"},{"id":93,"value":"md`Sometimes you need to load data from a remote server, or compute something expensive in a web worker. For that, cells can be defined asynchronously using [promises](https://developer.mozilla.org/docs/Web/JavaScript/Guide/Using_promises):`","pinned":false,"hidden":false,"mode":"ojs"},{"id":109,"value":"status = new Promise(resolve => {\n setTimeout(() => {\n resolve({resolved: new Date});\n }, 2000);\n})","pinned":true,"hidden":false,"mode":"ojs"},{"id":117,"value":"md`A cell that refers to a promise cell sees the value when it is resolved; this implicit await means that referencing cells don’t care whether the value is synchronous or not. Edit the *status* cell above to see the cell below update after two seconds.`","pinned":false,"hidden":false,"mode":"ojs"},{"id":112,"value":"status","pinned":true,"hidden":false,"mode":"ojs"},{"id":124,"value":"md`Promises are also useful for loading libraries from npm. Below, \\`require\\` returns a promise that resolves to the d3-fetch library:`","pinned":false,"hidden":false,"mode":"ojs"},{"id":126,"value":"d3 = require(\"d3-fetch@1\")","pinned":true,"hidden":false,"mode":"ojs"},{"id":102,"value":"md`If you prefer, you can use async and await explicitly:`","pinned":false,"hidden":false,"mode":"ojs"},{"id":104,"value":"countries = (await d3.tsv(\"https://cdn.jsdelivr.net/npm/world-atlas@1/world/110m.tsv\"))\n .sort((a, b) => b.pop_est - a.pop_est) // Sort by descending estimated population.\n .slice(0, 10) // Take the top ten.","pinned":true,"hidden":false,"mode":"ojs"},{"id":140,"value":"md`Cells can be defined as [generators](https://developer.mozilla.org/docs/Web/JavaScript/Guide/Iterators_and_Generators#Generators); a value is yielded up to sixty times a second.`","pinned":false,"hidden":false,"mode":"ojs"},{"id":147,"value":"i = {\n let i = 0;\n while (true) {\n yield ++i;\n }\n}","pinned":true,"hidden":false,"mode":"ojs"},{"id":158,"value":"`The current value of i is ${i}.`","pinned":true,"hidden":false,"mode":"ojs"},{"id":151,"value":"md`Any cell that refers to a generator cell sees its current value; the referencing cell is re-evaluated whenever the generator yields a new value. As you might guess, a generator can yield promises for [async iteration](https://github.com/tc39/proposal-async-iteration); referencing cells see the current resolved value.`","pinned":false,"hidden":false,"mode":"ojs"},{"id":168,"value":"date = {\n while (true) {\n yield new Promise(resolve => {\n setTimeout(() => resolve(new Date), 1000);\n });\n }\n}","pinned":true,"hidden":false,"mode":"ojs"},{"id":176,"value":"md`Combining these primitives—promises, generators and DOM—you can build custom user interfaces. Here’s a slider and a generator that yields the slider’s value:`","pinned":false,"hidden":false,"mode":"ojs"},{"id":181,"value":"slider = html``","pinned":true,"hidden":false,"mode":"ojs"},{"id":185,"value":"sliderValue = Generators.input(slider)","pinned":true,"hidden":false,"mode":"ojs"},{"id":197,"value":"md`Generators.input returns a generator that yields promises. The promise resolves whenever the associated input element emits an input event. You don’t need to implement that generator by hand, though. There’s a builtin viewof operator which exposes the current value of a given input element:`","pinned":false,"hidden":false,"mode":"ojs"},{"id":200,"value":"viewof value = html``","pinned":true,"hidden":false,"mode":"ojs"},{"id":203,"value":"value","pinned":true,"hidden":false,"mode":"ojs"},{"id":252,"value":"md`You can import cells from other notebooks. To demonstrate how custom user interfaces can expose arbitrary values to other cells, here’s a brushable scatterplot of cars showing the inverse relationship between horsepower and fuel efficiency.`","pinned":false,"hidden":false,"mode":"ojs"},{"id":242,"value":"import {viewof selection as cars} from \"@d3/brushable-scatterplot\"","pinned":true,"hidden":false,"mode":"ojs"},{"id":244,"value":"viewof cars","pinned":true,"hidden":false,"mode":"ojs"},{"id":248,"value":"cars","pinned":true,"hidden":false,"mode":"ojs"},{"id":264,"value":"md`${Array.from(new Set(cars.map(c => c.name)))\n .sort()\n .map(c => `* ${c}`)\n .join(\"\\n\")}`","pinned":true,"hidden":false,"mode":"ojs"}],"files":"$W31","stdlib":"1","titling":"auto"}