Add index property to Transform

This makes it possible to know which line is being transformed.

Fix: #611

readme.md

@@ -1373,7 +1373,33 @@ render(<Example />);
 
 Since `transform` function converts all characters to upper case, final output that's rendered to the terminal will be "HELLO WORLD", not "Hello World".
 
-#### transform(children)
+When the output wraps to multiple lines, it can be helpful to know which line is being processed.
+
+For example, to implement a hanging indent component, you can indent all the lines except for the first.
+
+```jsx
+import {render, Transform} from 'ink';
+
+const HangingIndent = ({ content, indent = 4, children, ...props }) => (
+	<Transform transform={(line, index) =>
+		index === 0 ? line : (' '.repeat(indent) + line)} {...props}>
+	{children}
+	</Transform>
+)
+
+const text =
+	'WHEN I WROTE the following pages, or rather the bulk of them, ' +
+	'I lived alone, in the woods, a mile from any neighbor, in a ' +
+	'house which I had built myself, on the shore of Walden Pond, ' +
+	'in Concord, Massachusetts, and earned my living by the labor ' +
+	'of my hands only. I lived there two years and two months. At ' +
+	'present I am a sojourner in civilized life again.'
+
+// Other text properties allowed as well
+render(<HangingIndent bold dimColor indent={4}>{text}</HangingIndent>)
+```
+
+#### transform(outputLine, index)
 
 Type: `Function`
 
@@ -1386,6 +1412,12 @@ Type: `string`
 
 Output of child components.
 
+##### index
+
+Type: `number`
+
+The zero-indexed line number of the line currently being transformed.
+
 ## Hooks
 
 ### useInput(inputHandler, options?)

src/components/Transform.tsx

@@ -4,7 +4,7 @@ export type Props = {
 	/**
 	 * Function which transforms children output. It accepts children and must return transformed children too.
 	 */
-	readonly transform: (children: string) => string;
+	readonly transform: (children: string, index: number) => string;
 
 	readonly children?: ReactNode;
 };

src/global.d.ts

@@ -28,6 +28,6 @@ declare namespace Ink {
 		style?: Styles;
 
 		// eslint-disable-next-line @typescript-eslint/naming-convention
-		internal_transform?: (children: string) => string;
+		internal_transform?: (children: string, index: number) => string;
 	};
 }

src/output.ts

@@ -187,7 +187,9 @@ export default class Output {
 
 				let offsetY = 0;
 
-				for (let line of lines) {
+				for (let index = 0; index < lines.length; index++) {
+					let line = lines[index];
+					if (line === undefined) continue;
 					const currentLine = output[y + offsetY];
 
 					// Line can be missing if `text` is taller than height of pre-initialized `this.output`
@@ -196,7 +198,7 @@ export default class Output {
 					}
 
 					for (const transformer of transformers) {
-						line = transformer(line);
+						line = transformer(line, index);
 					}
 
 					const characters = styledCharsFromTokens(tokenize(line));

src/render-node-to-output.ts

@@ -27,7 +27,7 @@ const applyPaddingToText = (node: DOMElement, text: string): string => {
 	return text;
 };
 
-export type OutputTransformer = (s: string) => string;
+export type OutputTransformer = (s: string, index: number) => string;
 
 // After nodes are laid out, render each to output object, which later gets rendered to terminal
 const renderNodeToOutput = (

src/squash-text-nodes.ts

@@ -9,32 +9,32 @@ import {type DOMElement} from './dom.js';
 const squashTextNodes = (node: DOMElement): string => {
 	let text = '';
 
-	if (node.childNodes.length > 0) {
-		for (const childNode of node.childNodes) {
-			let nodeText = '';
+	for (let index = 0; index < node.childNodes.length; index++) {
+		const childNode = node.childNodes[index];
+		if (childNode === undefined) continue;
+		let nodeText = '';
 
-			if (childNode.nodeName === '#text') {
-				nodeText = childNode.nodeValue;
-			} else {
-				if (
-					childNode.nodeName === 'ink-text' ||
-					childNode.nodeName === 'ink-virtual-text'
-				) {
-					nodeText = squashTextNodes(childNode);
-				}
-
-				// Since these text nodes are being concatenated, `Output` instance won't be able to
-				// apply children transform, so we have to do it manually here for each text node
-				if (
-					nodeText.length > 0 &&
-					typeof childNode.internal_transform === 'function'
-				) {
-					nodeText = childNode.internal_transform(nodeText);
-				}
+		if (childNode.nodeName === '#text') {
+			nodeText = childNode.nodeValue;
+		} else {
+			if (
+				childNode.nodeName === 'ink-text' ||
+				childNode.nodeName === 'ink-virtual-text'
+			) {
+				nodeText = squashTextNodes(childNode);
 			}
 
-			text += nodeText;
+			// Since these text nodes are being concatenated, `Output` instance won't be able to
+			// apply children transform, so we have to do it manually here for each text node
+			if (
+				nodeText.length > 0 &&
+				typeof childNode.internal_transform === 'function'
+			) {
+				nodeText = childNode.internal_transform(nodeText, index);
+			}
 		}
+
+		text += nodeText;
 	}
 
 	return text;

test/components.tsx

@@ -253,46 +253,58 @@ test('fragment', t => {
 
 test('transform children', t => {
 	const output = renderToString(
-		<Transform transform={(string: string) => `[${string}]`}>
+		<Transform transform={(string: string, index: number) => `[${index}: ${string}]`}>
 			<Text>
-				<Transform transform={(string: string) => `{${string}}`}>
+				<Transform transform={(string: string, index: number) => `{${index}: ${string}}`}>
 					<Text>test</Text>
 				</Transform>
 			</Text>
 		</Transform>
 	);
 
-	t.is(output, '[{test}]');
+	t.is(output, '[0: {0: test}]');
 });
 
 test('squash multiple text nodes', t => {
 	const output = renderToString(
-		<Transform transform={(string: string) => `[${string}]`}>
+		<Transform transform={(string: string, index: number) => `[${index}: ${string}]`}>
 			<Text>
-				<Transform transform={(string: string) => `{${string}}`}>
+				<Transform transform={(string: string, index: number) => `{${index}: ${string}}`}>
 					{/* prettier-ignore */}
 					<Text>hello{' '}world</Text>
 				</Transform>
 			</Text>
 		</Transform>
 	);
 
-	t.is(output, '[{hello world}]');
+	t.is(output, '[0: {0: hello world}]');
 });
 
+test('transform with multiple lines', t => {
+	const output = renderToString(
+		<Transform transform={(string: string, index: number) => `[${index}: ${string}]`}>
+			{/* prettier-ignore */}
+			<Text>hello{' '}world{'\n'}goodbye{' '}world</Text>
+		</Transform>
+	);
+
+	t.is(output, '[0: hello world]\n[1: goodbye world]');
+});
+
+
 test('squash multiple nested text nodes', t => {
 	const output = renderToString(
-		<Transform transform={(string: string) => `[${string}]`}>
+		<Transform transform={(string: string, index: number) => `[${index}: ${string}]`}>
 			<Text>
-				<Transform transform={(string: string) => `{${string}}`}>
+				<Transform transform={(string: string, index: number) => `{${index}: ${string}}`}>
 					hello
 					<Text> world</Text>
 				</Transform>
 			</Text>
 		</Transform>
 	);
 
-	t.is(output, '[{hello world}]');
+	t.is(output, '[0: {0: hello world}]');
 });
 
 test('squash empty `<Text>` nodes', t => {