repl: support previews by eager evaluating input

This adds input previews by using the inspectors eager evaluation
functionality.
It is implemented as additional line that is not counted towards
the actual input. In case no colors are supported, it will be visible
as comment. Otherwise it's grey.
It will be triggered on any line change. It is heavily tested against
edge cases and adheres to "dumb" terminals (previews are deactived
in that case).

Fixes: #20977

Author: BridgeAR

doc/api/repl.md

@@ -510,6 +510,10 @@ with REPL instances programmatically.
 <!-- YAML
 added: v0.1.91
 changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/30811
+    description: The `preview` option is available from now on. The input
+                 generates output previews from now on.
   - version: v12.0.0
     pr-url: https://github.com/nodejs/node/pull/26518
     description: The `terminal` option now follows the default description in
@@ -562,6 +566,8 @@ changes:
   * `breakEvalOnSigint` {boolean} Stop evaluating the current piece of code when
     `SIGINT` is received, such as when `Ctrl+C` is pressed. This cannot be used
     together with a custom `eval` function. **Default:** `false`.
+  * `preview` {boolean} Defines if the repl prints output previews or not.
+     **Default:** `true`. Always `false` in case `terminal` is falsy.
 * Returns: {repl.REPLServer}
 
 The `repl.start()` method creates and starts a [`repl.REPLServer`][] instance.

lib/internal/repl/utils.js

@@ -4,7 +4,10 @@ const {
   Symbol,
 } = primordials;
 
-const acorn = require('internal/deps/acorn/acorn/dist/acorn');
+const { MathMin } = primordials;
+
+const { tokTypes: tt, Parser: AcornParser } =
+  require('internal/deps/acorn/acorn/dist/acorn');
 const privateMethods =
   require('internal/deps/acorn-plugins/acorn-private-methods/index');
 const classFields =
@@ -13,7 +16,30 @@ const numericSeparator =
   require('internal/deps/acorn-plugins/acorn-numeric-separator/index');
 const staticClassFeatures =
   require('internal/deps/acorn-plugins/acorn-static-class-features/index');
-const { tokTypes: tt, Parser: AcornParser } = acorn;
+
+const { sendInspectorCommand } = require('internal/util/inspector');
+
+const {
+  ERR_INSPECTOR_NOT_AVAILABLE
+} = require('internal/errors').codes;
+
+const {
+  clearLine,
+  cursorTo,
+  moveCursor,
+} = require('readline');
+
+const { inspect } = require('util');
+
+const debug = require('internal/util/debuglog').debuglog('repl');
+
+const inspectOptions = {
+  depth: 1,
+  colors: false,
+  compact: true,
+  breakLength: Infinity
+};
+const inspectedOptions = inspect(inspectOptions, { colors: false });
 
 // If the error is that we've unexpectedly ended the input,
 // then let the user try to recover by adding more input.
@@ -91,7 +117,144 @@ function isRecoverableError(e, code) {
   }
 }
 
+function setupPreview(repl, contextSymbol, bufferSymbol, active) {
+  // Simple terminals can't handle previews.
+  if (process.env.TERM === 'dumb' || !active) {
+    return { showInputPreview() {}, clearPreview() {} };
+  }
+
+  let preview = null;
+  let lastPreview = '';
+
+  const clearPreview = () => {
+    if (preview !== null) {
+      moveCursor(repl.output, 0, 1);
+      clearLine(repl.output);
+      moveCursor(repl.output, 0, -1);
+      lastPreview = preview;
+      preview = null;
+    }
+  };
+
+  // This returns a code preview for arbitrary input code.
+  function getPreviewInput(input, callback) {
+    // For similar reasons as `defaultEval`, wrap expressions starting with a
+    // curly brace with parenthesis.
+    if (input.startsWith('{') && !input.endsWith(';')) {
+      input = `(${input})`;
+    }
+    sendInspectorCommand((session) => {
+      session.post('Runtime.evaluate', {
+        expression: input,
+        throwOnSideEffect: true,
+        timeout: 333,
+        contextId: repl[contextSymbol],
+      }, (error, preview) => {
+        if (error) {
+          callback(error);
+          return;
+        }
+        const { result } = preview;
+        if (result.value !== undefined) {
+          callback(null, inspect(result.value, inspectOptions));
+        // Ignore EvalErrors, SyntaxErrors and ReferenceErrors. It is not clear
+        // where they came from and if they are recoverable or not. Other errors
+        // may be inspected.
+        } else if (preview.exceptionDetails &&
+                   (result.className === 'EvalError' ||
+                     result.className === 'SyntaxError' ||
+                     result.className === 'ReferenceError')) {
+          callback(null, null);
+        } else if (result.objectId) {
+          session.post('Runtime.callFunctionOn', {
+            functionDeclaration: `(v) => util.inspect(v, ${inspectedOptions})`,
+            objectId: result.objectId,
+            arguments: [result]
+          }, (error, preview) => {
+            if (error) {
+              callback(error);
+            } else {
+              callback(null, preview.result.value);
+            }
+          });
+        } else {
+          // Either not serializable or undefined.
+          callback(null, result.unserializableValue || result.type);
+        }
+      });
+    }, () => callback(new ERR_INSPECTOR_NOT_AVAILABLE()));
+  }
+
+  const showInputPreview = () => {
+    // Prevent duplicated previews after a refresh.
+    if (preview !== null) {
+      return;
+    }
+
+    const line = repl.line.trim();
+
+    // Do not preview if the command is buffered or if the line is empty.
+    if (repl[bufferSymbol] || line === '') {
+      return;
+    }
+
+    getPreviewInput(line, (error, inspected) => {
+      // Ignore the output if the value is identical to the current line and the
+      // former preview is not identical to this preview.
+      if ((line === inspected && lastPreview !== inspected) ||
+          inspected === null) {
+        return;
+      }
+      if (error) {
+        debug('Error while generating preview', error);
+        return;
+      }
+      // Do not preview `undefined` if colors are deactivated or explicitly
+      // requested.
+      if (inspected === 'undefined' &&
+          (!repl.useColors || repl.ignoreUndefined)) {
+        return;
+      }
+
+      preview = inspected;
+
+      // Limit the output to maximum 250 characters. Otherwise it becomes a)
+      // difficult to read and b) non terminal REPLs would visualize the whole
+      // output.
+      const maxColumns = MathMin(repl.columns, 250);
+
+      if (inspected.length > maxColumns) {
+        inspected = `${inspected.slice(0, maxColumns - 6)}...`;
+      }
+      const lineBreakPos = inspected.indexOf('\n');
+      if (lineBreakPos !== -1) {
+        inspected = `${inspected.slice(0, lineBreakPos)}`;
+      }
+      const result = repl.useColors ?
+        `\u001b[90m${inspected}\u001b[39m` :
+        `// ${inspected}`;
+
+      repl.output.write(`\n${result}`);
+      moveCursor(repl.output, 0, -1);
+      cursorTo(repl.output, repl.cursor + repl._prompt.length);
+    });
+  };
+
+  // Refresh prints the whole screen again and the preview will be removed
+  // during that procedure. Print the preview again. This also makes sure
+  // the preview is always correct after resizing the terminal window.
+  const tmpRefresh = repl._refreshLine.bind(repl);
+  repl._refreshLine = () => {
+    preview = null;
+    tmpRefresh();
+    showInputPreview();
+  };
+
+  return { showInputPreview, clearPreview };
+}
+
 module.exports = {
   isRecoverableError,
-  kStandaloneREPL: Symbol('kStandaloneREPL')
+  kStandaloneREPL: Symbol('kStandaloneREPL'),
+  setupPreview
 };

lib/repl.js

@@ -98,7 +98,8 @@ const experimentalREPLAwait = require('internal/options').getOptionValue(
 );
 const {
   isRecoverableError,
-  kStandaloneREPL
+  kStandaloneREPL,
+  setupPreview,
 } = require('internal/repl/utils');
 const {
   getOwnNonIndexProperties,
@@ -204,6 +205,9 @@ function REPLServer(prompt,
     }
   }
 
+  const preview = options.terminal &&
+    (options.preview !== undefined ? !!options.preview : true);
+
   this.inputStream = options.input;
   this.outputStream = options.output;
   this.useColors = !!options.useColors;
@@ -804,9 +808,20 @@ function REPLServer(prompt,
     }
   });
 
+  const {
+    clearPreview,
+    showInputPreview
+  } = setupPreview(
+    this,
+    kContextId,
+    kBufferedCommandSymbol,
+    preview
+  );
+
   // Wrap readline tty to enable editor mode and pausing.
   const ttyWrite = self._ttyWrite.bind(self);
   self._ttyWrite = (d, key) => {
+    clearPreview();
     key = key || {};
     if (paused && !(self.breakEvalOnSigint && key.ctrl && key.name === 'c')) {
       pausedBuffer.push(['key', [d, key]]);
@@ -819,6 +834,7 @@ function REPLServer(prompt,
         self.clearLine();
       }
       ttyWrite(d, key);
+      showInputPreview();
       return;
     }
 

test/parallel/test-repl-history-navigation.js

@@ -46,28 +46,50 @@ ActionStream.prototype.readable = true;
 const ENTER = { name: 'enter' };
 const UP = { name: 'up' };
 const DOWN = { name: 'down' };
+const LEFT = { name: 'left' };
+const DELETE = { name: 'delete' };
 
 const prompt = '> ';
 
+const prev = process.features.inspector;
+
 const tests = [
   { // Creates few history to navigate for
     env: { NODE_REPL_HISTORY: defaultHistoryPath },
     test: [ 'let ab = 45', ENTER,
             '555 + 909', ENTER,
-            '{key : {key2 :[] }}', ENTER],
+            '{key : {key2 :[] }}', ENTER,
+            'Array(100).fill(1).map((e, i) => i ** i)', LEFT, LEFT, DELETE,
+            '2', ENTER],
     expected: [],
     clean: false
   },
   {
     env: { NODE_REPL_HISTORY: defaultHistoryPath },
-    test: [UP, UP, UP, UP, DOWN, DOWN, DOWN],
+    test: [UP, UP, UP, UP, UP, DOWN, DOWN, DOWN, DOWN],
     expected: [prompt,
+               `${prompt}Array(100).fill(1).map((e, i) => i ** 2)`,
+               prev && '\n// [ 0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, ' +
+                 '144, 169, 196, 225, 256, 289, 324, 361, 400, 441, 484, 529,' +
+                 ' 576, 625, 676, 729, 784, 841, 900, 961, 1024, 1089, 1156, ' +
+                 '1225, 1296, 1369, 1444, 1521, 1600, 1681, 1764, 1849, 1936,' +
+                 ' 2025, 2116, 2209, ...',
                `${prompt}{key : {key2 :[] }}`,
+               prev && '\n// { key: { key2: [] } }',
                `${prompt}555 + 909`,
+               prev && '\n// 1464',
                `${prompt}let ab = 45`,
                `${prompt}555 + 909`,
+               prev && '\n// 1464',
                `${prompt}{key : {key2 :[] }}`,
-               prompt],
+               prev && '\n// { key: { key2: [] } }',
+               `${prompt}Array(100).fill(1).map((e, i) => i ** 2)`,
+               prev && '\n// [ 0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, ' +
+                 '144, 169, 196, 225, 256, 289, 324, 361, 400, 441, 484, 529,' +
+                 ' 576, 625, 676, 729, 784, 841, 900, 961, 1024, 1089, 1156, ' +
+                 '1225, 1296, 1369, 1444, 1521, 1600, 1681, 1764, 1849, 1936,' +
+                 ' 2025, 2116, 2209, ...',
+               prompt].filter((e) => typeof e === 'string'),
     clean: true
   }
 ];

test/parallel/test-repl-multiline.js

@@ -3,34 +3,44 @@ const common = require('../common');
 const ArrayStream = require('../common/arraystream');
 const assert = require('assert');
 const repl = require('repl');
-const inputStream = new ArrayStream();
-const outputStream = new ArrayStream();
-const input = ['const foo = {', '};', 'foo;'];
-let output = '';
+const input = ['const foo = {', '};', 'foo'];
 
-outputStream.write = (data) => { output += data.replace('\r', ''); };
+function run({ useColors }) {
+  const inputStream = new ArrayStream();
+  const outputStream = new ArrayStream();
+  let output = '';
 
-const r = repl.start({
-  prompt: '',
-  input: inputStream,
-  output: outputStream,
-  terminal: true,
-  useColors: false
-});
+  outputStream.write = (data) => { output += data.replace('\r', ''); };
 
-r.on('exit', common.mustCall(() => {
-  const actual = output.split('\n');
+  const r = repl.start({
+    prompt: '',
+    input: inputStream,
+    output: outputStream,
+    terminal: true,
+    useColors
+  });
 
-  // Validate the output, which contains terminal escape codes.
-  assert.strictEqual(actual.length, 6);
-  assert.ok(actual[0].endsWith(input[0]));
-  assert.ok(actual[1].includes('... '));
-  assert.ok(actual[1].endsWith(input[1]));
-  assert.strictEqual(actual[2], 'undefined');
-  assert.ok(actual[3].endsWith(input[2]));
-  assert.strictEqual(actual[4], '{}');
-  // Ignore the last line, which is nothing but escape codes.
-}));
+  r.on('exit', common.mustCall(() => {
+    const actual = output.split('\n');
 
-inputStream.run(input);
-r.close();
+    // Validate the output, which contains terminal escape codes.
+    assert.strictEqual(actual.length, 6 + process.features.inspector);
+    assert.ok(actual[0].endsWith(input[0]));
+    assert.ok(actual[1].includes('... '));
+    assert.ok(actual[1].endsWith(input[1]));
+    assert.ok(actual[2].includes('undefined'));
+    assert.ok(actual[3].endsWith(input[2]));
+    if (process.features.inspector) {
+      assert.ok(actual[4].includes(actual[5]));
+      assert.strictEqual(actual[4].includes('//'), !useColors);
+    }
+    assert.strictEqual(actual[4 + process.features.inspector], '{}');
+    // Ignore the last line, which is nothing but escape codes.
+  }));
+
+  inputStream.run(input);
+  r.close();
+}
+
+run({ useColors: true });
+run({ useColors: false });