1 files changed, 165 insertions, 3 deletions

diff --git a/lib/internal/repl/utils.js b/lib/internal/repl/utils.js
index 6c011e8533..c4280c1d1f 100644
--- a/lib/internal/repl/utils.js
+++ b/lib/internal/repl/utils.js
@@ -1,10 +1,12 @@
 'use strict';
 
 const {
+  MathMin,
   Symbol,
 } = primordials;
 
-const acorn = require('internal/deps/acorn/acorn/dist/acorn');
+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 +15,30 @@ 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 +116,144 @@ 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
 };