repl: make last error available as `_error`

This is pretty useful when trying to inspect the last
error caught by a REPL, and is made to be analogous to `_`,
which contains the last successful completion value.

PR-URL: https://github.com/nodejs/node/pull/18919
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Vladimir de Turckheim <vlad2t@hotmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Shingo Inoue <leko.noor@gmail.com>

doc/api/repl.md

@@ -142,6 +142,12 @@ global or scoped variable, the input `fs` will be evaluated on-demand as
 ```
 
 #### Assignment of the `_` (underscore) variable
+<!-- YAML
+changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/18919
+    description: Added `_error` support.
+-->
 
 The default evaluator will, by default, assign the result of the most recently
 evaluated expression to the special variable `_` (underscore).
@@ -162,6 +168,17 @@ Expression assignment to _ now disabled.
 4
 ```
 
+Similarly, `_error` will refer to the last seen error, if there was any.
+Explicitly setting `_error` to a value will disable this behavior.
+
+<!-- eslint-skip -->
+```js
+> throw new Error('foo');
+Error: foo
+> _error.message
+'foo'
+```
+
 ### Custom Evaluation Functions
 
 When a new `repl.REPLServer` is created, a custom evaluation function may be

lib/repl.js

@@ -156,6 +156,8 @@ function REPLServer(prompt,
   self.replMode = replMode || exports.REPL_MODE_SLOPPY;
   self.underscoreAssigned = false;
   self.last = undefined;
+  self.underscoreErrAssigned = false;
+  self.lastError = undefined;
   self.breakEvalOnSigint = !!breakEvalOnSigint;
   self.editorMode = false;
   // Context id for use with the inspector protocol.
@@ -388,6 +390,8 @@ function REPLServer(prompt,
       internalUtil.decorateErrorStack(e);
     Error.prepareStackTrace = pstrace;
     const isError = internalUtil.isError(e);
+    if (!self.underscoreErrAssigned)
+      self.lastError = e;
     if (e instanceof SyntaxError && e.stack) {
       // remove repl:line-number and stack trace
       e.stack = e.stack
@@ -796,6 +800,7 @@ REPLServer.prototype.createContext = function() {
 REPLServer.prototype.resetContext = function() {
   this.context = this.createContext();
   this.underscoreAssigned = false;
+  this.underscoreErrAssigned = false;
   this.lines = [];
   this.lines.level = [];
 
@@ -811,6 +816,19 @@ REPLServer.prototype.resetContext = function() {
     }
   });
 
+  Object.defineProperty(this.context, '_error', {
+    configurable: true,
+    get: () => this.lastError,
+    set: (value) => {
+      this.lastError = value;
+      if (!this.underscoreErrAssigned) {
+        this.underscoreErrAssigned = true;
+        this.outputStream.write(
+          'Expression assignment to _error now disabled.\n');
+      }
+    }
+  });
+
   // Allow REPL extensions to extend the new context
   this.emit('reset', this.context);
 };

test/parallel/test-repl-underscore.js

@@ -10,6 +10,7 @@ testStrictMode();
 testResetContext();
 testResetContextGlobal();
 testMagicMode();
+testError();
 
 function testSloppyMode() {
   const r = initRepl(repl.REPL_MODE_SLOPPY);
@@ -153,6 +154,73 @@ function testResetContextGlobal() {
   delete global.require;
 }
 
+function testError() {
+  const r = initRepl(repl.REPL_MODE_STRICT);
+
+  r.write(`_error;                                // initial value undefined
+           throw new Error('foo');                // throws error
+           _error;                                // shows error
+           fs.readdirSync('/nonexistent?');       // throws error, sync
+           _error.code;                           // shows error code
+           _error.syscall;                        // shows error syscall
+           setImmediate(() => { throw new Error('baz'); }); undefined;
+                                                  // throws error, async
+           `);
+
+  setImmediate(() => {
+    const lines = r.output.accum.trim().split('\n');
+    const expectedLines = [
+      'undefined',
+
+      // The error, both from the original throw and the `_error` echo.
+      'Error: foo',
+      'Error: foo',
+
+      // The sync error, with individual property echoes
+      /Error: ENOENT: no such file or directory, scandir '.*nonexistent.*'/,
+      /fs\.readdirSync/,
+      "'ENOENT'",
+      "'scandir'",
+
+      // Dummy 'undefined' from the explicit silencer + one from the comment
+      'undefined',
+      'undefined',
+
+      // The message from the original throw
+      'Error: baz',
+      /setImmediate/,
+      /^    at/,
+      /^    at/,
+      /^    at/,
+      /^    at/,
+    ];
+    for (const line of lines) {
+      const expected = expectedLines.shift();
+      if (typeof expected === 'string')
+        assert.strictEqual(line, expected);
+      else
+        assert(expected.test(line), `${line} should match ${expected}`);
+    }
+    assert.strictEqual(expectedLines.length, 0);
+
+    // Reset output, check that '_error' is the asynchronously caught error.
+    r.output.accum = '';
+    r.write(`_error.message                 // show the message
+             _error = 0;                    // disable auto-assignment
+             throw new Error('quux');       // new error
+             _error;                        // should not see the new error
+             `);
+
+    assertOutput(r.output, [
+      "'baz'",
+      'Expression assignment to _error now disabled.',
+      '0',
+      'Error: quux',
+      '0'
+    ]);
+  });
+}
+
 function initRepl(mode, useGlobal) {
   const inputStream = new stream.PassThrough();
   const outputStream = new stream.PassThrough();