Fix REPL bug with previous line carryover (#1480)

* Fix expressions carrying over from the previous line

* Simplify semicolon insertion logic and avoid issues with line number

* fix-previous-line-carryover: Add tests, simplify code, remove dead code

* Minor refactor of ASI and and test tweak

* Fix linting issue

* Avoid adding a semicolon if one already exists, since TS emits double-semicolons
as empty statements and this *might* affect stack trace line numbers at
some point

* lint-fix

* Add test case for avoiding double-semicolons; also fix tests to allow console.log() capture

Co-authored-by: Andrew Bradley <cspotcode@gmail.com>

src/repl.ts

@@ -510,6 +510,18 @@ function appendCompileAndEvalInput(options: {
     undo();
   } else {
     state.output = output;
+
+    // Insert a semicolon to make sure that the code doesn't interact with the next line,
+    // for example to prevent `2\n+ 2` from producing 4.
+    // This is safe since the output will not change since we can only get here with successful inputs,
+    // and adding a semicolon to the end of a successful input won't ever change the output.
+    state.input = state.input.replace(
+      /([^\n\s])([\n\s]*)$/,
+      (all, lastChar, whitespace) => {
+        if (lastChar !== ';') return `${lastChar};${whitespace}`;
+        return all;
+      }
+    );
   }
 
   let commands: Array<{ mustAwait?: true; execCommand: () => any }> = [];
@@ -586,15 +598,6 @@ function appendToEvalState(state: EvalState, input: string) {
   const undoOutput = state.output;
   const undoLines = state.lines;
 
-  // Handle ASI issues with TypeScript re-evaluation.
-  if (
-    undoInput.charAt(undoInput.length - 1) === '\n' &&
-    /^\s*[\/\[(`-]/.test(input) &&
-    !/;\s*$/.test(undoInput)
-  ) {
-    state.input = `${state.input.slice(0, -1)};\n`;
-  }
-
   state.input += input;
   state.lines += lineCount(input);
   state.version++;

src/test/repl/repl.spec.ts

@@ -288,3 +288,125 @@ test.suite(
     });
   }
 );
+
+test.suite(
+  'REPL inputs are syntactically independent of each other',
+  (test) => {
+    // Serial because it's timing-sensitive
+    test.serial(
+      'arithmetic operators are independent of previous values',
+      async (t) => {
+        const { stdout, stderr } = await t.context.executeInRepl(
+          `9
+          + 3
+          7
+          - 3
+          3
+          * 7\n.break
+          100
+          / 2\n.break
+          5
+          ** 2\n.break
+          console.log('done!')
+          `,
+          {
+            registerHooks: true,
+            startInternalOptions: { useGlobal: false },
+            waitPattern: 'done!\nundefined\n>',
+          }
+        );
+        expect(stdout).not.toContain('12');
+        expect(stdout).not.toContain('4');
+        expect(stdout).not.toContain('21');
+        expect(stdout).not.toContain('50');
+        expect(stdout).not.toContain('25');
+        expect(stdout).toContain('3');
+        expect(stdout).toContain('-3');
+      }
+    );
+
+    // Serial because it's timing-sensitive
+    test.serial(
+      'automatically inserted semicolons do not appear in error messages at the end',
+      async (t) => {
+        const { stdout, stderr } = await t.context.executeInRepl(
+          `(
+          a
+          console.log('done!')`,
+          {
+            registerHooks: true,
+            startInternalOptions: { useGlobal: false },
+            waitPattern: 'done!\nundefined\n>',
+          }
+        );
+        expect(stderr).toContain("error TS1005: ')' expected.");
+        expect(stderr).not.toContain(';');
+      }
+    );
+
+    // Serial because it's timing-sensitive
+    test.serial(
+      'automatically inserted semicolons do not appear in error messages at the start',
+      async (t) => {
+        const { stdout, stderr } = await t.context.executeInRepl(
+          `)
+          console.log('done!')`,
+          {
+            registerHooks: true,
+            startInternalOptions: { useGlobal: false },
+            waitPattern: 'done!\nundefined\n>',
+          }
+        );
+        expect(stderr).toContain(
+          'error TS1128: Declaration or statement expected.'
+        );
+        expect(stderr).toContain(')');
+        expect(stderr).not.toContain(';');
+      }
+    );
+
+    // Serial because it's timing-sensitive
+    test.serial(
+      'automatically inserted semicolons do not break function calls',
+      async (t) => {
+        const { stdout, stderr } = await t.context.executeInRepl(
+          `function foo(a: number) {
+              return a + 1;
+          }
+          foo(
+            1
+          )`,
+          {
+            registerHooks: true,
+            startInternalOptions: { useGlobal: false },
+            waitPattern: '2\n>',
+          }
+        );
+        expect(stderr).toBe('');
+        expect(stdout).toContain('2');
+      }
+    );
+
+    // Serial because it's timing-sensitive
+    test.serial(
+      'automatically inserted semicolons do not affect subsequent line numbers',
+      async (t) => {
+        // If first line of input ends in a semicolon, should not add a second semicolon.
+        // That will cause an extra blank line in the compiled output which will
+        // offset the stack line number.
+        const { stdout, stderr } = await t.context.executeInRepl(
+          `1;
+          new Error().stack!.split('\\n')[1]
+          console.log('done!')`,
+          {
+            registerHooks: true,
+            startInternalOptions: { useGlobal: false },
+            waitPattern: 'done!',
+          }
+        );
+        expect(stderr).toBe('');
+        expect(stdout).toContain(":1:1'\n");
+      }
+    );
+  }
+);