Reporter: handle objects with cycles

Updating `TapReporter` with the ability to handle objects with circular
references. This is needed for proper stringification of actual and
expected values that contain cycles.

Fixes #104

lib/reporters/TapReporter.js

@@ -30,6 +30,9 @@ const hasOwn = Object.hasOwnProperty;
  * - bare unquoted text, for simple one-line strings.
  * - JSON (quoted text), for complex one-line strings.
  * - YAML Block, for complex multi-line strings.
+ *
+ * Objects with cyclical references will be stringifed as
+ * "[Circular]" as they cannot otherwise be represented.
  */
 function prettyYamlValue (value, indent = 4) {
   if (value === undefined) {
@@ -116,7 +119,46 @@ function prettyYamlValue (value, indent = 4) {
   }
 
   // Handle null, boolean, array, and object
-  return JSON.stringify(value, null, 2);
+  return JSON.stringify(decycledShallowClone(value), null, 2);
+}
+
+/**
+ * Creates a shallow clone of an object where cycles have
+ * been replaced with "[Circular]".
+ */
+function decycledShallowClone (object, ancestors = []) {
+  if (ancestors.indexOf(object) !== -1) {
+    return '[Circular]';
+  }
+
+  let clone;
+
+  const type = Object.prototype.toString
+    .call(object)
+    .replace(/^\[.+\s(.+?)]$/, '$1')
+    .toLowerCase();
+
+  switch (type) {
+    case 'array':
+      ancestors.push(object);
+      clone = object.map(function (element) {
+        return decycledShallowClone(element, ancestors);
+      });
+      ancestors.pop();
+      break;
+    case 'object':
+      ancestors.push(object);
+      clone = {};
+      Object.keys(object).forEach(function (key) {
+        clone[key] = decycledShallowClone(object[key], ancestors);
+      });
+      ancestors.pop();
+      break;
+    default:
+      clone = object;
+  }
+
+  return clone;
 }
 
 module.exports = class TapReporter {

test/fixtures/unit.js

@@ -11,6 +11,39 @@ function copyErrors (testEnd) {
   return testEnd;
 }
 
+/**
+ * Creates an object that has a cyclical reference.
+ */
+function createCyclical () {
+  const cyclical = { a: 'example' };
+  cyclical.cycle = cyclical;
+  return cyclical;
+}
+
+/**
+ * Creates an object that has a cyclical reference in a subobject.
+ */
+function createSubobjectCyclical () {
+  const cyclical = { a: 'example', sub: {} };
+  cyclical.sub.cycle = cyclical;
+  return cyclical;
+}
+
+/**
+ * Creates an object that references another object more
+ * than once in an acyclical way.
+ */
+function createDuplicateAcyclical () {
+  const duplicate = {
+    example: 'value'
+  };
+  return {
+    a: duplicate,
+    b: duplicate,
+    c: 'unique'
+  };
+}
+
 module.exports = {
   passingTestStart: {
     name: 'pass',
@@ -195,6 +228,79 @@ module.exports = {
   message: failed
   severity: failed
   actual  : []
+  expected: expected
+  ...`,
+  actualCyclical: copyErrors({
+    name: 'Failing',
+    suiteName: undefined,
+    fullName: ['Failing'],
+    status: 'failed',
+    runtime: 0,
+    errors: [{
+      passed: false,
+      actual: createCyclical(),
+      expected: 'expected'
+    }],
+    assertions: null
+  }),
+  actualCyclicalTap: `  ---
+  message: failed
+  severity: failed
+  actual  : {
+  "a": "example",
+  "cycle": "[Circular]"
+}
+  expected: expected
+  ...`,
+  actualSubobjectCyclical: copyErrors({
+    name: 'Failing',
+    suiteName: undefined,
+    fullName: ['Failing'],
+    status: 'failed',
+    runtime: 0,
+    errors: [{
+      passed: false,
+      actual: createSubobjectCyclical(),
+      expected: 'expected'
+    }],
+    assertions: null
+  }),
+  actualSubobjectCyclicalTap: `  ---
+  message: failed
+  severity: failed
+  actual  : {
+  "a": "example",
+  "sub": {
+    "cycle": "[Circular]"
+  }
+}
+  expected: expected
+  ...`,
+  actualDuplicateAcyclic: copyErrors({
+    name: 'Failing',
+    suiteName: undefined,
+    fullName: ['Failing'],
+    status: 'failed',
+    runtime: 0,
+    errors: [{
+      passed: false,
+      actual: createDuplicateAcyclical(),
+      expected: 'expected'
+    }],
+    assertions: null
+  }),
+  actualDuplicateAcyclicTap: `  ---
+  message: failed
+  severity: failed
+  actual  : {
+  "a": {
+    "example": "value"
+  },
+  "b": {
+    "example": "value"
+  },
+  "c": "unique"
+}
   expected: expected
   ...`,
   expectedUndefinedTest: copyErrors({
@@ -223,6 +329,19 @@ module.exports = {
     }],
     assertions: null
   }),
+  expectedCircularTest: copyErrors({
+    name: 'fail',
+    suiteName: undefined,
+    fullName: [],
+    status: 'failed',
+    runtime: 0,
+    errors: [{
+      passed: false,
+      actual: 'actual',
+      expected: createCyclical()
+    }],
+    assertions: null
+  }),
   skippedTest: {
     name: 'skip',
     suiteName: undefined,

test/unit/tap-reporter.js

@@ -115,6 +115,21 @@ QUnit.module('TapReporter', hooks => {
     assert.equal(spy.args[1][0], data.actualArrayTap);
   });
 
+  test('output actual assertion value of a cyclical structure', assert => {
+    emitter.emit('testEnd', data.actualCyclical);
+    assert.equal(spy.args[1][0], data.actualCyclicalTap);
+  });
+
+  test('output actual assertion value of a subobject cyclical structure', assert => {
+    emitter.emit('testEnd', data.actualSubobjectCyclical);
+    assert.equal(spy.args[1][0], data.actualSubobjectCyclicalTap);
+  });
+
+  test('output actual assertion value of an acyclical structure', assert => {
+    emitter.emit('testEnd', data.actualDuplicateAcyclic);
+    assert.equal(spy.args[1][0], data.actualDuplicateAcyclicTap);
+  });
+
   test('output expected assertion of undefined', assert => {
     emitter.emit('testEnd', data.expectedUndefinedTest);
     assert.true(spy.calledWithMatch(/^ {2}expected: undefined$/m));
@@ -125,6 +140,11 @@ QUnit.module('TapReporter', hooks => {
     assert.true(spy.calledWithMatch(/^ {2}expected: 0$/m));
   });
 
+  test('output expected assertion of a circular structure', assert => {
+    emitter.emit('testEnd', data.expectedCircularTest);
+    assert.true(spy.calledWithMatch(/^ {2}expected: \{\n {2}"a": "example",\n {2}"cycle": "\[Circular\]"\n\}$/m));
+  });
+
   test('output the total number of tests', assert => {
     const summary = '1..6';
     const passCount = '# pass 3';