javascript - Jasmine toEqual 失败但打印两个对象是相同的

Question

In this Jasmine test我比较了两个几乎相同的对象，唯一的区别是第二个对象多了一个未定义的成员。

describe('Testing', function () {

  it('should compare two objects', function () {


    var obj1 = {a: 1, b: 2 };

    var obj2 = {a: 1, b: 2, c: undefined };

    console.log(JSON.stringify(obj1));
    console.log(JSON.stringify(obj2));

    expect(obj1).toEqual(obj2);


  });

测试失败，但是使用 JSON.stringify 打印两个对象会导致两个相同的输出。

{"a":1,"b":2}
{"a":1,"b":2}

浏览对象可以发现差异，但在复杂的对象中，这并不容易。关于如何处理这个问题有什么建议吗？

Answer 1

您的问题基于两个误解:

1. 在 obj2 中，c 是一个已定义的属性，其值为 undefined。
2. stringify() 未根据规范序列化 undefined - 您的测试不安全

两个对象不相等。

关于 Jasmine toEqual()

toEqual使用内部 util.equals()对于 a 和 b 中定义的所有 可枚举 键，它将逐键比较对象键。

经过一些类型检查后，它进入了 comparing the keys of the object

关于定义属性

查看 ECMAscript 规范。当您创建对象文字时，将调用此内部 Put 方法:

11.1.5 Object Initialiser

[...]

The production PropertyNameAndValueList : PropertyAssignment is evaluated as follows:

1. Let obj be the result of creating a new object as if by the expression new Object() where Object is the standard built-in constructor with that name.
2. Let propId be the result of evaluating PropertyAssignment.
3. Call the [[DefineOwnProperty]] internal method of obj with arguments propId.name, propId.descriptor, and false.
4. Return obj.

[...]

The production PropertyAssignment : PropertyName : AssignmentExpression is evaluated as follows:

1. Let propName be the result of evaluating PropertyName.
2. Let exprValue be the result of evaluating AssignmentExpression.
3. Let propValue be GetValue(exprValue).
4. Let desc be the Property Descriptor{[[Value]]: propValue, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}

类似的通过成员表达式定义属性:

8.12.5 [[Put]] ( P, V, Throw ) When the [[Put]] internal method of O is called with property P, value V, and Boolean flag Throw, the following steps are taken:

[...]

6. Else, create a named data property named P on object O as follows

   a. Let newDesc be the Property Descriptor {[[Value]]: V, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.

   b. Call the [[DefineOwnProperty]] internal method of O passing P, newDesc, and Throw as arguments.

DefineOwnProperty 的实现，在 8.12.9 [[DefineOwnProperty]] (P, Desc, Throw) 中有描述，不再赘述。还有 MDN says即使是默认值也是 undefined。

检查:

> var obj2 = {a: 1, b: 2, c: undefined };
> obj2.hasOwnProperty("c");
< true

关于stringify()

看看 ECMAscript spec on stringify()或在 JSON spec :

(来源:json.org)

这是 ECMAscript 规范的一部分:

6. Else a. Let K be an internal List of Strings consisting of the names of all the own properties of value whose [[Enumerable]] attribute is true. The ordering of the Strings should be the same as that used by the Object.keys standard built-in function.

他们说，该对象的枚举属性应符合 Object.keys() 的顺序(和结果)。让我们测试一下......

> var obj2 = {a: 1, b: 2, c: undefined };
> Object.keys(obj2);
< ["a", "b", "c"]

嗯，他们是对的!

然后是 Str() 函数，它定义了处理 undefined 值的行为。有几个 If Type() ... 步骤不适用于 undefined 值，以

结尾

11. Return undefined.

当被对象序列化器调用时:

8. For each element P of K.

   a. Let strP be the result of calling the abstract operation Str with arguments P and value.

   b. If strP is not undefined

   [...]