javascript - javascript中括号与 `this`绑定(bind)的效果

Question

我遇到一个非常棘手的案例:

class C {
  // class method are implicit in strict mode by default
  static method() { return this === undefined; }  
}

C.method(); // => false
(0,C.method)(); // => true

为什么 (0, C.method) 在上面的例子中改变了 this 的绑定(bind)？

Answer 1

那是因为C.method 返回了一个引用

{ base: C, referencedName: "method", strict: strictFlag }

当你call it , JS获取函数使用GetValue使用该引用，并提供引用的基数 (C) 作为 this 值。

<i>CallExpression</i> : <i>MemberExpression</i> <i>Arguments</i>

 1. Let <i>ref</i> be the result of evaluating <i>MemberExpression</i>. // <-- The reference
 2. Let <i>func</i> be ? <a href="http://www.ecma-international.org/ecma-262/7.0/#sec-getvalue" rel="noreferrer noopener nofollow">GetValue</a>(<i>ref</i>).                          // <-- The function
 4. If <a href="http://www.ecma-international.org/ecma-262/7.0/#sec-ecmascript-data-types-and-values" rel="noreferrer noopener nofollow">Type</a>(<i>ref</i>) is <a href="http://www.ecma-international.org/ecma-262/7.0/#sec-reference-specification-type" rel="noreferrer noopener nofollow">Reference</a>, then
    a. If IsPropertyReference(<i>ref</i>) is <b>true</b>, then
       i. Let <i>thisValue</i> be <a href="http://www.ecma-international.org/ecma-262/7.0/#sec-getthisvalue" rel="noreferrer noopener nofollow">GetThisValue</a>(<i>ref</i>).             // <-- C

但是，当您使用 comma operator ，您直接获得函数，而不是引用。

<i>Expression</i> : <i>Expression</i> <b>,</b> <i>AssignmentExpression</i>

 1. Let <i>lref</i> be the result of evaluating <i>Expression</i>.
 2. Perform ? <a href="http://www.ecma-international.org/ecma-262/7.0/#sec-getvalue" rel="noreferrer noopener nofollow">GetValue</a>(<i>lref</i>).                             // <-- 0
 3. Let <i>rref</i> be the result of evaluating <i>AssignmentExpression</i>.
 4. Return ? <a href="http://www.ecma-international.org/ecma-262/7.0/#sec-getvalue" rel="noreferrer noopener nofollow">GetValue</a>(<i>rref</i>).                              // <-- The function

由于没有引用，JS 无法知道基础对象，所以当你调用它时提供undefined作为this值。

<i>CallExpression</i> : <i>MemberExpression</i> <i>Arguments</i>

 1. Let <i>ref</i> be the result of evaluating <i>MemberExpression</i>. // <-- The function
 2. Let <i>func</i> be ? <a href="http://www.ecma-international.org/ecma-262/7.0/#sec-getvalue" rel="noreferrer noopener nofollow">GetValue</a>(<i>ref</i>).                          // <-- The function
 5. Else <a href="http://www.ecma-international.org/ecma-262/7.0/#sec-ecmascript-data-types-and-values" rel="noreferrer noopener nofollow">Type</a>(<i>ref</i>) is not <a href="http://www.ecma-international.org/ecma-262/7.0/#sec-reference-specification-type" rel="noreferrer noopener nofollow">Reference</a>,
    1. Let <i>thisValue</i> be <b>undefined</b>.                        // <-- undefined