javascript - Javascript 中函数参数的求值顺序是什么？

Question

根据我的测试，它总是从左到右

>> console.log( console.log(1), console.log(2) );
1
2
undefined undefined

但我在 ECMAScript 标准中找不到确认这一点的相关部分。

Answer 1

JavaScript 中的所有运算符都从左到右计算其操作数，包括函数调用运算符。首先评估要调用的函数，然后按从左到右的顺序评估实际参数。

Section 11.2.3是相关的规范部分。

11.2.3 Function Calls

...

2 Let func be GetValue(ref).

3 Let argList be the result of evaluating Arguments, producing an internal list of argument values (see 11.2.4).

...

你可以看到ArgumentList产生式是左递归的

11.2.4 Argument lists

...

The production ArgumentList : ArgumentList , AssignmentExpression is evaluated as follows

和 ArgumentList 在 AssignmentExpression 之前计算，如下所示。

在 EcmaScript 3 下，一些比较运算符( < 、 <= 、 > 、 >= )自 a<=b 起从右到左计算。定义为!(b<a) ，但这被广泛认为是一个规范错误，主要解释器并没有以这种方式实现它，并且它在 EcmaScript 5 中得到了修复。

来自语言规范:

11.8.5 The Abstract Relational Comparison Algorithm # Ⓣ

The comparison x < y, where x and y are values, produces true, false, or undefined (which indicates that at least one operand is NaN). In addition to x and y the algorithm takes a Boolean flag named LeftFirst as a parameter. The flag is used to control the order in which operations with potentially visible side-effects are performed upon x and y. It is necessary because ECMAScript specifies left to right evaluation of expressions. The default value of LeftFirst is true and indicates that the x parameter corresponds to an expression that occurs to the left of the y parameter’s corresponding expression. If LeftFirst is false, the reverse is the case and operations must be performed upon y before x. Such a comparison is performed as follows: