javascript 为什么javascript在if语句中接受逗号？
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Why does javascript accept commas in if statements?
提问by Matt
I stumbled across some javascript syntax that seemed like it should produce a parse error of some kind but doesn't:
我偶然发现了一些 javascript 语法，它们似乎应该会产生某种解析错误，但不会：
if (true, true) {console.log('splendid')} else {console.log('horrid')} // splendid
if (true, false) {console.log('splendid')} else {console.log('horrid')} // horrid
It seems only the last expression affects the logic, though all expressions are executed:
尽管所有表达式都被执行，但似乎只有最后一个表达式会影响逻辑：
if (console.log('super'), true) {console.log('splendid')} // super splendid
Anyone know why that is valid javascript syntax? Is there any practical use for it?
有人知道为什么这是有效的 javascript 语法吗？它有什么实际用途吗？
采纳答案by Ignacio Vazquez-Abrams
The comma operator chains multiple expressions together, and the result of the operation is the value of the last operand. The only real use for it is when you need multiple side effects to occur, such as assignment or function calls.
逗号运算符将多个表达式链接在一起，运算的结果是最后一个操作数的值。它唯一真正的用途是当您需要发生多个副作用时，例如赋值或函数调用。
回答by Adam Ayres
The comma operator evaluates each of its operands (from left to right) and returns the value of the last operand.
逗号运算符计算其每个操作数（从左到右）并返回最后一个操作数的值。
https://developer.mozilla.org/en/Core_JavaScript_1.5_Guide/Expressions_and_Operators#comma_operator
回答by Matt Briggs
commas in javascript are actually pretty arcane. The coolest use I have seen is this
javascript 中的逗号实际上非常神秘。我见过的最酷的用法是这个
while(doSomething(), checkIfSomethingHappened());
the most common would be the way var is used in modern js
最常见的是 var 在现代 js 中的使用方式
var foo = 1,
bar = 2;
回答by Suroot
This is also the same as in most other programming languages where you might have multiple iterators in a loop.
这也与在循环中可能有多个迭代器的大多数其他编程语言相同。
int x,y;
for(x = 0, y = 0; x < 10 || y < 100; x++, y++) {
....
}
回答by vinyll
I permits to do operations and comparisons in the same context.
我允许在相同的上下文中进行操作和比较。
Example:
例子：
if(a = 2, a > 1) console.log('a is', a)
