javascript - 了解JavaScript逗号运算符

Question

This article by Angus Croll解释JavaScript逗号运算符，如下所示：

//(LHE: left hand expression, RHE right hand expression)

LHE && RHE
1. Always evaluate LHE
2. If LHE is true, evaluate RHE

LHE || RHE
1. Always evaluate LHE
2. If LHE is false, evaluate RHE

LHE, RHE
1. Always evaluate LHE
2. Always evaluate RHE

但是，我使用以下代码进行了jsfiddle测试enter link description here，并且如果运算符为&&，则LHE似乎必须用括号括起来。

// Works fine
(function one(window) {
    window.testOne = function testOne() {
        alert("Test one");
    }, testOne();
})(window);


// Works, but JSHint complains at *:
// "Expected an assignment or function call but saw instead an expression"
(function two(window) { 
    (window.testTwo = function testTwo() {
        alert("Test two");
    }) && testTwo(); // *
})(window);


// Looks good to JSHint, but fails at runtime:
// "ReferenceError: Can't find variable: testThree"
(function three(window) {
    window.testThree = function testThree() {
        alert("Test three");
    } && testThree();
})(window);

您能否解释为什么testOne（使用,）不需要在第一个表达式的圆括号，而testTwo（使用&&）却需要括号？为什么JSHint认为test()不是函数调用？

Answer 1

这是运算符优先级的情况。您使用的运算符具有以下优先级：&&，||，=，,。

这意味着var ... = ... && ...等效于var ... = (... && ...)，但var ... = ... , ....等效于(var ... = ...) , ....。

例如，您可以检查优先级here。