javascript - if 和 while 语句的条件部分中的赋值

Question

这是 Crockford 的引用:

Assignment Expressions

Avoid doing assignments in the condition part of if and while statements.

Is

if (a = b) {

a correct statement? Or was

if (a == b) {

intended? Avoid constructs that cannot easily be determined to be correct.

// Here is an assignment as I understand it:
var var1 = "foo";
var var2 = "bar";

// And what is shown in Crockford's quote isn't an assignment.

那么，克罗克福德的引言是什么意思？

Answer 1

这意味着

if (a = b) {

实际上是一个赋值，将b的值赋给a，这通常不是人们想要的。

那段话只是告诉你要注意，只有当你真正想这么做时才这样做，但你通常不会这样做。通常人们想在那里进行比较，所以

if (a == b) {

反例:如果运行以下命令，只有在确认弹出窗口中输入“确定”时，它才会打印 true，否则不会打印。

if(a = confirm('test')) { console.log(a) }

请注意，有一个赋值，我们正在 if 内打印 a 的值。