这是什么JS语法?表达式中的作业?(x！=空& amp; amp;(y = x)) [英] What is this JS syntax? Assignment in expression? (x != null && (y = x))

问题描述

我正在使用此JS插件，并且遇到了一些以前从未见过的语法.我了解它在做什么，但是我不确定它为什么起作用.

I'm working with this JS plugin, and I've encountered some syntax I've never seen before. I understand what it's doing, but I'm not sure why it works.

以下是其中一个实例的示例:

Here's an example of one instance of it:

settings.maxId != null && (params.max_id = settings.maxId);

这是否只是利用条件和单= =的优势?这是JS的通用语法吗?

Is this just taking advantage of conditionals and the single = ? Is this common syntax for JS?

推荐答案

在JavaScript中， = 运算符是表达式并评估分配的值.因为它是一个 expression ，所以它可以在允许表达的任何地方使用，即使它会产生副作用.因此:

In JavaScript the = operator is an expression and evaluates the assigned value. Because it is an expression it can be used anywhere an expression is allowed even though it causes a side-effect. Thus:

settings.maxId != null && (params.max_id = settings.maxId)

意思是:如果 settings.maxId 不为null，则(，然后只有，因为&& 为

Means: If settings.maxId is not null then (and only then, since && is short circuiting) evaluate the right-expression (params.max_id = settings.maxId) which in turn causes the value of settings.maxId to be assigned to params.max_id. This is much more clearly written as:

if (settings.maxId != null) {
  params.max_id = settings.maxId
}

快乐的编码.

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