为什么匹配的子字符串返回“未定义”在JavaScript中？ [英] Why is a matched substring returning "undefined" in JavaScript?

问题描述

 在JavaScript中使用一些正则表达式时，我遇到了一个奇怪的行为（Firefox 3在Windows上） var str =format_％A; 
 var format = /(?:*|s)format_(.*?)(?:\s|$)/.exec(str）; 
 
 console.log（format）; // [format_％A，％A] 
 console.log（format [0]）; //format_undefined
 console.log（format [1]）; //未定义
  

正则表达式没有任何问题。正如您所看到的，它已经匹配了第一个 console.log 调用中的正确部分。

Internet Explorer 7和Chrome都表现得如预期： format [1] 返回％A（好吧，Internet Explorer 7正确地做了一些事情是有点意外的...） / b>

这是Firefox中的一个bug，还是一些我不知道的功能？

  console.log（我的名字是％A，John）; //我的名字是John
  

参见 console.log（）文档了解详情。 ％A和任何其他未公开的占位符似乎都和％o一样。

I came across a strange behaviour when doing some regular expressions in JavaScript today (Firefox 3 on Windows Vista).

var str = "format_%A";
var format = /(?:^|\s)format_(.*?)(?:\s|$)/.exec(str);

console.log(format);    // ["format_%A", "%A"]
console.log(format[0]); // "format_undefined"
console.log(format[1]); // Undefined

There's nothing wrong with the regular expression. As you can see, it has matched the correct part in the first console.log call.

Internet Explorer 7 and Chrome both behave as expected: format[1] returns "%A" (well, Internet Explorer 7 doing something right was a bit unexpected...)

Is this a bug in Firefox, or some "feature" I don't know about?

解决方案

This is because console.log() works like printf(). The first argument to console.log() is actually a format string which may be followed with additional arguments. %A is a placeholder. For example:

console.log("My name is %A", "John"); // My name is "John"

See console.log() documentation for details. %A and any other undocumented placeholders seem to do the same as %o.

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