Javascript 为什么 IndexOf 返回 -1？

Why does IndexOf return -1?

提问by Adnan Khan

I am learning Javascript and don't understand why the indexOf below returns -1:

我正在学习 Javascript 并且不明白为什么下面的 indexOf 返回 -1：

var string = "The quick brown fox jumps over the lazy dog";

console.log (string.indexOf("good"));

回答by Interrobang

-1 means "no match found".

-1 表示“未找到匹配项”。

The reason it returns -1 instead of "false" is that a needle at the beginning of the string would be at position 0, which is equivalent to false in Javascript. So returning -1 ensures that you know there is not actually a match.

它返回 -1 而不是 "false" 的原因是字符串开头的针将位于位置 0，这相当于 Javascript 中的 false。因此，返回 -1 可确保您知道实际上没有匹配项。

回答by mpen

-1 means no match found. "good" is not in that sentence. This is documentedbehaviour.

-1 表示未找到匹配项。“好”不在那个句子中。这是记录在案的行为。

The indexOf()method returns the first index at which a given element can be found in the array, or -1if it is not present.

该indexOf()方法返回可以在数组中找到给定元素的第一个索引，或者-1如果它不存在。

回答by Mauvis Ledford

Because arrays are 0 based, returning 0 would mean starting from the first character was matched; 1, the second character, and so on. This means anything 0 and up would be a true or "found" response. To keep everything in the integer category, -1 signifies no match found.

因为数组是基于 0 的，所以返回 0 意味着从匹配的第一个字符开始；1、第二个字符，依此类推。这意味着 0 及以上的任何内容都是真实或“找到”的响应。要将所有内容保留在整数类别中，-1 表示未找到匹配项。

回答by VietNg

There is another reason for indexOf to return -1 when no match is found. Consider the code below:

indexOf 在找不到匹配项时返回 -1 的另一个原因。考虑下面的代码：

if (~str.indexOf(pattern)){
  console.log('found')
}else{
  console.log('not found')
}

Because ~(-1) = 0 so the fact that indexOf returning -1 makes it easier to write if...else using ~.

因为 ~(-1) = 0 所以 indexOf 返回 -1 的事实使得使用 ~ 编写 if...else 变得更容易。

回答by farzadgo

Related issue; even if the subject word exists in the sentence (semantically speaking) or any sequence of letters matching partially, indexOf() would return the index of its first matching letter. e.g.

相关问题；即使主题词存在于句子中（从语义上讲）或任何部分匹配的字母序列，indexOf() 也会返回其第一个匹配字母的索引。例如

const string = "The quick brown fox jumps over the lazy dog";
console.log(string.length);
// expected output: 43

console.log (string.indexOf("fox"));
// expected output: 16

console.log (string.indexOf(" q"));
// expected output: 3

But if the partial sequence of letters is not matching any part of the string array, the method will return -1, the same case with "good";

但是如果部分字母序列与字符串数组的任何部分都不匹配，则该方法将返回-1，与“good”的情况相同；

console.log (string.indexOf("foxy"));
// expected output: -1

console.log (string.indexOf("good"));
// expected output: -1

Just in case, turning it to an array of sub-strings (here the words) is possible with split() method, splitting the long string by its spaces (punctuations);

以防万一，可以使用 split() 方法将其转换为子字符串数组（此处为单词），将长字符串按空格（标点符号）拆分；

const words = string.split(' ');
console.log(words);
// expected output: ["The", "quick", "brown", "fox", "jumps", "over", "the", "lazy", "dog"]

console.log(words.indexOf("fox"));
// expected output: 3

回答by numegil

The search never finds what it's looking for ("good" isn't in the sentence), and -1 is the default return value.

搜索永远找不到它要找的东西（“good”不在句子中），-1 是默认返回值。

http://www.w3schools.com/jsref/jsref_indexof.asp

http://www.w3schools.com/jsref/jsref_indexof.asp