String.prototype.substring()

¥Syntax

substring(indexStart)
substring(indexStart, indexEnd)

¥Parameters

indexStart

要包含在返回的子字符串中的第一个字符的索引。

indexEnd Optional

要从返回的子字符串中排除的第一个字符的索引。

¥Return value

包含给定字符串的指定部分的新字符串。

¥A new string containing the specified part of the given string.

¥Description

substring() 提取从 indexStart 到 indexEnd 的字符（但不包括 indexEnd）。尤其：

¥substring() extracts characters from indexStart up to but not including indexEnd. In particular:

• 如果省略 indexEnd，则 substring() 将字符提取到字符串末尾。
• 如果 indexStart 等于 indexEnd，则 substring() 返回空字符串。
• 如果 indexStart 大于 indexEnd，则 substring() 的效果就好像两个参数交换了；请参见下面的示例。

任何小于 0 或大于 str.length 的参数值将分别被视为 0 和 str.length。

¥Any argument value that is less than 0 or greater than str.length is treated as if it were 0 and str.length, respectively.

任何 NaN 的参数值都被视为 0。

¥Any argument value that is NaN is treated as if it were 0.

¥Examples

¥Using substring()

以下示例使用 substring() 显示字符串 "Mozilla" 中的字符：

¥The following example uses substring() to display characters from the string "Mozilla":

const anyString = "Mozilla";

console.log(anyString.substring(0, 1)); // "M"
console.log(anyString.substring(1, 0)); // "M"

console.log(anyString.substring(0, 6)); // "Mozill"

console.log(anyString.substring(4)); // "lla"
console.log(anyString.substring(4, 7)); // "lla"
console.log(anyString.substring(7, 4)); // "lla"

console.log(anyString.substring(0, 7)); // "Mozilla"
console.log(anyString.substring(0, 10)); // "Mozilla"

¥Using substring() with length property

以下示例使用 substring() 方法和 length 属性来提取特定字符串的最后一个字符。鉴于你不需要像上面的示例中那样知道开始和结束索引，因此这种方法可能更容易记住。

¥The following example uses the substring() method and length property to extract the last characters of a particular string. This method may be easier to remember, given that you don't need to know the starting and ending indices as you would in the above examples.

const text = "Mozilla";

// Takes 4 last characters of string
console.log(text.substring(text.length - 4)); // prints "illa"

// Takes 5 last characters of string
console.log(text.substring(text.length - 5)); // prints "zilla"

¥The difference between substring() and substr()

substring() 和 substr() 方法之间存在细微差别，因此你应该小心不要将它们混淆。

¥There are subtle differences between the substring() and substr() methods, so you should be careful not to get them confused.

• substr() 的两个参数是 start 和 length，而 substring() 的两个参数是 start 和 end。
• 如果 substr() 的 start 索引为负数，则它会换行到字符串的末尾，而 substring() 会将其钳位到 0。
• substr() 中的负长度被视为零，而如果 end 小于 start，则 substring() 将交换两个索引。

此外，substr() 被认为是 ECMAScript 中的遗留功能，因此最好尽可能避免使用它。

¥Furthermore, substr() is considered a legacy feature in ECMAScript, so it is best to avoid using it if possible.

const text = "Mozilla";
console.log(text.substring(2, 5)); // "zil"
console.log(text.substr(2, 3)); // "zil"

¥Differences between substring() and slice()

substring() 和 slice() 方法几乎相同，但两者之间存在一些细微的差异，特别是在处理否定参数的方式上。

¥The substring() and slice() methods are almost identical, but there are a couple of subtle differences between the two, especially in the way negative arguments are dealt with.

如果 indexStart 大于 indexEnd，则 substring() 方法会交换其两个参数，这意味着仍返回字符串。如果是这种情况，slice() 方法将返回空字符串。

¥The substring() method swaps its two arguments if indexStart is greater than indexEnd, meaning that a string is still returned. The slice() method returns an empty string if this is the case.

const text = "Mozilla";
console.log(text.substring(5, 2)); // "zil"
console.log(text.slice(5, 2)); // ""

如果其中一个或两个参数为负数或 NaN，则 substring() 方法将它们视为 0。

¥If either or both of the arguments are negative or NaN, the substring() method treats them as if they were 0.

console.log(text.substring(-5, 2)); // "Mo"
console.log(text.substring(-5, -2)); // ""

slice() 还将 NaN 参数视为 0，但当给它负值时，它会从字符串末尾向后计数以查找索引。

¥slice() also treats NaN arguments as 0, but when it is given negative values it counts backwards from the end of the string to find the indexes.

console.log(text.slice(-5, 2)); // ""
console.log(text.slice(-5, -2)); // "zil"

有关负数的更多示例，请参阅 slice() 页。

¥See the slice() page for more examples with negative numbers.

¥Replacing a substring within a string

以下示例替换字符串中的子字符串。它将替换单个字符和子字符串。示例末尾的函数调用从原始字符串 Brave New World 创建字符串 Brave New Web。

¥The following example replaces a substring within a string. It will replace both individual characters and substrings. The function call at the end of the example creates a string Brave New Web from the original string Brave New World.

// Replaces oldS with newS in the string fullS
function replaceString(oldS, newS, fullS) {
  for (let i = 0; i < fullS.length; ++i) {
    if (fullS.substring(i, i + oldS.length) === oldS) {
      fullS =
        fullS.substring(0, i) +
        newS +
        fullS.substring(i + oldS.length, fullS.length);
    }
  }
  return fullS;
}

replaceString("World", "Web", "Brave New World");

请注意，如果 oldS 本身是 newS 的子字符串，这可能会导致无限循环 - 例如，如果你尝试在此处将 "World" 替换为 "OtherWorld"。

¥Note that this can result in an infinite loop if oldS is itself a substring of newS — for example, if you attempted to replace "World" with "OtherWorld" here.

更好的替换字符串的方法如下：

¥A better method for replacing strings is as follows:

function replaceString(oldS, newS, fullS) {
  return fullS.split(oldS).join(newS);
}

上面的代码作为子字符串操作的示例。如果你需要替换子字符串，大多数时候你会想要使用 String.prototype.replace()。

¥The code above serves as an example for substring operations. If you need to replace substrings, most of the time you will want to use String.prototype.replace().

¥See also

• String.prototype.substr()
• String.prototype.slice()