在此页

• 句法
• 描述
• Polyfill
• 范例
• 规范
• 浏览器兼容性
• 另请参阅
• 相关话题

substr() method returns a portion of the string, starting at the specified index and extending for a given number of characters afterwards.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

句法

str.substr(start[, length])
					

参数

start

The index of the first character to include in the returned substring.

length

Optional. The number of characters to extract.

返回值

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

描述

substr() extracts length characters from a str , counting from the start index.

• 若 start is a positive number, the index starts counting at the start of the string. Its value is capped at str .length .
• 若 start is a negative number, the index starts counting from the end of the string. Its value is capped at - str .length .
• Note: In Microsoft JScript, negative values of the start argument are not considered to refer to the end of the string.
• 若 length 被省略， substr() extracts characters to the end of the string.
• 若 length is undefined , substr() extracts characters to the end of the string.
• 若 length is a negative number, it is treated as 0 .
• For both start and length , NaN is treated as 0 .

Polyfill

Microsoft's JScript does not support negative values for the start index. To use this feature in JScript, you can use the following code:

// only run when the substr() function is broken
if ('ab'.substr(-1) != 'b') {
  /**
   *  Get the substring of a string
   *  @param  {integer}  start   where to start the substring
   *  @param  {integer}  length  how many characters to return
   *  @return {string}
   */
  String.prototype.substr = function(substr) {
    return function(start, length) {
      // call the original method
      return substr.call(this,
      	// did we get a negative start, calculate how much it is from the beginning of the string
        // adjust the start parameter for negative value
        start < 0 ? this.length + start : start,
        length)
    }
  }(String.prototype.substr);
}
					

范例

Using substr()

var aString = 'Mozilla';
console.log(aString.substr(0, 1));   // 'M'
console.log(aString.substr(1, 0));   // ''
console.log(aString.substr(-1, 1));  // 'a'
console.log(aString.substr(1, -1));  // ''
console.log(aString.substr(-3));     // 'lla'
console.log(aString.substr(1));      // 'ozilla'
console.log(aString.substr(-20, 2)); // 'Mo'
console.log(aString.substr(20, 2));  // ''
					

规范

规范
ECMAScript (ECMA-262) The definition of 'String.prototype.substr' in that specification.

浏览器兼容性

The compatibility table in this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request. 更新 GitHub 上的兼容性数据

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
substr 弃用	Chrome 1	Edge 12	Firefox 1	IE 4	Opera 4	Safari 1	WebView Android 1	Chrome Android 18	Firefox Android 4	Opera Android 10.1	Safari iOS 1	Samsung Internet Android 1.0	nodejs 0.1.100

图例

完整支持

完整支持

弃用。不要用于新网站。

弃用。不要用于新网站。

另请参阅

• String.prototype.slice()
• String.prototype.substring()

元数据

• 最后修改： Jul 11, 2020

相关话题

1. 标准内置对象
2. String
3. 特性
   1. String 长度
4. 方法
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   2. String.fromCodePoint()
   3. String.prototype.anchor()
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   5. String.prototype.blink()
   6. String.prototype.bold()
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   14. String.prototype.fontsize()
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   16. String.prototype.indexOf()
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   18. String.prototype.lastIndexOf()
   19. String.prototype.link()
   20. String.prototype.localeCompare()
   21. String.prototype.match()
   22. String.prototype.matchAll()
   23. String.prototype.normalize()
   24. String.prototype.padEnd()
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   26. String.prototype.repeat()
   27. String.prototype.replace()
   28. String.prototype.replaceAll()
   29. String.prototype.search()
   30. String.prototype.slice()
   31. String.prototype.small()
   32. String.prototype.split()
   33. String.prototype.startsWith()
   34. String.prototype.strike()
   35. String.prototype.sub()
   36. String.prototype.substr()
   37. String.prototype.substring()
   38. String.prototype.sup()
   39. String.prototype.toLocaleLowerCase()
   40. String.prototype.toLocaleUpperCase()
   41. String.prototype.toLowerCase()
   42. String.prototype.toSource()
   43. String.prototype.toString()
   44. String.prototype.toUpperCase()
   45. String.prototype.trim()
   46. String.prototype.trimEnd()
   47. String.prototype.trimStart()
   48. String.prototype.valueOf()
   49. String.prototype[@@iterator]()
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7. 特性
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