String.prototype.substr()

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

Syntax

str.substr(start[, length])

Parameters

start

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

length

Optional. The number of characters to extract.

Return value

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

Description

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

• If start is a positive number, the index starts counting at the start of the string. Its value is capped at str.length.
• If 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.
• If length is omitted, substr() extracts characters to the end of the string.
• If length is undefined, substr() extracts characters to the end of the string.
• If 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);
}

Examples

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));  // ''

Specifications

Browser compatibility

See also

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