JavaScript String prototype substr

Gets a substring beginning at the specified location and having the specified length.

Syntax

str.substr(start[, length])

Parameters

start

Location at which to begin extracting characters. If a negative number is given, it is treated as strLength + start where strLength is the length of the string (for example, if start is -3 it is treated as strLength - 3.)

length

Optional. The number of characters to extract.

MDN link | MSDN link

Returns

The characters in a string beginning at the specified location through the specified number of characters.

Description

start is a character index. The index of the first character is 0, and the index of the last character is 1 less than the length of the string. substr() begins extracting characters at start and collects length characters (unless it reaches the end of the string first, in which case it will return fewer).

If start is positive and is greater than or equal to the length of the string, substr() returns an empty string.

If start is negative, substr() uses it as a character index from the end of the string. If start is negative and abs(start) is larger than the length of the string, substr() uses 0 as the start index. Note: the described handling of negative values of the start argument is not supported by Microsoft JScript.

If length is 0 or negative, substr() returns an empty string. If length is omitted, substr() extracts characters to the end of the string.

Examples

var str = 'abcdefghij';

console.log('(1, 2): '   + str.substr(1, 2));   // '(1, 2): bc'
console.log('(-3, 2): '  + str.substr(-3, 2));  // '(-3, 2): hi'
console.log('(-3): '     + str.substr(-3));     // '(-3): hij'
console.log('(1): '      + str.substr(1));      // '(1): bcdefghij'
console.log('(-20, 2): ' + str.substr(-20, 2)); // '(-20, 2): ab'
console.log('(20, 2): '  + str.substr(20, 2));  // '(20, 2): '

var s = "The quick brown fox jumps over the lazy dog.";
var ss = s.substr(10, 5);  
document.write("[" + ss + "] <br>");

ss = s.substr(10);
document.write("[" + ss + "] <br>");

ss = s.substr(10, -5);
document.write("[" + ss + "] <br>");

// Output:
// [brown] 
// [brown fox jumps over the lazy dog.] 
// []