String.prototype.substring()

Summary

The substring() method returns a subset of a string between one index and another, or through the end of the string.

Syntax

str.substring(indexA[, indexB])

Parameters

indexA

An integer between 0 and the length of the string.

indexB

(optional) An integer between 0 and the length of the string.

Description

substring extracts characters from indexA up to but not including indexB. In particular:

• If indexA equals indexB, substring returns an empty string.
• If indexB is omitted, substring extracts characters to the end of the string.
• If either argument is less than 0 or is NaN, it is treated as if it were 0.
• If either argument is greater than stringName.length, it is treated as if it were stringName.length.

If indexA is larger than indexB, then the effect of substring is as if the two arguments were swapped; for example, str.substring(1, 0) == str.substring(0, 1).

Examples

Example: Using substring

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

// assumes a print function is defined
var anyString = "Mozilla";

// Displays "Moz"
console.log(anyString.substring(0,3));
console.log(anyString.substring(3,0));

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

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

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

Example: Replacing a substring within a string

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 changes the string "Brave New World" into "Brave New Web".

function replaceString(oldS, newS, fullS) {
// Replaces oldS with newS in the string fullS
  for (var 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");

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. A better method for replacing strings is as follows:

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

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().

Specifications

Browser compatibility

See also

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