String.prototype.substring()

indexStart

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

indexEnd Optional

The index of the first character to exclude from the returned substring.

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

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

• If indexEnd is omitted, substring() extracts characters to the end of the string.
• If indexStart is equal to indexEnd, substring() returns an empty string.
• If indexStart is greater than indexEnd, then the effect of substring() is as if the two arguments were swapped; see example below.

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

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

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'

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"

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

• The two parameters of substr() are start and length, while for substring(), they are start and end.
• substr()'s start index will wrap to the end of the string if it is negative, while substring() will clamp it to 0.
• Negative lengths in substr() are treated as zero, while substring() will swap the two indexes if end is less than start.

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"

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.

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)); // ""

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() 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"

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

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 to Brave New Web.

// 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");

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().

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