String.prototype.substring()

const str = "Mozilla";

console.log(str.substring(1, 3));
// Expected output: "oz"

console.log(str.substring(2));
// Expected output: "zilla"

str.substring(indexStart[, indexEnd])

indexStart

indexEnd

. ( .)

.

Description

substring() indexStart indexEnd . .

• indexEnd , substring() .
• indexStart indexEnd , substring() .
• indexStart indexEnd , substring() . .

0 0, stringName.length , stringName.length . NaN 0 .

Examples

Using substring()

The following example uses substring() to display characters from the string 'Mozilla':

var anyString = "Mozilla";

// Displays 'M'
console.log(anyString.substring(0, 1));
console.log(anyString.substring(1, 0));

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

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

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

Using substring() with length property

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.

// Displays 'illa' the last 4 characters
var anyString = "Mozilla";
var anyString4 = anyString.substring(anyString.length - 4);
console.log(anyString4);

// Displays 'zilla' the last 5 characters
var anyString = "Mozilla";
var anyString5 = anyString.substring(anyString.length - 5);
console.log(anyString5);

The difference between substring() and substr()

There's a subtle difference between the substring() and substr() methods, so you should be careful not to get them confused.

The arguments of substring() represent the starting and ending indexes, while the arguments of substr() represent the starting index and the number of characters to include in the returned string.

var text = "Mozilla";
console.log(text.substring(2, 5)); // => "zil"
console.log(text.substr(2, 3)); // => "zil"

Differences between substring() and slice()

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.

var 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.

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 to Brave New Web.

// Replaces oldS with newS in the string fullS
function replaceString(oldS, newS, 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().

See also

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