String.prototype.substring()
Baseline Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
The substring()
method of String
values returns the part of this string from the start index up to and excluding the end index, or to the end of the string if no end index is supplied.
Try it
Syntax
substring(indexStart)
substring(indexStart, indexEnd)
Parameters
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.
Return value
A new string containing the specified part of the given string.
Description
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 toindexEnd
,substring()
returns an empty string. - If
indexStart
is greater thanindexEnd
, then the effect ofsubstring()
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
.
Examples
Using substring()
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"
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.
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"
The difference between substring() and substr()
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()
arestart
andlength
, while forsubstring()
, they arestart
andend
. substr()
'sstart
index will wrap to the end of the string if it is negative, whilesubstring()
will clamp it to0
.- Negative lengths in
substr()
are treated as zero, whilesubstring()
will swap the two indexes ifend
is less thanstart
.
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"
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.
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.
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 creates a string Brave New Web
from the original string Brave New World
.
// 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()
.
Specifications
Specification |
---|
ECMAScript Language Specification # sec-string.prototype.substring |
Browser compatibility
BCD tables only load in the browser