String.prototype.substr()
Deprecated: This feature is no longer recommended. Though some browsers might still support it, it may have already been removed from the relevant web standards, may be in the process of being dropped, or may only be kept for compatibility purposes. Avoid using it, and update existing code if possible; see the compatibility table at the bottom of this page to guide your decision. Be aware that this feature may cease to work at any time.
The substr()
method of String
values returns a portion of this string, starting at the specified index and extending for a given number of characters afterwards.
Note: substr()
is not part of the main ECMAScript specification — it's defined in Annex B: Additional ECMAScript Features for Web Browsers, which is normative optional for non-browser runtimes. Therefore, people are advised to use the standard String.prototype.substring()
and String.prototype.slice()
methods instead to make their code maximally cross-platform friendly. The String.prototype.substring()
page has some comparisons between the three methods.
Try it
Syntax
substr(start)
substr(start, length)
Parameters
Return value
A new string containing the specified part of the given string.
Description
A string's substr()
method extracts length
characters from the string, counting from the start
index.
- If
start >= str.length
, an empty string is returned. - If
start < 0
, the index starts counting from the end of the string. More formally, in this case the substring starts atmax(start + str.length, 0)
. - If
start
is omitted orundefined
, it's treated as0
. - If
length
is omitted orundefined
, or ifstart + length >= str.length
,substr()
extracts characters to the end of the string. - If
length < 0
, an empty string is returned. - For both
start
andlength
,NaN
is treated as0
.
Although you are encouraged to avoid using substr()
, there is no trivial way to migrate substr()
to either slice()
or substring()
in legacy code without essentially writing a polyfill for substr()
. For example, str.substr(a, l)
, str.slice(a, a + l)
, and str.substring(a, a + l)
all have different results when str = "01234", a = 1, l = -2
— substr()
returns an empty string, slice()
returns "123"
, while substring()
returns "0"
. The actual refactoring path depends on the knowledge of the range of a
and l
.
Examples
Using substr()
const aString = "Mozilla";
console.log(aString.substr(0, 1)); // 'M'
console.log(aString.substr(1, 0)); // ''
console.log(aString.substr(-1, 1)); // 'a'
console.log(aString.substr(1, -1)); // ''
console.log(aString.substr(-3)); // 'lla'
console.log(aString.substr(1)); // 'ozilla'
console.log(aString.substr(-20, 2)); // 'Mo'
console.log(aString.substr(20, 2)); // ''
Specifications
Specification |
---|
ECMAScript Language Specification # sec-string.prototype.substr |
Browser compatibility
BCD tables only load in the browser