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
------


```js
substr(start)
substr(start, length)
```

### Parameters

`start`

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

`length` Optional

The number of characters to extract.

### 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 at `max(start + str.length, 0)`.
* If `start` is omitted or `undefined`, it's treated as `0`.
* If `length` is omitted or `undefined`, or if `start + length >= str.length`, `substr()` extracts characters to the end of the string.
* If `length < 0`, an empty string is returned.
* For both `start` and `length`, `NaN` is treated as `0`.

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()


```js
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
---------------------

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See also
--------

* Polyfill of `String.prototype.substr` in `core-js`
* `String.prototype.slice()`
* `String.prototype.substring()`