ES6/ES7 String.prototype.at polyfill

A robust & optimized ES3-compatible polyfill for the String.prototype.at proposal for ECMAScript 6/7.

Spec bug ticket: https://bugs.ecmascript.org/show_bug.cgi?id=2073

Spec proposal for String.prototype.at(pos)

NOTE: Returns a single-element String containing the code point at element position pos in the String value resulting from converting the this object to a String. If there is no element at that position, the result is the empty String. The result is a String value, not a String object.

When the at method is called with one argument pos, the following steps are taken:

1. Let O be RequireObjectCoercible(this value).
2. Let S be ToString(O).
3. ReturnIfAbrupt(S).
4. Let position be ToInteger(pos).
5. ReturnIfAbrupt(position).
6. Let size be the number of elements in S.
7. If position < 0 or position ≥ size, return the empty String.
8. Let first be the code unit at index position in the String S.
9. Let cuFirst be the code unit value of the element at index 0 in the String first.
10. If cuFirst < 0xD800 or cuFirst > 0xDBFF or position + 1 = size, then return first.
11. Let cuSecond be the code unit value of the element at index position + 1 in the String S.
12. If cuSecond < 0xDC00 or cuSecond > 0xDFFF, then return first.
13. Let second be the code unit at index position + 1 in the string S.
14. Let cp be (first – 0xD800) × 0x400 + (second – 0xDC00) + 0x10000.
15. Return the elements of the UTF-16 Encoding (clause 6) of cp.

NOTE: The at function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

Installation

In a browser:

<script src="at.js"></script>

Via npm:

npm install string.prototype.at

Then, in Node.js:

require('string.prototype.at');

// On Windows and on Mac systems with default settings, case doesn’t matter,
// which allows you to do this instead:
require('String.prototype.at');

Notes

Polyfills and test suites for String.fromCodePoint, String.prototype.codePointAt are available, too.

Author

Mathias Bynens

License

This polyfill is available under the MIT license.