# JavaScript runtime features for CloudFront Functions
<a name="functions-javascript-runtime-features"></a>

The CloudFront Functions JavaScript runtime environment is compliant with [ECMAScript (ES) version 5.1](https://www.ecma-international.org/ecma-262/5.1/) and also supports some features of ES versions 6 through 12.

For the most up-to-date features, we recommend that you use JavaScript runtime 2.0. 

The JavaScript runtime 2.0 features has the following changes compared to 1.0:
+ Buffer module methods are available
+ The following non-standard string prototype methods are not available:
  + `String.prototype.bytesFrom()`
  + `String.prototype.fromBytes()`
  + `String.prototype.fromUTF8()`
  + `String.prototype.toBytes()`
  + `String.prototype.toUTF8()`
+ The cryptographic module has the following changes:
  + `hash.digest()` – Return type is changed to `Buffer` if no encoding is provided
  + `hmac.digest()` – Return type is changed to `Buffer` if no encoding is provided
+ For more information about additional new features, see [JavaScript runtime 2.0 features for CloudFront Functions](functions-javascript-runtime-20.md).

**Topics**
+ [JavaScript runtime 1.0 features](functions-javascript-runtime-10.md)
+ [JavaScript runtime 2.0 features](functions-javascript-runtime-20.md)