diff --git a/docs/spec/broker.md b/docs/spec/broker.md
index 34db7f9cb94..ea46ad9c858 100644
--- a/docs/spec/broker.md
+++ b/docs/spec/broker.md
@@ -40,10 +40,45 @@ default Broker upon creation if no Broker is specified by the user.
 A Trigger MAY be created before its assigned Broker exists. A Trigger SHOULD
 progress to Ready when its assigned Broker exists and is Ready.
 
+#### Event filters
+
+Implementations of Trigger MUST support both attributes filter and jsExpression
+filter.
+
 The attributes filter specifying a list of key-value pairs MUST be supported by
 Trigger. Events that pass the attributes filter MUST include context or
 extension attributes that match all key-value pairs exactly.
 
+The jsExpression filter specifying a Javascript expression MUST be supported by
+Trigger. Implementations MUST support evaluation of Javascript expressions
+following the standard
+[EcmaScript 5.1](http://www.ecma-international.org/ecma-262/5.1/), although they
+MAY support [EcmaScript 6](http://www.ecma-international.org/ecma-262/6.0/) or
+greater. If the evaluation of the expression returns a Javascript value
+coercible to the boolean `true`, then the trigger filter MUST pass the event.
+Implementations MUST make the event accessible, within the expression, through
+the variable `event` and its context attributes and extensions through the
+Javascript dot notation `.`. For example the `id` attribute of a CloudEvent
+should be accessible with `event.id`. Implementations MUST coerce CloudEvent
+types to JS types. For example the value of an attribute with type `Timestamp`
+should be translated to the JS type `Date`.
+
+An example jsExpression is:
+
+```js
+event.id.indexOf("francesco") != -1 &&
+  event.time != null &&
+  event.time.getFullYear() >= 2020 &&
+  event.exta != null;
+```
+
+This expression MUST pass the event iff the event id contains the string
+`francesco`, event time has a year greater or equal to 2020 and event contains
+an extension named `exta`.
+
+If a Trigger specify both attributes and jsExpression filters, then the event
+MUST pass only and only if both filters pass the event.
+
 ## Data Plane
 
 ### Ingress
diff --git a/docs/spec/spec.md b/docs/spec/spec.md
index 232e76e8e9d..8435214efec 100644
--- a/docs/spec/spec.md
+++ b/docs/spec/spec.md
@@ -243,6 +243,7 @@ retrieved from ref.
 
 ### TriggerFilter
 
-| Field      | Type              | Description                                                                         | Constraints |
-| ---------- | ----------------- | ----------------------------------------------------------------------------------- | ----------- |
-| attributes | map[string]string | A filter specifying which events match this trigger. Matches exactly on the fields. |             |
+| Field        | Type              | Description                                                                                                               | Constraints |
+| ------------ | ----------------- | ------------------------------------------------------------------------------------------------------------------------- | ----------- |
+| attributes   | map[string]string | A filter specifying which events match this trigger. Matches exactly on the fields.                                       |             |
+| jsExpression | string            | Javascript expression used for filtering events. If not specified, the behaviour is the same as an expression always true |             |