the following is demo

<script>
  export default {
    props: ["userList", "sessionIndex", "session", "search"],
    filters: {
      selection (list, sear) {
        return list.filter(item => item.name.indexOf(sear) > -1)
      }
    }
  }
</script>

<template>
    <div class="m-list">
        <ul>
            <li v-for="item in userList | selection(search)" :class="{ active: session.userId === item.id }">
                <img class="avatar" width="30" height="30" :alt="item.name" :src="item.img">
                <p class="name">{{item.name}}

            </li>
        </ul>
    </div>
</template>

console error message is as follows

Vue warn: Property or method "selection" is not defined on the instance but referenced during render. Make sure that this property is reactive, either in the data option, or for class-based components, by initializing the property.

Error in render: "TypeError: _vm.selection is not a function"

TypeError: _vm.selection is not a function