How can I share a method between components in Vue.js?

I found this technique to be more simple/satisfactory, as I prefer composition over inheritance:

src/shared.js

export default {
  foo: function() { alert("foo!") }
}

src/yourcomponent.vue

<template>...</template>

<script>
  import shared from './shared'

  export default {
    created() { 
      this.foo = shared.foo // now you can call this.foo() (in your functions/template)
    }
  }
</script>

This will also allow you to write Vue-agnostic tests.

NOTE: if you need foo to run in Vue-scope replace this.foo = shared.foo with this.foo = shared.foo.bind(this)