It's declared this way:

let vm = new Vue({
  data() {
    return {
      testText: "The quick brown fox jumps over the lazy dog",
      planck : {
        "name": "Planck",
        "fonts" : [
            "400"
        ],
        "tags" : ["sans-serif"],
    },
    bizon : {
        "name": "Bizon",
        "fonts" : [
            "400"
        ],
        "tags" : ["sans-serif"],
    },
    archive : {
        "name": "Archive",
        "fonts" : [
            "400"
        ],
        "tags" : ["sans-serif"],
    },
    typefaceList : [
      this.archive,
      this.planck,
      this.bizon
    ]
    }
  }
}).$mount('#root')