VueJS: variable is undefined inside computed only

Don't use arrow function ()=>{} for computed, it will cause the wrong context (not current Vue instance).

Change to function () {} then it should work fine.

And for methods, watch, you should follow same rules.

computed: {
    dummyText: function () { // change to function () {}
        if (this.address.length > 0 && this.nothingFound) { // This will return error
            return 'There is no such address'
        } else {
            return 'Keep typing'
        }
    }
},

You can also use es2015 shorthand for a method function:

computed: {
    dummyText() {
        return this.address.length > 0 && this.nothingFound ? 'There is no such address' : 'Keep typing';
    }
}