Use

The following illustrates what ~some.key would return given different data structures:

// A non-observable JS object:
const data1 = { some: { key: "value" } };

//-> "value"

// A non-observable JS object w/ a function at the end
const data2 = { some: { key: function() {
    return "value";
} } };

//-> "value"

// A non-observable JS object with intermediate functions:
const data3 = { some: function() {
    return { key: "value" };
} };

//-> "value"

// A observable can-map
const data4 = { some: new DefineMap( { key: "value" } ) };

//-> canCompute("value")

// A method on an observable can-map that reads observables
const Some = DefineMap.extend( {
    value: "string",
    key: function() {
        return this.value;
    }
} );
const data5 = { some: new Some( { value: "value" } ) };

//-> compute(function(){ return this.value; })

Notice that ~ should only be used once in a value lookup expression.