Function observe

static/vuejs/vue.runtime.common.js:789–809

* Attempt to create an observer instance for a value, * returns the new observer if successfully observed, * or the existing observer if the value already has one.

(value, asRootData)

 * or the existing observer if the value already has one.
 */
function observe (value, asRootData) {
  if (!isObject(value)) {
    return
  }
  var ob;
  if (hasOwn(value, '__ob__') && value.__ob__ instanceof Observer) {
    ob = value.__ob__;
  } else if (
    observerState.shouldConvert &&
    !isServerRendering() &&
    (Array.isArray(value) || isPlainObject(value)) &&
    Object.isExtensible(value) &&
    !value._isVue
  ) {
    ob = new Observer(value);
  }
  if (asRootData && ob) {
    ob.vmCount++;
  }
  return ob
}

/**
 * Define a reactive property on an Object.