fix: handle more edge cases in looseEqual()

[jacobmllr95]

See vuejs/vue#9527 details.

[ayush987goyal]

Hey! @yyx990803 Should this function (and some other functions) in vModel.ts be exported so that it can be unit tested?

[jacobmllr95]

@yyx990803 Maybe you can revisit this when you find some time.
For more details see: vuejs/vue#9527

[wangfengming]

given a and b

const a = {x: undefined}
const b = {y: 1}

the previous logic of looseEqual(a, b) will return true because of:

const keysA = Object.keys(a)
const keysB = Object.keys(b)

keysA.length === keysB.length && keysA.every((key) => looseEqual(a[key], b[key]))

---

should check the key set of a and b to be equal

const a = {x: undefined}
const b = {y: 1}

const keysA = Object.keys(a)
const keysB = Object.keys(b)

keysA.length === keysB.length
&& keysA.every((key) => hasOwnProp(b, key))
&& keysB.every((key) => hasOwnProp(a, key))
&& keysA.every((key) => looseEqual(a[key], b[key]))

[wangfengming]

found that you use aHasKey and bHasKey to ensure the key set to be equal. great!