[9.5.2] The query string doesn't cleanup undefined value

[nghiepdev]

Bug report

Describe the bug

Navigating router via push:

router.push({
  pathname: "/page-a",
  query: {
    param1: "ok",
    param2: undefined
  }
})

// /pages/page-2
const router = useRouter();

console.log(router.query);

Result

{
  param1: "ok",
  param2: "undefined"
}

In the page-a: The param2 is an undefined string value.

Expected behavior

Result

{
  param1: "ok"
}

The param2 should be an undefined value.

System information

• OS: [macOS]
• Browser (if applies) [chrome]
• Version of Next.js: [9.5.2]
• Version of Node.js: [14]

Additional context

#15378

[Janpot]

Keep in mind that in next@<9.5, the result would be

{
  param1: "ok",
  param2: ""
}

and not

{
  param1: "ok"
}

edit:

In fact, been testing values for query from pre vs post #15378. and the results are quite different, i.e.

  router.push({
      pathname: "/query",
      query: {
        param1: '',
        param2: undefined,
        param3: null,
        param4: 0,
        param5: false,
        param6: [],
        param7: {},
        param8: NaN,
        param9: new Date(1234),
        param10: /hello/
      }
    })

pre #15378 this results in:

{
  "param1": "",
  "param2": "",
  "param3": "",
  "param4": "0",
  "param5": "false",
  "param7": "",
  "param8": "",
  "param9": "",
  "param10": ""
}

post #15378 we get:

{
  "param1": "",
  "param2": "undefined",
  "param3": "null",
  "param4": "0",
  "param5": "false",
  "param7": "[object Object]",
  "param8": "NaN",
  "param9": "Thu Jan 01 1970 01:00:01 GMT+0100 (Central European Standard Time)",
  "param10": "/hello/"
} 

looks like these only agree on "", 0, false and [] .

[balazsorban44]

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