Slightly less code (still un-golfed as that does not appear to be the
stated requirement described at OP) using the same/similar pattern.

let input_json_string = `{"z":null}`;
const convertJSONNullToUndefined = json => {
  const o = JSON.parse(json);
  for (const key in o) if (o[key] === null) o[key] = void 0;
  return o
}

convertJSONNullToUndefined(input_json_string); // do destructuring stuff
```
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Slightly less code (still un-golfed as that does not appear to be the
stated requirement described at OP) using the same/similar pattern.


```
let input_json_string = `{"z":null}`;
const convertJSONNullToUndefined = json => {
  const o = JSON.parse(json);
  for (const key in o) if (o[key] === null) o[key] = void 0;
  return o;
}

convertJSONNullToUndefined(input_json_string); // do destructuring stuff
```

Alternatively a single use function expecting a single input parameter that is a valid JSON string consisting of a ```{"key":"value"}```

```
const convertJSONNullToUndefined = (json, o = JSON.parse(json), [k] = Object.keys(o), res = o[k] !== null ? o : {[k]: void 0}) => res;

const { z = 'a' } = convertJSONNullToUndefined(`{"z":null}`);
```

Since ```.find()``` returns ```undefined``` if the value is not matched

```
const [z = 'a'] = [Object.values(JSON.parse(`{"z":null}`)).find(value => value !== null)];
```

or 

```
const [z = 'a'] = Object.values(JSON.parse(`{"z":null}`)).filter(value => value !== null);
```

Slightly less code (still un-golfed as that does not appear to be the
stated requirement described at OP) using the same/similar pattern.


```
let input_json_string = `{"z":null}`;
const convertJSONNullToUndefined = json => {
  const o = JSON.parse(json);
  for (const key in o) if (o[key] === null) o[key] = void 0;
  return o;
}

convertJSONNullToUndefined(input_json_string); // do destructuring stuff
```

Alternatively a single use function expecting a single input parameter that is a valid JSON string consisting of a ```{"key":"value"}```

```
const convertJSONNullToUndefined = (json, o = JSON.parse(json), [k] = Object.keys(o), res = o[k] !== null ? o : {[k]: void 0}) => res;

const { z = 'a' } = convertJSONNullToUndefined(`{"z":null}`);
```

Since ```.find()``` returns ```undefined``` if the value is not matched

```
const [z = 'a'] = [Object.values(JSON.parse(`{"z":null}`)).find(value => value !== null)];
```

Slightly less code (still un-golfed as that does not appear to be the
stated requirement described at OP) using the same/similar pattern.


```
let input_json_string = `{"z":null}`;
const convertJSONNullToUndefined = json => {
  const o = JSON.parse(json);
  for (const key in o) if (o[key] === null) o[key] = void 0;
  return o;
}

convertJSONNullToUndefined(input_json_string); // do destructuring stuff
```

Alternatively a single use function expecting a single input parameter that is a valid JSON string consisting of a ```{"key":"value"}```

```
const convertJSONNullToUndefined = (json, o = JSON.parse(json), [k] = Object.keys(o), res = o[k] !== null ? o : {[k]: void 0}) => res;

const { z = 'a' } = convertJSONNullToUndefined(`{"z":null}`);
```

Since ```.find()``` returns ```undefined``` 

```
const [z = 'a'] = [Object.values(JSON.parse(`{"z":null}`)).find(value => value !== null)];
```

Slightly less code (still un-golfed as that does not appear to be the
stated requirement described at OP) using the same/similar pattern.


```
let input_json_string = `{"z":null}`;
const convertJSONNullToUndefined = json => {
  const o = JSON.parse(json);
  for (const key in o) if (o[key] === null) o[key] = void 0;
  return o;
}

convertJSONNullToUndefined(input_json_string); // do destructuring stuff
```

Alternatively a single use function expecting a single input parameter that is a valid JSON string consisting of a ```{"key":"value"}```

```
const convertJSONNullToUndefined = (json, o = JSON.parse(json), [k] = Object.keys(o), res = o[k] !== null ? o : {[k]: void 0}) => res;

const { z = 'a' } = convertJSONNullToUndefined(`{"z":null}`);
```

Slightly less code (still un-golfed as that does not appear to be the
stated requirement described at OP) using the same/similar pattern.


```
let input_json_string = `{"z":null}`;
const convertJSONNullToUndefined = json => {
  const o = JSON.parse(json);
  for (const key in o) if (o[key] === null) o[key] = void 0;
  return o;
}

convertJSONNullToUndefined(input_json_string); // do destructuring stuff
```

Alternatively a single use function expecting a single input parameter that is a valid JSON string consisting of a ```{"key":"value"}```

```
const convertJSONNullToUndefined = (json, o = JSON.parse(json), [k] = Object.keys(o), res = o[k] !== null ? o : {[k]: void 0}) => res;

const { z = 'a' } = fn(`{"z":null}`);
```

Slightly less code (still un-golfed as that does not appear to be the
stated requirement described at OP) using the same/similar pattern.


```
let input_json_string = `{"z":null}`;
const convertJSONNullToUndefined = json => {
  const o = JSON.parse(json);
  for (const key in o) if (o[key] === null) o[key] = void 0;
  return o;
}

convertJSONNullToUndefined(input_json_string); // do destructuring stuff
```

Alternatively a single use function expecting a single input parameter that is a valid JSON string consisting of a ```{"key":"value"}```

```
const convertJSONNullToUndefined = (json, o = JSON.parse(o), [k] = Object.keys(o), res = o[k] !== null ? o : {[k]: void 0}) => res;

const { z = 'a' } = fn(`{"z":null}`);
```

Slightly less code (still un-golfed as that does not appear to be the
stated requirement described at OP) using the same/similar pattern.


```
let input_json_string = `{"z":null}`;
const convertJSONNullToUndefined = json => {
  const o = JSON.parse(json);
  for (const key in o) if (o[key] === null) o[key] = void 0;
  return o;
}

convertJSONNullToUndefined(input_json_string); // do destructuring stuff
```

Slightly less code (still un-golfed as that does not appear to be the
stated requirement described at OP) using the same/similar pattern.


```
let input_json_string = `{"z":null}`;
const convertJSONNullToUndefined = json => {
  const o = JSON.parse(json);
  for (const key in o) if (o[key] === null) o[key] = void 0;
  return o
}

convertJSONNullToUndefined(input_json_string); // do destructuring stuff
```