This is a modernized, esm-only fork of [qs](https://github.com/ljharb/qs) without unnecessary polyfills. This fork is not endorsed by [Jordan Harband](https://github.com/ljharb), the lead maintainer of qs.

# qs-esm

A querystring parsing and stringifying library with some added security.

Original Maintainer of qs (not of this fork): [Jordan Harband](https://github.com/ljharb)

The **qs** module was originally created and maintained by [TJ Holowaychuk](https://github.com/visionmedia/node-querystring).

## Migrate from qs

Install `qs-esm`, uninstall `qs` and `@types/qs`. Then, replace `import qs from 'qs';` with `import * as qs from 'qs-esm';`.

## Usage

```javascript
import * as qs from 'qs-esm'
import assert from 'assert'

const obj = qs.parse('a=c')
assert.deepEqual(obj, { a: 'c' })

const str = qs.stringify(obj)
assert.equal(str, 'a=c')
```

### Parsing Objects

[](#preventEval)

```javascript
qs.parse(string, [options])
```

**qs** allows you to create nested objects within your query strings, by surrounding the name of sub-keys with square brackets `[]`.
For example, the string `'foo[bar]=baz'` converts to:

```javascript
assert.deepEqual(qs.parse('foo[bar]=baz'), {
  foo: {
    bar: 'baz',
  },
})
```

When using the `plainObjects` option the parsed value is returned as a null object, created via `Object.create(null)` and as such you should be aware that prototype methods will not exist on it and a user may set those names to whatever value they like:

```javascript
var nullObject = qs.parse('a[hasOwnProperty]=b', { plainObjects: true })
assert.deepEqual(nullObject, { a: { hasOwnProperty: 'b' } })
```

By default parameters that would overwrite properties on the object prototype are ignored, if you wish to keep the data from those fields either use `plainObjects` as mentioned above, or set `allowPrototypes` to `true` which will allow user input to overwrite those properties.
_WARNING_ It is generally a bad idea to enable this option as it can cause problems when attempting to use the properties that have been overwritten.
Always be careful with this option.

```javascript
var protoObject = qs.parse('a[hasOwnProperty]=b', { allowPrototypes: true })
assert.deepEqual(protoObject, { a: { hasOwnProperty: 'b' } })
```

URI encoded strings work too:

```javascript
assert.deepEqual(qs.parse('a%5Bb%5D=c'), {
  a: { b: 'c' },
})
```

You can also nest your objects, like `'foo[bar][baz]=foobarbaz'`:

```javascript
assert.deepEqual(qs.parse('foo[bar][baz]=foobarbaz'), {
  foo: {
    bar: {
      baz: 'foobarbaz',
    },
  },
})
```

By default, when nesting objects **qs** will only parse up to 5 children deep.
This means if you attempt to parse a string like `'a[b][c][d][e][f][g][h][i]=j'` your resulting object will be:

```javascript
var expected = {
  a: {
    b: {
      c: {
        d: {
          e: {
            f: {
              '[g][h][i]': 'j',
            },
          },
        },
      },
    },
  },
}
var string = 'a[b][c][d][e][f][g][h][i]=j'
assert.deepEqual(qs.parse(string), expected)
```

This depth can be overridden by passing a `depth` option to `qs.parse(string, [options])`:

```javascript
var deep = qs.parse('a[b][c][d][e][f][g][h][i]=j', { depth: 1 })
assert.deepEqual(deep, { a: { b: { '[c][d][e][f][g][h][i]': 'j' } } })
```

The depth limit helps mitigate abuse when **qs** is used to parse user input, and it is recommended to keep it a reasonably small number.

For similar reasons, by default **qs** will only parse up to 1000 parameters. This can be overridden by passing a `parameterLimit` option:

```javascript
var limited = qs.parse('a=b&c=d', { parameterLimit: 1 })
assert.deepEqual(limited, { a: 'b' })
```

To bypass the leading question mark, use `ignoreQueryPrefix`:

```javascript
var prefixed = qs.parse('?a=b&c=d', { ignoreQueryPrefix: true })
assert.deepEqual(prefixed, { a: 'b', c: 'd' })
```

An optional delimiter can also be passed:

```javascript
var delimited = qs.parse('a=b;c=d', { delimiter: ';' })
assert.deepEqual(delimited, { a: 'b', c: 'd' })
```

Delimiters can be a regular expression too:

```javascript
var regexed = qs.parse('a=b;c=d,e=f', { delimiter: /[;,]/ })
assert.deepEqual(regexed, { a: 'b', c: 'd', e: 'f' })
```

Option `allowDots` can be used to enable dot notation:

```javascript
var withDots = qs.parse('a.b=c', { allowDots: true })
assert.deepEqual(withDots, { a: { b: 'c' } })
```

Option `decodeDotInKeys` can be used to decode dots in keys
Note: it implies `allowDots`, so `parse` will error if you set `decodeDotInKeys` to `true`, and `allowDots` to `false`.

```javascript
var withDots = qs.parse('name%252Eobj.first=John&name%252Eobj.last=Doe', { decodeDotInKeys: true })
assert.deepEqual(withDots, { 'name.obj': { first: 'John', last: 'Doe' } })
```

Option `allowEmptyArrays` can be used to allowing empty array values in object

```javascript
var withEmptyArrays = qs.parse('foo[]&bar=baz', { allowEmptyArrays: true })
assert.deepEqual(withEmptyArrays, { foo: [], bar: 'baz' })
```

Option `duplicates` can be used to change the behavior when duplicate keys are encountered

```javascript
assert.deepEqual(qs.parse('foo=bar&foo=baz'), { foo: ['bar', 'baz'] })
assert.deepEqual(qs.parse('foo=bar&foo=baz', { duplicates: 'combine' }), { foo: ['bar', 'baz'] })
assert.deepEqual(qs.parse('foo=bar&foo=baz', { duplicates: 'first' }), { foo: 'bar' })
assert.deepEqual(qs.parse('foo=bar&foo=baz', { duplicates: 'last' }), { foo: 'baz' })
```

If you have to deal with legacy browsers or services, there's also support for decoding percent-encoded octets as iso-8859-1:

```javascript
var oldCharset = qs.parse('a=%A7', { charset: 'iso-8859-1' })
assert.deepEqual(oldCharset, { a: '§' })
```

Some services add an initial `utf8=✓` value to forms so that old Internet Explorer versions are more likely to submit the form as utf-8.
Additionally, the server can check the value against wrong encodings of the checkmark character and detect that a query string or `application/x-www-form-urlencoded` body was _not_ sent as utf-8, eg. if the form had an `accept-charset` parameter or the containing page had a different character set.

**qs** supports this mechanism via the `charsetSentinel` option.
If specified, the `utf8` parameter will be omitted from the returned object.
It will be used to switch to `iso-8859-1`/`utf-8` mode depending on how the checkmark is encoded.

**Important**: When you specify both the `charset` option and the `charsetSentinel` option, the `charset` will be overridden when the request contains a `utf8` parameter from which the actual charset can be deduced.
In that sense the `charset` will behave as the default charset rather than the authoritative charset.

```javascript
var detectedAsUtf8 = qs.parse('utf8=%E2%9C%93&a=%C3%B8', {
  charset: 'iso-8859-1',
  charsetSentinel: true,
})
assert.deepEqual(detectedAsUtf8, { a: 'ø' })

// Browsers encode the checkmark as ✓ when submitting as iso-8859-1:
var detectedAsIso8859_1 = qs.parse('utf8=%26%2310003%3B&a=%F8', {
  charset: 'utf-8',
  charsetSentinel: true,
})
assert.deepEqual(detectedAsIso8859_1, { a: 'ø' })
```

If you want to decode the `&#...;` syntax to the actual character, you can specify the `interpretNumericEntities` option as well:

```javascript
var detectedAsIso8859_1 = qs.parse('a=%26%239786%3B', {
  charset: 'iso-8859-1',
  interpretNumericEntities: true,
})
assert.deepEqual(detectedAsIso8859_1, { a: '☺' })
```

It also works when the charset has been detected in `charsetSentinel` mode.

### Parsing Arrays

**qs** can also parse arrays using a similar `[]` notation:

```javascript
var withArray = qs.parse('a[]=b&a[]=c')
assert.deepEqual(withArray, { a: ['b', 'c'] })
```

You may specify an index as well:

```javascript
var withIndexes = qs.parse('a[1]=c&a[0]=b')
assert.deepEqual(withIndexes, { a: ['b', 'c'] })
```

Note that the only difference between an index in an array and a key in an object is that the value between the brackets must be a number to create an array.
When creating arrays with specific indices, **qs** will compact a sparse array to only the existing values preserving their order:

```javascript
var noSparse = qs.parse('a[1]=b&a[15]=c')
assert.deepEqual(noSparse, { a: ['b', 'c'] })
```

You may also use `allowSparse` option to parse sparse arrays:

```javascript
var sparseArray = qs.parse('a[1]=2&a[3]=5', { allowSparse: true })
assert.deepEqual(sparseArray, { a: [, '2', , '5'] })
```

Note that an empty string is also a value, and will be preserved:

```javascript
var withEmptyString = qs.parse('a[]=&a[]=b')
assert.deepEqual(withEmptyString, { a: ['', 'b'] })

var withIndexedEmptyString = qs.parse('a[0]=b&a[1]=&a[2]=c')
assert.deepEqual(withIndexedEmptyString, { a: ['b', '', 'c'] })
```

**qs** will also limit specifying indices in an array to a maximum index of `20`.
Any array members with an index of greater than `20` will instead be converted to an object with the index as the key.
This is needed to handle cases when someone sent, for example, `a[999999999]` and it will take significant time to iterate over this huge array.

```javascript
var withMaxIndex = qs.parse('a[100]=b')
assert.deepEqual(withMaxIndex, { a: { 100: 'b' } })
```

This limit can be overridden by passing an `arrayLimit` option:

```javascript
var withArrayLimit = qs.parse('a[1]=b', { arrayLimit: 0 })
assert.deepEqual(withArrayLimit, { a: { 1: 'b' } })
```

To disable array parsing entirely, set `parseArrays` to `false`.

```javascript
var noParsingArrays = qs.parse('a[]=b', { parseArrays: false })
assert.deepEqual(noParsingArrays, { a: { 0: 'b' } })
```

If you mix notations, **qs** will merge the two items into an object:

```javascript
var mixedNotation = qs.parse('a[0]=b&a[b]=c')
assert.deepEqual(mixedNotation, { a: { 0: 'b', b: 'c' } })
```

You can also create arrays of objects:

```javascript
var arraysOfObjects = qs.parse('a[][b]=c')
assert.deepEqual(arraysOfObjects, { a: [{ b: 'c' }] })
```

Some people use comma to join array, **qs** can parse it:

```javascript
var arraysOfObjects = qs.parse('a=b,c', { comma: true })
assert.deepEqual(arraysOfObjects, { a: ['b', 'c'] })
```

(_this cannot convert nested objects, such as `a={b:1},{c:d}`_)

### Parsing primitive/scalar values (numbers, booleans, null, etc)

By default, all values are parsed as strings.
This behavior will not change and is explained in [issue #91](https://github.com/ljharb/qs/issues/91).

```javascript
var primitiveValues = qs.parse('a=15&b=true&c=null')
assert.deepEqual(primitiveValues, { a: '15', b: 'true', c: 'null' })
```

If you wish to auto-convert values which look like numbers, booleans, and other values into their primitive counterparts, you can use the [query-types Express JS middleware](https://github.com/xpepermint/query-types) which will auto-convert all request query parameters.

### Stringifying

[](#preventEval)

```javascript
qs.stringify(object, [options])
```

When stringifying, **qs** by default URI encodes output. Objects are stringified as you would expect:

```javascript
assert.equal(qs.stringify({ a: 'b' }), 'a=b')
assert.equal(qs.stringify({ a: { b: 'c' } }), 'a%5Bb%5D=c')
```

This encoding can be disabled by setting the `encode` option to `false`:

```javascript
var unencoded = qs.stringify({ a: { b: 'c' } }, { encode: false })
assert.equal(unencoded, 'a[b]=c')
```

Encoding can be disabled for keys by setting the `encodeValuesOnly` option to `true`:

```javascript
var encodedValues = qs.stringify(
  { a: 'b', c: ['d', 'e=f'], f: [['g'], ['h']] },
  { encodeValuesOnly: true },
)
assert.equal(encodedValues, 'a=b&c[0]=d&c[1]=e%3Df&f[0][0]=g&f[1][0]=h')
```

This encoding can also be replaced by a custom encoding method set as `encoder` option:

```javascript
var encoded = qs.stringify(
  { a: { b: 'c' } },
  {
    encoder: function (str) {
      // Passed in values `a`, `b`, `c`
      return // Return encoded string
    },
  },
)
```

_(Note: the `encoder` option does not apply if `encode` is `false`)_

Analogue to the `encoder` there is a `decoder` option for `parse` to override decoding of properties and values:

```javascript
var decoded = qs.parse('x=z', {
  decoder: function (str) {
    // Passed in values `x`, `z`
    return // Return decoded string
  },
})
```

You can encode keys and values using different logic by using the type argument provided to the encoder:

```javascript
var encoded = qs.stringify(
  { a: { b: 'c' } },
  {
    encoder: function (str, defaultEncoder, charset, type) {
      if (type === 'key') {
        return // Encoded key
      } else if (type === 'value') {
        return // Encoded value
      }
    },
  },
)
```

The type argument is also provided to the decoder:

```javascript
var decoded = qs.parse('x=z', {
  decoder: function (str, defaultDecoder, charset, type) {
    if (type === 'key') {
      return // Decoded key
    } else if (type === 'value') {
      return // Decoded value
    }
  },
})
```

Examples beyond this point will be shown as though the output is not URI encoded for clarity.
Please note that the return values in these cases _will_ be URI encoded during real usage.

When arrays are stringified, they follow the `arrayFormat` option, which defaults to `indices`:

```javascript
qs.stringify({ a: ['b', 'c', 'd'] })
// 'a[0]=b&a[1]=c&a[2]=d'
```

You may override this by setting the `indices` option to `false`, or to be more explicit, the `arrayFormat` option to `repeat`:

```javascript
qs.stringify({ a: ['b', 'c', 'd'] }, { indices: false })
// 'a=b&a=c&a=d'
```

You may use the `arrayFormat` option to specify the format of the output array:

```javascript
qs.stringify({ a: ['b', 'c'] }, { arrayFormat: 'indices' })
// 'a[0]=b&a[1]=c'
qs.stringify({ a: ['b', 'c'] }, { arrayFormat: 'brackets' })
// 'a[]=b&a[]=c'
qs.stringify({ a: ['b', 'c'] }, { arrayFormat: 'repeat' })
// 'a=b&a=c'
qs.stringify({ a: ['b', 'c'] }, { arrayFormat: 'comma' })
// 'a=b,c'
```

Note: when using `arrayFormat` set to `'comma'`, you can also pass the `commaRoundTrip` option set to `true` or `false`, to append `[]` on single-item arrays, so that they can round trip through a parse.

When objects are stringified, by default they use bracket notation:

```javascript
qs.stringify({ a: { b: { c: 'd', e: 'f' } } })
// 'a[b][c]=d&a[b][e]=f'
```

You may override this to use dot notation by setting the `allowDots` option to `true`:

```javascript
qs.stringify({ a: { b: { c: 'd', e: 'f' } } }, { allowDots: true })
// 'a.b.c=d&a.b.e=f'
```

You may encode the dot notation in the keys of object with option `encodeDotInKeys` by setting it to `true`:
Note: it implies `allowDots`, so `stringify` will error if you set `decodeDotInKeys` to `true`, and `allowDots` to `false`.
Caveat: when `encodeValuesOnly` is `true` as well as `encodeDotInKeys`, only dots in keys and nothing else will be encoded.

```javascript
qs.stringify(
  { 'name.obj': { first: 'John', last: 'Doe' } },
  { allowDots: true, encodeDotInKeys: true },
)
// 'name%252Eobj.first=John&name%252Eobj.last=Doe'
```

You may allow empty array values by setting the `allowEmptyArrays` option to `true`:

```javascript
qs.stringify({ foo: [], bar: 'baz' }, { allowEmptyArrays: true })
// 'foo[]&bar=baz'
```

Empty strings and null values will omit the value, but the equals sign (=) remains in place:

```javascript
assert.equal(qs.stringify({ a: '' }), 'a=')
```

Key with no values (such as an empty object or array) will return nothing:

```javascript
assert.equal(qs.stringify({ a: [] }), '')
assert.equal(qs.stringify({ a: {} }), '')
assert.equal(qs.stringify({ a: [{}] }), '')
assert.equal(qs.stringify({ a: { b: [] } }), '')
assert.equal(qs.stringify({ a: { b: {} } }), '')
```

Properties that are set to `undefined` will be omitted entirely:

```javascript
assert.equal(qs.stringify({ a: null, b: undefined }), 'a=')
```

The query string may optionally be prepended with a question mark:

```javascript
assert.equal(qs.stringify({ a: 'b', c: 'd' }, { addQueryPrefix: true }), '?a=b&c=d')
```

The delimiter may be overridden with stringify as well:

```javascript
assert.equal(qs.stringify({ a: 'b', c: 'd' }, { delimiter: ';' }), 'a=b;c=d')
```

If you only want to override the serialization of `Date` objects, you can provide a `serializeDate` option:

```javascript
var date = new Date(7)
assert.equal(qs.stringify({ a: date }), 'a=1970-01-01T00:00:00.007Z'.replace(/:/g, '%3A'))
assert.equal(
  qs.stringify(
    { a: date },
    {
      serializeDate: function (d) {
        return d.getTime()
      },
    },
  ),
  'a=7',
)
```

You may use the `sort` option to affect the order of parameter keys:

```javascript
function alphabeticalSort(a, b) {
  return a.localeCompare(b)
}
assert.equal(qs.stringify({ a: 'c', z: 'y', b: 'f' }, { sort: alphabeticalSort }), 'a=c&b=f&z=y')
```

Finally, you can use the `filter` option to restrict which keys will be included in the stringified output.
If you pass a function, it will be called for each key to obtain the replacement value.
Otherwise, if you pass an array, it will be used to select properties and array indices for stringification:

```javascript
function filterFunc(prefix, value) {
  if (prefix == 'b') {
    // Return an `undefined` value to omit a property.
    return
  }
  if (prefix == 'e[f]') {
    return value.getTime()
  }
  if (prefix == 'e[g][0]') {
    return value * 2
  }
  return value
}
qs.stringify({ a: 'b', c: 'd', e: { f: new Date(123), g: [2] } }, { filter: filterFunc })
// 'a=b&c=d&e[f]=123&e[g][0]=4'
qs.stringify({ a: 'b', c: 'd', e: 'f' }, { filter: ['a', 'e'] })
// 'a=b&e=f'
qs.stringify({ a: ['b', 'c', 'd'], e: 'f' }, { filter: ['a', 0, 2] })
// 'a[0]=b&a[2]=d'
```

You could also use `filter` to inject custom serialization for user defined types.
Consider you're working with some api that expects query strings of the format for ranges:

```
https://domain.com/endpoint?range=30...70
```

For which you model as:

```javascript
class Range {
  constructor(from, to) {
    this.from = from
    this.to = to
  }
}
```

You could _inject_ a custom serializer to handle values of this type:

```javascript
qs.stringify(
  {
    range: new Range(30, 70),
  },
  {
    filter: (prefix, value) => {
      if (value instanceof Range) {
        return `${value.from}...${value.to}`
      }
      // serialize the usual way
      return value
    },
  },
)
// range=30...70
```

### Handling of `null` values

By default, `null` values are treated like empty strings:

```javascript
var withNull = qs.stringify({ a: null, b: '' })
assert.equal(withNull, 'a=&b=')
```

Parsing does not distinguish between parameters with and without equal signs.
Both are converted to empty strings.

```javascript
var equalsInsensitive = qs.parse('a&b=')
assert.deepEqual(equalsInsensitive, { a: '', b: '' })
```

To distinguish between `null` values and empty strings use the `strictNullHandling` flag. In the result string the `null`
values have no `=` sign:

```javascript
var strictNull = qs.stringify({ a: null, b: '' }, { strictNullHandling: true })
assert.equal(strictNull, 'a&b=')
```

To parse values without `=` back to `null` use the `strictNullHandling` flag:

```javascript
var parsedStrictNull = qs.parse('a&b=', { strictNullHandling: true })
assert.deepEqual(parsedStrictNull, { a: null, b: '' })
```

To completely skip rendering keys with `null` values, use the `skipNulls` flag:

```javascript
var nullsSkipped = qs.stringify({ a: 'b', c: null }, { skipNulls: true })
assert.equal(nullsSkipped, 'a=b')
```

If you're communicating with legacy systems, you can switch to `iso-8859-1` using the `charset` option:

```javascript
var iso = qs.stringify({ æ: 'æ' }, { charset: 'iso-8859-1' })
assert.equal(iso, '%E6=%E6')
```

Characters that don't exist in `iso-8859-1` will be converted to numeric entities, similar to what browsers do:

```javascript
var numeric = qs.stringify({ a: '☺' }, { charset: 'iso-8859-1' })
assert.equal(numeric, 'a=%26%239786%3B')
```

You can use the `charsetSentinel` option to announce the character by including an `utf8=✓` parameter with the proper encoding if the checkmark, similar to what Ruby on Rails and others do when submitting forms.

```javascript
var sentinel = qs.stringify({ a: '☺' }, { charsetSentinel: true })
assert.equal(sentinel, 'utf8=%E2%9C%93&a=%E2%98%BA')

var isoSentinel = qs.stringify({ a: 'æ' }, { charsetSentinel: true, charset: 'iso-8859-1' })
assert.equal(isoSentinel, 'utf8=%26%2310003%3B&a=%E6')
```

### Dealing with special character sets

By default the encoding and decoding of characters is done in `utf-8`, and `iso-8859-1` support is also built in via the `charset` parameter.

If you wish to encode querystrings to a different character set (i.e.
[Shift JIS](https://en.wikipedia.org/wiki/Shift_JIS)) you can use the
[`qs-iconv`](https://github.com/martinheidegger/qs-iconv) library:

```javascript
var encoder = require('qs-iconv/encoder')('shift_jis')
var shiftJISEncoded = qs.stringify({ a: 'こんにちは！' }, { encoder: encoder })
assert.equal(shiftJISEncoded, 'a=%82%B1%82%F1%82%C9%82%BF%82%CD%81I')
```

This also works for decoding of query strings:

```javascript
var decoder = require('qs-iconv/decoder')('shift_jis')
var obj = qs.parse('a=%82%B1%82%F1%82%C9%82%BF%82%CD%81I', { decoder: decoder })
assert.deepEqual(obj, { a: 'こんにちは！' })
```

### RFC 3986 and RFC 1738 space encoding

RFC3986 used as default option and encodes ' ' to _%20_ which is backward compatible.
In the same time, output can be stringified as per RFC1738 with ' ' equal to '+'.

```
assert.equal(qs.stringify({ a: 'b c' }), 'a=b%20c');
assert.equal(qs.stringify({ a: 'b c' }, { format : 'RFC3986' }), 'a=b%20c');
assert.equal(qs.stringify({ a: 'b c' }, { format : 'RFC1738' }), 'a=b+c');
```