# qs
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[](https://bestpractices.coreinfrastructure.org/projects/9058)
[][package-url]
A querystring parsing and stringifying library with some added security.
Lead Maintainer: [Jordan Harband](https://github.com/ljharb)
The **qs** module was originally created and maintained by [TJ Holowaychuk](https://github.com/visionmedia/node-querystring).
## Usage
var qs = require('qs');
var assert = require('assert');
var obj = qs.parse('a=c');
assert.deepEqual(obj, { a: 'c' });
var str = qs.stringify(obj);
assert.equal(str, 'a=c');
### Parsing Objects
[](#preventEval)
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:
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 `{ __proto__: 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:
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.
var protoObject = qs.parse('a[hasOwnProperty]=b', { allowPrototypes: true });
assert.deepEqual(protoObject, { a: { hasOwnProperty: 'b' } });
URI encoded strings work too:
assert.deepEqual(qs.parse('a%5Bb%5D=c'), {
a: { b: 'c' }
});
You can also nest your objects, like `'foo[bar][baz]=foobarbaz'`:
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:
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])`:
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' } } });
You can configure **qs** to throw an error when parsing nested input beyond this depth using the `strictDepth` option (defaulted to false):
try {
qs.parse('a[b][c][d][e][f][g][h][i]=j', { depth: 1, strictDepth: true });
} catch (err) {
assert(err instanceof RangeError);
assert.strictEqual(err.message, 'Input depth exceeded depth option of 1 and strictDepth is true');
}
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. The strictDepth option adds a layer of protection by throwing an error when the limit is exceeded, allowing you to catch and handle such cases.
For similar reasons, by default **qs** will only parse up to 1000 parameters. This can be overridden by passing a `parameterLimit` option:
var limited = qs.parse('a=b&c=d', { parameterLimit: 1 });
assert.deepEqual(limited, { a: 'b' });
If you want an error to be thrown whenever the a limit is exceeded (eg, `parameterLimit`, `arrayLimit`), set the `throwOnLimitExceeded` option to `true`. This option will generate a descriptive error if the query string exceeds a configured limit.
try {
qs.parse('a=1&b=2&c=3&d=4', { parameterLimit: 3, throwOnLimitExceeded: true });
} catch (err) {
assert(err instanceof Error);
assert.strictEqual(err.message, 'Parameter limit exceeded. Only 3 parameters allowed.');
}
When `throwOnLimitExceeded` is set to `false` (default), **qs** will parse up to the specified `parameterLimit` and ignore the rest without throwing an error.
To bypass the leading question mark, use `ignoreQueryPrefix`:
var prefixed = qs.parse('?a=b&c=d', { ignoreQueryPrefix: true });
assert.deepEqual(prefixed, { a: 'b', c: 'd' });
An optional delimiter can also be passed:
var delimited = qs.parse('a=b;c=d', { delimiter: ';' });
assert.deepEqual(delimited, { a: 'b', c: 'd' });
Delimiters can be a regular expression too:
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:
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`.
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 allow empty array values in an object
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
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' });
Note that keys with bracket notation (`[]`) always combine into arrays, regardless of the `duplicates` setting:
assert.deepEqual(qs.parse('a=1&a=2&b[]=1&b[]=2', { duplicates: 'last' }), { a: '2', b: ['1', '2'] });
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.
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:
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:
var withArray = qs.parse('a[]=b&a[]=c');
assert.deepEqual(withArray, { a: ['b', 'c'] });
You may specify an index as well:
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:
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:
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:
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 arrays to a maximum of `20` elements.
Any array members with an index of `20` or greater 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.
var withMaxIndex = qs.parse('a[100]=b');
assert.deepEqual(withMaxIndex, { a: { '100': 'b' } });
This limit can be overridden by passing an `arrayLimit` option:
var withArrayLimit = qs.parse('a[1]=b', { arrayLimit: 0 });
assert.deepEqual(withArrayLimit, { a: { '1': 'b' } });
If you want to throw an error whenever the array limit is exceeded, set the `throwOnLimitExceeded` option to `true`. This option will generate a descriptive error if the query string exceeds a configured limit.
try {
qs.parse('a[1]=b', { arrayLimit: 0, throwOnLimitExceeded: true });
} catch (err) {
assert(err instanceof Error);
assert.strictEqual(err.message, 'Array limit exceeded. Only 0 elements allowed in an array.');
}
When `throwOnLimitExceeded` is set to `false` (default), **qs** will parse up to the specified `arrayLimit` and if the limit is exceeded, the array will instead be converted to an object with the index as the key
To prevent array syntax (`a[]`, `a[0]`) from being parsed as arrays, set `parseArrays` to `false`.
Note that duplicate keys (e.g. `a=b&a=c`) may still produce arrays when `duplicates` is `'combine'` (the default).
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:
var mixedNotation = qs.parse('a[0]=b&a[b]=c');
assert.deepEqual(mixedNotation, { a: { '0': 'b', b: 'c' } });
When a key appears as both a plain value and an object, **qs** will by default wrap the conflicting values in an array (`strictMerge` defaults to `true`):
assert.deepEqual(qs.parse('a[b]=c&a=d'), { a: [{ b: 'c' }, 'd'] });
assert.deepEqual(qs.parse('a=d&a[b]=c'), { a: ['d', { b: 'c' }] });
To restore the legacy behavior (where the primitive is used as a key with value `true`), set `strictMerge` to `false`:
assert.deepEqual(qs.parse('a[b]=c&a=d', { strictMerge: false }), { a: { b: 'c', d: true } });
You can also create arrays of objects:
var arraysOfObjects = qs.parse('a[][b]=c');
assert.deepEqual(arraysOfObjects, { a: [{ b: 'c' }] });
Some people use comma to join array, **qs** can parse it:
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).
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)
qs.stringify(object, [options]);
When stringifying, **qs** by default URI encodes output. Objects are stringified as you would expect:
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`:
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`:
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:
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:
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:
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:
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.
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`:
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:
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:
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`:
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.
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`:
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:
assert.equal(qs.stringify({ a: '' }), 'a=');
Key with no values (such as an empty object or array) will return nothing:
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:
assert.equal(qs.stringify({ a: null, b: undefined }), 'a=');
The query string may optionally be prepended with a question mark:
assert.equal(qs.stringify({ a: 'b', c: 'd' }, { addQueryPrefix: true }), '?a=b&c=d');
Note that when the output is an empty string, the prefix will not be added:
assert.equal(qs.stringify({}, { addQueryPrefix: true }), '');
The delimiter may be overridden with stringify as well:
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:
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:
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:
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:
class Range {
constructor(from, to) {
this.from = from;
this.to = to;
}
}
You could _inject_ a custom serializer to handle values of this type:
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:
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.
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:
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:
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:
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:
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:
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.
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
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:
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:
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');
## Security
Please email [@ljharb](https://github.com/ljharb) or see https://tidelift.com/security if you have a potential security vulnerability to report.
## qs for enterprise
Available as part of the Tidelift Subscription
## Acknowledgements
qs logo by [NUMI](https://github.com/numi-hq/open-design):
[
](https://numi.tech/?ref=qs)