qs

What is qs?

The qs package is a query string parsing and stringifying library with some added security. It is often used to parse and stringify query strings, handling complex data structures within query strings.

What are qs's main functionalities?

Parsing query strings

This feature allows you to convert a query string into an object, making it easy to access the values.

const qs = require('qs');
const query = qs.parse('a=c&b=d');
console.log(query); // { a: 'c', b: 'd' }

Stringifying objects

This feature enables you to take an object and convert it into a query string, which can be used in URLs.

const qs = require('qs');
const stringified = qs.stringify({ a: 'c', b: 'd' });
console.log(stringified); // 'a=c&b=d'

Handling arrays and objects

qs can parse and stringify nested objects and arrays, allowing for complex data structures in query strings.

const qs = require('qs');
const query = qs.parse('a[]=b&a[]=c');
console.log(query); // { a: ['b', 'c'] }

Custom parsing options

You can customize the parsing behavior by providing options such as delimiters, allowing for flexibility in the format of query strings.

const qs = require('qs');
const query = qs.parse('a=c&b=d', { delimiter: ';' });
console.log(query); // { 'a=c&b=d': '' }

Other packages similar to qs

query-string

query-string is a robust library for parsing and stringifying URL query strings. It is designed to be simpler than qs and does not support nested objects, but it is tree-shakable and can be used in both Node.js and the browser.

url-search-params

url-search-params is a polyfill for the URLSearchParams API, which is a native web API for handling query strings. It is not as feature-rich as qs, but it provides a standard way for parsing and stringifying query strings in the browser.

README

qs

A querystring parsing and stringifying library with some added security.

Lead Maintainer: Jordan Harband

The qs module was originally created and maintained by TJ Holowaychuk.

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

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 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:

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' } } });

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:

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

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' } });

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'] });

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 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:

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' } });

To disable array parsing entirely, set parseArrays to false.

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' } });

You can also create arrays of objects:

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

Stringifying

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');

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
}})

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, by default they are given explicit indices:

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:

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'

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'

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 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'

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');

Dealing with special character sets

By default the encoding and decoding of characters is done in utf-8. If you wish to encode querystrings to a different character set (i.e. Shift JIS) you can use the 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');