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: Nathan LaFreniere

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

Usage

var Qs = require('qs');

var obj = Qs.parse('a=c');    // { a: 'c' }
var str = Qs.stringify(obj);  // '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 [], or prefixing the sub-key with a dot .. For example, the string 'foo[bar]=baz' converts to:

{
  foo: {
    bar: 'baz'
  }
}

When using the plainObjects option the parsed value is returned as a plain 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:

Qs.parse('a.hasOwnProperty=b', { plainObjects: true });
// { 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.

Qs.parse('a.hasOwnProperty=b', { allowPrototypes: true });
// { a: { hasOwnProperty: 'b' } }

URI encoded strings work too:

Qs.parse('a%5Bb%5D=c');
// { a: { b: 'c' } }

You can also nest your objects, like '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:

{
  a: {
    b: {
      c: {
        d: {
          e: {
            f: {
              '[g][h][i]': 'j'
            }
          }
        }
      }
    }
  }
}

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

Qs.parse('a[b][c][d][e][f][g][h][i]=j', { depth: 1 });
// { 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:

Qs.parse('a=b&c=d', { parameterLimit: 1 });
// { a: 'b' }

An optional delimiter can also be passed:

Qs.parse('a=b;c=d', { delimiter: ';' });
// { a: 'b', c: 'd' }

Delimiters can be a regular expression too:

Qs.parse('a=b;c=d,e=f', { delimiter: /[;,]/ });
// { a: 'b', c: 'd', e: 'f' }

Option allowDots can be used to disable dot notation:

Qs.parse('a.b=c', { allowDots: false });
// { 'a.b': 'c' } }

Parsing Arrays

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

Qs.parse('a[]=b&a[]=c');
// { a: ['b', 'c'] }

You may specify an index as well:

Qs.parse('a[1]=c&a[0]=b');
// { 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:

Qs.parse('a[1]=b&a[15]=c');
// { a: ['b', 'c'] }

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

Qs.parse('a[]=&a[]=b');
// { a: ['', 'b'] }
Qs.parse('a[0]=b&a[1]=&a[2]=c');
// { 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:

Qs.parse('a[100]=b');
// { a: { '100': 'b' } }

This limit can be overridden by passing an arrayLimit option:

Qs.parse('a[1]=b', { arrayLimit: 0 });
// { a: { '1': 'b' } }

To disable array parsing entirely, set parseArrays to false.

Qs.parse('a[]=b', { parseArrays: false });
// { a: { '0': 'b' } }

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

Qs.parse('a[0]=b&a[b]=c');
// { a: { '0': 'b', b: 'c' } }

You can also create arrays of objects:

Qs.parse('a[][b]=c');
// { a: [{ b: 'c' }] }

Stringifying

Qs.stringify(object, [options]);

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

Qs.stringify({ a: 'b' });
// 'a=b'
Qs.stringify({ a: { b: 'c' } });
// 'a%5Bb%5D=c'

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'

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

Qs.stringify({ a: '' });
// 'a='

Properties that are set to undefined will be omitted entirely:

Qs.stringify({ a: null, b: undefined });
// 'a='

The delimiter may be overridden with stringify as well:

Qs.stringify({ a: 'b', c: 'd' }, { delimiter: ';' });
// 'a=b;c=d'

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:

Qs.stringify({ a: null, b: '' });
// 'a=&b='

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

Qs.parse('a&b=')
// { a: '', b: '' }

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

Qs.stringify({ a: null, b: '' }, { strictNullHandling: true });
// 'a&b='

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

Qs.parse('a&b=', { strictNullHandling: true });
// { a: null, b: '' }

Changelog

4.0.0

• #98 make returning plain objects and allowing prototype overwriting properties optional