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.

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:

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:

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.

URI encoded strings work too:

You can also nest your objects, like '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[c][d][e][f][g][h][i]=j' your resulting object will be:

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

You can configure qs to throw an error when parsing nested input beyond this depth using the strictDepth option (defaulted to false):

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:

To bypass the leading question mark, use ignoreQueryPrefix:

An optional delimiter can also be passed:

Delimiters can be a regular expression too:

Option allowDots can be used to enable dot notation:

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.

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

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

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

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.

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

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

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

You may specify an index as well:

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:

You may also use allowSparse option to parse sparse arrays:

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

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.

This limit can be overridden by passing an arrayLimit option:

To disable array parsing entirely, set parseArrays to false.

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

You can also create arrays of objects:

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

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

By default, all values are parsed as strings. This behavior will not change and is explained in issue #91.

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 which will auto-convert all request query parameters.

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

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

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

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

(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:

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

The type argument is also provided to the decoder:

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:

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

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

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:

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

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.

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

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

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

Properties that are set to undefined will be omitted entirely:

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

The delimiter may be overridden with stringify as well:

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

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

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:

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:

For which you model as:

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

By default, null values are treated like empty strings:

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

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

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

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

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

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

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.

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) you can use the qs-iconv library:

This also works for decoding of query strings:

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 '+'.

Please email @ljharb or see https://tidelift.com/security if you have a potential security vulnerability to report.

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