Powerful pattern matching for JavaScript

npm install missmatch
9 downloads in the last week
34 downloads in the last month


Pattern matching for JavaScript. It allows you to match any kind of JavaScript value (including arrays and objects) against patterns, bind values to names and execute handler functions, when a pattern matches. In general this is useful in cases where you receive some value but can't be sure how it is exactly composed. It might be some nested array or object. Pattern matching lets you test your input against composition-patterns, decompose it and bind the properties you are interested in to variables. Patterns and handler functions are entered as JavaScript objects where patterns are the keys (strings) and handlers are the values (functions).

  • License: MIT
  • Contributions, Issue Reports & Suggestions welcome!

Patterns are composed in a simple and concise syntax

  • 'a' means array
  • 'o' means object
  • 'n' means numeric
  • 's' means string
  • 'S' means non-blank string
  • 'b' means boolean
  • 'f' means function
  • 'd' means date
  • 'r' means regular expression
  • '|' means the rest of a list
  • '_' means wildcard (match anything)

Nested patterns

Patterns may be arbitrarily nested.

  • a(n, n) matches an array with exactly two numbers (e.g. [1,2]).

  • a(a, a(n)) matches an array that is composed of two nested arrays. The second array is required to contain exactly one number.

  • a(a, o(.x, .y)) matches an array that is composed of an array and then an object. The object is required to contain at least the two properties 'x' and 'y'.

Binding values

  • a(n@x, n@y) matches an array that is composed of exactly two numbers where the first one is bound to the variable 'x' and the second one to 'y'. They can be used in the handler function of the pattern:
mm.match([2,3], {   
'a(n@x, n@y)': function () { return this.x * this.y; },
'_': function () { /* Match all */ }

Note that bound variables must always be accessed using 'this' in the handler function.

Returning values from a (matching) pattern

The right side of a pattern may be a function or any other value. If it is a function and the pattern matches, then the result of the function is returned. If it is a non-function value and the pattern matches, then this value is returned. In earlier version (pre 0.1.0) only handler functions were allowed.

// Handler function
mm.match(42, {   
'n@x': function () { return this.x; }

// Return a value when the pattern matches
mm.match(42, {   
'n': 42

Matching objects

  • o matches any object.

  • o(.x) matches an object that is required to have a property namend 'x' which must belong to the object itself and must not be a part of the prototype chain.

  • o(:x) matches an object that is required to have a property named 'x' which may also be part of the prototype chain.

  • o(:x, .y) matches an object that is required to have (at least) two properties 'x' and 'y'. 'y' is required to belong to the object itself while 'x' may be part of the prototype cain.

Matching object properties by type

  • o(.x:n) matches an object with at least a property 'x' which is required to be a number. You can also bind the number's value:

    • o(.x:n@x)
  • o(.coord:o(.x:n@x, .y:n@y)) matches an object that has (at least) a property 'coord'. 'coord' is further required to be an object and provide (at least) the properties 'x' and 'y'. They are then bound to variables and can be used in the handler function.


Literals can be matched for strings, numbers, booleans and dates. Literals may also be bound to names.

  • n(121.5) denotes the numeric literal 121.5.

  • s("a_str") denotes the string literal 'a_str'.

  • s(/^a/) requires the string to match the regular expression /^a/.

  • b(true) denotes a boolean literal that only matches 'true' values.

  • d("2012/2/28") denotes the date literal '2012/2/28'. Every valid JavaScript date string is accepted in the pattern. Time information may also be specified: d("2012/03/01 02:03:45")

Literal lists

  • n(1,2,3) matches if one of the numbers 1, 2 or 3 occurs.

  • s("a", "b") matches if a string "a" or a string "b" occurs.

  • n(1,2,3)@x matches if one of the numbers 1, 2 or 3 occurs and binds the actual value to the variable 'x'.

  • d("2012/2/28", "2012/3/01") matches if one of the specified dates occur.

The rest of an array

  • a(n,n,n|) matches an array that is required to contain at least three numeric values, but may also contain more.

  • a(n,n,n|@r) matches the same array, but binds the rest (everything but the first three items) to the variable 'r'.

Note that the rest of an array (if there is a rest left to be matched) will always be an array itself even if there is only one item left to match the rest to.

Matching the arguments of the enclosing function

You can match against the calling function's arguments without passing the arguments object. It's a nice way to write dispatching or generic functions. (deprecated as of 0.1.0)

function plus() {
    // Function arguments are treated like arrays.
    // If we pass in two numbers, we ADD them:
    'a(n@a,n@b)': function () { return this.a + this.b; },

    // If we pass in two booleans we AND them:
    'a(b@a,b@b)': function () { return this.a && this.b; }    

plus(2,4);          // 6 
plus(true, false);  // false


The API consists of four functions:

  • match takes a value to match and a JavaScript Object containing the patterns and handlers. If one of the patterns matches, then it's handler function is executed.

  • matchJSON is equivalent to match(JSON.parse(obj_to_match), {...})

  • matchArgs lets you match against the calling function's arguments. You don't have to pass the arguments object, only the patterns. Function arguments are matched like arrays.

  • compile compiles a single pattern (string) to a function. The function can be executed on some input value and will return an object with the properties 'result' and 'context'. If the pattern matched the input, 'result' will be true and 'context' will be an object containing all bindings.


MissMatch can be used in the Browser, with Node.js and possibly with other server-side JavaScript Engines like Rhino (haven't tested that yet). It has no dependencies.


<script type="text/javascript" src="/path/to/MissMatch.js"></script>

The functions 'match', 'matchJSON', ',matchArgs' and 'compile' are all bound to an object with the name 'mm'.


  • npm install missmatch
  • (optional) npm test missmatch

Version History

  • 0.1.1

    • can match non-blank strings (with S)
  • 0.1.0

    • Non-function handler arguments allowed
    • matchArgs is now deprecated
  • 0.0.5 (Bugfix release)

    • capital 'E' accepted in numeric expressions (e.g. 3E-5)
    • code revised
  • 0.0.4:

    • can match strings with regular expressions (e.g. s(/^a/, /[0-9]/)).
    • can match regular expression objects.
    • code revised.
  • 0.0.3:

    • improved performance.
    • can match function arguments nicely (matchArgs).
    • can match date objects.
    • version string added (mm.version).
  • 0.0.2:

    • Can match properties in the prototype chain (with ':' instead of '.').
    • Better support for valid variable and property names ($, _ and numbers allowed).
    • Will throw an exception if the same name is bound multiple times. In 0.0.1 The value was silently overwritten.
    • Improved parser error messages.
  • 0.0.1: Initial Release.

npm loves you