Same same, but different

samsam is a collection of predicate and comparison functions useful for identifiying the type of values and to compare values with varying degrees of strictness.

samsam is a general-purpose library with no dependencies. It works in browsers (including old and rowdy ones, like IE6) and Node. It will define itself as an AMD module if you want it to (i.e. if there’s a define function available).

samsam was originally extracted from the referee assertion library, which ships with the Buster.JS testing framework.

Predicate functions


Returns true if object is an arguments object, false otherwise.


Returns true if object is a DOM element node


Returns true if the object is a Date, or date-like. Duck typing of date objects work by checking that the object has a getTime function whose return value equals the return value from the object’s valueOf.


Returns true if passed in value is NaN. Unlike the globally available isNaN, this method does not attempt to coerce its argument to a number, instead it checks that the number is equal to itself.


Returns true if value is -0.

Comparison functions

identical(x, y)

Strict equality check according to EcmaScript Harmony’s egal.

From the Harmony wiki:

An egal function simply makes available the internal SameValue function from section 9.12 of the ES5 spec. If two values are egal, then they are not observably distinguishable.

identical returns true when === is true, except for -0 and +0, where it returns false. Additionally, it returns true when NaN is compared to itself.

deepEqual(obj1, obj2)

Deep equal comparison. Two values are “deep equal” if:

  • They are identical
  • They are both date objects representing the same time
  • They are both primitives and a == b
  • They are both arrays containing elements that are all deepEqual
  • They are objects with the same set of properties, and each property in obj1 is deepEqual to the corresponding property in obj2

match(object, matcher)

Partial equality check. Compares object with matcher according a wide set of rules:

String matcher

In its simplest form, match performs a case insensitive substring match. When the matcher is a string, object is converted to a string, and the function returns true if the matcher is a case-insensitive substring of object as a string.

samsam.match("Give me something", "Give"); //true
samsam.match("Give me something", "sumptn"); // false
samsam.match({ toString: function () { return "yeah"; } }, "Yeah!"); // true

The last example is not symmetric. When the matcher is a string, the object is coerced to a string - in this case using toString. Changing the order of the arguments would cause the matcher to be an object, in which case different rules apply (see below).

Boolean matcher

Performs a strict (i.e. ===) match with the object. So, only true matches true, and only false matches false.

Regular expression matcher

When the matcher is a regular expression, the function will pass if object.test(matcher) is true. match is written in a generic way, so any object with a test method will be used as a matcher this way.

samsam.match("Give me something", /^[a-z\s]$/i); // true
samsam.match("Give me something", /[0-9]/); // false
samsam.match({ toString: function () { return "yeah!"; } }, /yeah/); // true
samsam.match(234, /[a-z]/); // false

Number matcher

When the matcher is a number, the assertion will pass if object == matcher.

samsam.match("123", 123); // true
samsam.match("Give me something", 425); // false
samsam.match({ toString: function () { return "42"; } }, 42); // true
samsam.match(234, 1234); // false

Function matcher

When the matcher is a function, it is called with object as its only argument. match returns true if the function returns true. A strict match is performed against the return value, so a boolean true is required, truthy is not enough.

// true
samsam.match("123", function (exp) {
    return exp == "123";
// false
samsam.match("Give me something", function () {
    return "ok";
// true
    toString: function () {
        return "42";
}, function () { return true; });
// false
samsam.match(234, function () {});

Object matcher

As mentioned above, if an object matcher defines a test method, match will return true if matcher.test(object) returns truthy.

If the matcher does not have a test method, a recursive match is performed. If all properties of matcher matches corresponding properties in object, match returns true. Note that the object matcher does not care if the number of properties in the two objects are the same - only if all properties in the matcher recursively matches ones in object.

// true
samsam.match("123", {
    test: function (arg) {
        return arg == 123;
// false
samsam.match({}, { prop: 42 });
// true
    name: "Chris",
    profession: "Programmer"
}, {
    name: "Chris"
// false
samsam.match(234, { name: "Chris" });

DOM elements

match can be very helpful when comparing DOM elements, because it allows you to compare several properties with one call:

var el = document.getElementById("myEl");
samsam.match(el, {
    tagName: "h2",
    className: "item",
    innerHTML: "Howdy"