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jsprim: utilities for primitive JavaScript types

This module provides miscellaneous facilities for working with strings, numbers, dates, and objects and arrays of these basic types.


Creates a deep copy of a primitive type, object, or array of primitive types.

deepEqual(obj1, obj2)

Returns whether two objects are equal.


Returns true if the given object has no properties and false otherwise. This is O(1) (unlike Object.keys(obj).length === 0, which is O(N)).

hasKey(obj, key)

Returns true if the given object has an enumerable, non-inherited property called key. For information on enumerability and ownership of properties, see the MDN documentation.

forEachKey(obj, callback)

Like Array.forEach, but iterates enumerable, owned properties of an object rather than elements of an array. Equivalent to:

for (var key in obj) {
        if (, key)) {
                callback(key, obj[key]);

flattenObject(obj, depth)

Flattens an object up to a given level of nesting, returning an array of arrays of length "depth + 1", where the first "depth" elements correspond to flattened columns and the last element contains the remaining object . For example:

    'I': {
        'A': {
            'i': {
                'datum1': [ 1, 2 ],
                'datum2': [ 3, 4 ]
            'ii': {
                'datum1': [ 3, 4 ]
        'B': {
            'i': {
                'datum1': [ 5, 6 ]
            'ii': {
                'datum1': [ 7, 8 ],
                'datum2': [ 3, 4 ],
            'iii': {
    'II': {
        'A': {
            'i': {
                'datum1': [ 1, 2 ],
                'datum2': [ 3, 4 ]
}, 3)


    [ 'I',  'A', 'i',   { 'datum1': [ 1, 2 ], 'datum2': [ 3, 4 ] } ],
    [ 'I',  'A', 'ii',  { 'datum1': [ 3, 4 ] } ],
    [ 'I',  'B', 'i',   { 'datum1': [ 5, 6 ] } ],
    [ 'I',  'B', 'ii',  { 'datum1': [ 7, 8 ], 'datum2': [ 3, 4 ] } ],
    [ 'I',  'B', 'iii', {} ],
    [ 'II', 'A', 'i',   { 'datum1': [ 1, 2 ], 'datum2': [ 3, 4 ] } ]

This function is strict: "depth" must be a non-negative integer and "obj" must be a non-null object with at least "depth" levels of nesting under all keys.

flattenIter(obj, depth, func)

This is similar to flattenObject except that instead of returning an array, this function invokes func(entry) for each entry in the array that flattenObject would return. flattenIter(obj, depth, func) is logically equivalent to flattenObject(obj, depth).forEach(func). Importantly, this version never constructs the full array. Its memory usage is O(depth) rather than O(n) (where n is the number of flattened elements).

There's another difference between flattenObject and flattenIter that's related to the special case where depth === 0. In this case, flattenObject omits the array wrapping obj (which is regrettable).

pluck(obj, key)

Fetch nested property "key" from object "obj", traversing objects as needed. For example, pluck(obj, "") is roughly equivalent to, except that:

  1. If traversal fails, the resulting value is undefined, and no error is thrown. For example, pluck({}, "") is just undefined.
  2. If "obj" has property "key" directly (without traversing), the corresponding property is returned. For example, pluck({ '': 1 }, '') is 1, not undefined. This is also true recursively, so pluck({ 'a': { '': 1 } }, '') is also 1, not undefined.


Returns an element from "array" selected uniformly at random. If "array" is empty, throws an Error.

startsWith(str, prefix)

Returns true if the given string starts with the given prefix and false otherwise.

endsWith(str, suffix)

Returns true if the given string ends with the given suffix and false otherwise.

parseInteger(str, options)

Parses the contents of str (a string) as an integer. On success, the integer value is returned (as a number). On failure, an error is returned describing why parsing failed.

By default, leading and trailing whitespace characters are not allowed, nor are trailing characters that are not part of the numeric representation. This behaviour can be toggled by using the options below. The empty string ('') is not considered valid input. If the return value cannot be precisely represented as a number (i.e., is smaller than Number.MIN_SAFE_INTEGER or larger than Number.MAX_SAFE_INTEGER), an error is returned. Additionally, the string '-0' will be parsed as the integer 0, instead of as the IEEE floating point value -0.

This function accepts both upper and lowercase characters for digits, similar to parseInt(), Number(), and strtol(3C).

The following may be specified in options:

Option Type Default Meaning
base number 10 numeric base (radix) to use, in the range 2 to 36
allowSign boolean true whether to interpret any leading + (positive) and - (negative) characters
allowImprecise boolean false whether to accept values that may have lost precision (past MAX_SAFE_INTEGER or below MIN_SAFE_INTEGER)
allowPrefix boolean false whether to interpret the prefixes 0b (base 2), 0o (base 8), 0t (base 10), or 0x (base 16)
allowTrailing boolean false whether to ignore trailing characters
trimWhitespace boolean false whether to trim any leading or trailing whitespace/line terminators
leadingZeroIsOctal boolean false whether a leading zero indicates octal

Note that if base is unspecified, and allowPrefix or leadingZeroIsOctal are, then the leading characters can change the default base from 10. If base is explicitly specified and allowPrefix is true, then the prefix will only be accepted if it matches the specified base. base and leadingZeroIsOctal cannot be used together.

Context: It's tricky to parse integers with JavaScript's built-in facilities for several reasons:

  • parseInt() and Number() by default allow the base to be specified in the input string by a prefix (e.g., 0x for hex).
  • parseInt() allows trailing nonnumeric characters.
  • Number(str) returns 0 when str is the empty string ('').
  • Both functions return incorrect values when the input string represents a valid integer outside the range of integers that can be represented precisely. Specifically, parseInt('9007199254740993') returns 9007199254740992.
  • Both functions always accept - and + signs before the digit.
  • Some older JavaScript engines always interpret a leading 0 as indicating octal, which can be surprising when parsing input from users who expect a leading zero to be insignificant.

While each of these may be desirable in some contexts, there are also times when none of them are wanted. parseInteger() grants greater control over what input's permissible.


Converts a Date object to an ISO8601 date string of the form "YYYY-MM-DDTHH:MM:SS.sssZ". This format is not customizable.


Parses a date expressed as a string, as either a number of milliseconds since the epoch or any string format that Date accepts, giving preference to the former where these two sets overlap (e.g., strings containing small numbers).

hrtimeDiff(timeA, timeB)

Given two hrtime readings (as from Node's process.hrtime()), where timeA is later than timeB, compute the difference and return that as an hrtime. It is illegal to invoke this for a pair of times where timeB is newer than timeA.

hrtimeAdd(timeA, timeB)

Add two hrtime intervals (as from Node's process.hrtime()), returning a new hrtime interval array. This function does not modify either input argument.

hrtimeAccum(timeA, timeB)

Add two hrtime intervals (as from Node's process.hrtime()), storing the result in timeA. This function overwrites (and returns) the first argument passed in.

hrtimeNanosec(timeA), hrtimeMicrosec(timeA), hrtimeMillisec(timeA)

This suite of functions converts a hrtime interval (as from Node's process.hrtime()) into a scalar number of nanoseconds, microseconds or milliseconds. Results are truncated, as with Math.floor().

validateJsonObject(schema, object)

Uses JSON validation (via JSV) to validate the given object against the given schema. On success, returns null. On failure, returns (does not throw) a useful Error object.

extraProperties(object, allowed)

Check an object for unexpected properties. Accepts the object to check, and an array of allowed property name strings. If extra properties are detected, an array of extra property names is returned. If no properties other than those in the allowed list are present on the object, the returned array will be of zero length.

mergeObjects(provided, overrides, defaults)

Merge properties from objects "provided", "overrides", and "defaults". The intended use case is for functions that accept named arguments in an "args" object, but want to provide some default values and override other values. In that case, "provided" is what the caller specified, "overrides" are what the function wants to override, and "defaults" contains default values.

The function starts with the values in "defaults", overrides them with the values in "provided", and then overrides those with the values in "overrides". For convenience, any of these objects may be falsey, in which case they will be ignored. The input objects are never modified, but properties in the returned object are not deep-copied.

For example:

mergeObjects(undefined, { 'objectMode': true }, { 'highWaterMark': 0 })


{ 'objectMode': true, 'highWaterMark': 0 }

For another example:

    { 'highWaterMark': 16, 'objectMode': 7 }, /* from caller */
    { 'objectMode': true },                   /* overrides */
    { 'highWaterMark': 0 });                  /* default */


{ 'objectMode': true, 'highWaterMark': 16 }


Code should be "make check" clean. This target assumes that jsl and jsstyle are on your path.

New tests should generally accompany new functions and bug fixes. The tests should pass cleanly (run tests/basic.js).