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lexpjs is a Lightweight EXPression parser and evaluator for JavaScript. It is intended to be functional replacement for the use of eval(), which can present significant security issues.

lexpjs supports simple mathemtical expressions for addition, subtraction, multiplication, division, modulus, bitwise operations, and logical operations. It has a small library of built-in functions (abs, cos, sin, floor, ceil, round, etc.). The syntax is common "infix" expression notation, very similar to JavaScript itself.

Through a passed-in context table, lexpjs supports named variables, and custom functions. See the documentation below for how to implement these.

lexpjs is offered under the MIT License.

Known Issues

  • lexpjs uses a UMD wrapper for compatibility with node, CommonJS, AMD, etc., but it has not been tested in all of these environments.

Bug Reports and Contributions

I like bug reports. I like help. I like making things better. If you have suggestions or bug reports please use use GitHub Issues. If you have a contribution, have at it! Please try to follow the coding style to keep it consistent, and use spaces rather than tabs (4 space indenting). Please use the develop branch as the origin for your new branch and changes.

The Basics

compile( expressionString )

The compile() function accepts a single argument, the string the containing the expression to be parsed. If parsing of the expression succeeds, the function returns a JavaScript object containing the parse tree that is used as input to run() later. If parsing fails, the function throws an exception.

Example (node.js):

const lexpjs = require( "./lexp.js" );

try {
    pp = lexp.compile('2 + 3 * 4 + 5");
} catch (e) {
    console.log("Parsing failed:", e);

run( parsedResult [, executionContext ] )

The run() function executes the parsed expression. It takes an optional executionContext argument, which is an object containing pre-defined symbol (variable) names and definable functions. See get_context() below.

run() returns the result of the expression evaluation as an array, one value for each subexpression in the parsed string (see "Syntax" above). If evaluation fails, an exception is thrown.

Example (browser):

import * as lexpjs from './lexp.js';

var pp = lexpjs.compile("8*8");
console.log(;  // prints 64

pp = lexpjs.compile("8 * range");
var rr =; // throws ReferenceError because "range" is not defined

evaluate( expressionString [, executionContext ] )

The evaluate() function performs the work of compile() and run() in one step. The function result (or exception thrown) is the same as that for run() above (and in fact evaluate() is implemented simply as return run( compile( expressionString ), executionContext )).

var rr = lexpjs.evaluate("floor(3.1415*100)"); // rr would be 314

get_context( [variables] )

Creates a context to house and preserve local variables, defined functions, etc. If several expressions need to be evaluated, and the results from each shared with the next, creating a context first and passing it to each evaluate() or run() call will facilitate this sharing.

The optional parameter variables is an object containing key/value pairs of predefined variables and their values.

Consider the following:

var cc = lexpjs.get_context( { pi: 3.14159265, name: "alpha" } )
console.log( lexpjs.evaluate( "area=pi * 4 * 4", cc );
console.log( lexpjs.evaluate( "'Half the area is ' + area / 2" );

This will print two lines:

Half the area is 25.1327412

Notice in the above example that the context has carried the computed area from the first evaluate() call into the second.

define_var( context, name, value )

Variables can be created in a context by passing them in to get_context(), or they can be created or modified after by calling define_var(). The arguments should be obvious: context is the context in which to make the assignment (previously created by get_context()), and the name and value are the name and value of the variable, respectively.

define_func_impl( context, name, func )

Special/custom functions needed by your expressions can be defined on a context by calling define_func_impl(). The func parameter is simply a reference to a function that takes the context in which the call is being made followed by the arguments your expression function needs; the implementation must return a value.

// Custom functions defined using four common JavaScript syntaxes.
var cc = lexpjs.get_context();

/* By passing a reference to a function */
function r2d( ctx, radians ) {
    return radians * 180.0 / Math.PI;
lexpjs.define_func_impl( cc, 'rad_to_deg', r2d );

/* By passing a closure */
lexpjs.define_func_impl( cc, 'deg_to_rad', function( ctx, deg ) {
    return deg * Math.PI / 180.0;

/* By passing arrow function */
lexpjs.define_func_impl( cc, 'square', ( ctx, value ) => { return value * value; } );

The context in which the call is made will be the context in which the function is referenced and the call is made, not necessarily that at which the function was defined. A function defined in global scope but called from a descendent scope will have the descendent scope passed as its argument here. The context is passed for reentrancy in your application, as a handle, and is not intended to be used to access local variables or variables in other scopes; this latter possibility should be avoided (i.e. your function should be passed all values it needs to function).

Your function may return any primitive type, null, Infinity, NaN, or an array or object containing any of these. If the return value is undefined, null will be substituted. Your function may not return any other type, including function, user-defined class or instance, RegExp, Promise, Map or Set (or descendents), etc. Basically, if it can't be represented natively in JSON, it's not valid to return from your function.

Your function must check its own arguments for validity, if necessary. Any exception thrown by your function will be passed through unmodified.

NOTE: Functions can also be defined using expression syntax as further documented below (see the define statement in Statements).

Expressions Syntax

lexpjs's expression syntax is similar to the "infix" expression syntax used by most common languages (C, Java, JavaScript, etc.). The simplest expression is simply a numeric constant, such as 1234. This is a complete expression, the result value of which is 1234. Negative numbers begin with a - sign, such as -1234. Numbers may have decimal points and decimal digits: -12.34. Numbers may also be given in scientific format: 1.234e3 is equal to 1234 (1.234 x 103). Hexadecimal integers may be entered by prefixing with 0x; for example, 0x20 is decimal 32. Likewise binary integers can be prefixed with 0b, and octal with 0o.

Strings are represented as characters surrounded by matching double-quotes ("), single quotes ('), or back-ticks (`).

Boolean values true and false are represented by the reserved words true and false, respectively.

The reserved word null evaluates to the null value (basically means "no value"). The reserved word pi evaluates to the JavaScript Math.PI, approximately 3.14159265.

Identifiers are names that represent values. An identifier must begin with an upper- or lowercase alphabetic character, and may follow with any combination of alphanumeric characters and underscore. Thus myLastSignal is a valid identifier, but 023lastSignal is not, and nor is just another name!.

Functions are identifiers followed by a paren-enclosed list of expressions as its arguments (or empty for no arguments). The maximum value of a series of numbers, for example, can be found using the max function like this: max( 1, -2, pi, lastElement ).

The expression language includes a set of operators. Multiplication is performed by *, so that 3 * 4 yields 12. Division uses /, while addition and subtraction use + and -, respectively, as one might expect. The full list of operators is given below, in order of precedence. Operators with higher precedence are performed before operators with lower precedence, so that expressions like 3 + 4 * 2 yield 11, not 14. The precedence of mathemetical operators follows the Order of Operations we are taught in elementary school. Precedence can be controlled using parentheses, so per the previous example, the result 14 could be arrived at using (3 + 4) * 2.

String concatenation is performed using the + operator; if either operand is a string, the result will be a concatenated string in which the non-string operand was converted to its string representation (i.e. "123" + 456 results in the string "123456"). The null value is coerced to an empty string (so null + "abc" results in "abc").

In addition to the mathematical operators, there are relational operators: ==, !=, >, >=, < and <= all return true if their operands are equal, not equal, etc. In addition, the two special relational operators === and !== check equality/inequality not just of value, but of data type, such that "3" == 3 is true, but "3" === 3 is false (because the left operand is string type, and the right a number).

Note that equality/inequality comparison of arrays or objects (non-primitive types), such as array1 == array2 does not perform a "deep inspection" of the array/object and is effectively not a valid comparison for most practical purposes. Rather, it determines if the two operands are the same object in memory (as JavaScript does), and thus is most likely false and not truly a relevant comparison. Specifically, the following expressions are expected to be false: [1,2,3] == [1,2,3], { abc:1, def:2 } == { abc:1, def:2 } because although they are equivalent in terms of their contents, they are not, in fact, the same object in memory. The more complex s=[1,2,3], t=s, s == t is true, however, because s and t do refer to the same object in memory.

The boolean operators are && for and and || for or, such that false && true is false and false || true is true. The ! unary boolean operator negates its right-side operand, so !true is false.

The bitwise operators, following "C" (and Java, and JavaScript, and others) are & for bitwise AND, | for bitwise OR, and ^ for exclusive-OR (XOR).

The array element accessor is square brackets [] and should contain the array index. Arrays in expressions are zero-based, so the first element of an array is [0]. If the index given is less than 0, a runtime error occurs. If the index is positive or zero but off the end of the array, null is returned.

The member access operator "dot" (.) is used to traverse objects. For example, referring to the power state of an entity contain in an object may be entity.attributes.power_switch.state, which starts with the entity object, drops to the list of attributes within it, and then the "power_switch" capability within the attributes, and finally to the "state" value. The right-side operand of the dot operator must be an identifier, so it may not contain special characters. If a member name contains any non-identifier characters, the array access syntax can be used with a string: entity.attributes['forbidden-name'].value.

The ternary operator pair ? : common to C, C++ and Java (and others) is available: <boolean> ? <true-expression> : <false-expression>. If the boolean expression given is true, the true expression is evaluated; otherwise, the false expression is evaluated.

The coalesce operators, borrowed from C#, are ??, ?#, ?. and ?[. Coalesce operators help handle null values in the middle of complex expressions more gracefully. For example, value ?? 0 will result in the value of the variable value if it is not null, but if it is null, will yield 0. The numeric coalesce operator ?# provides a quick test if the left operand is (or can be) a number (integer or real), and if so returns the numeric value; if not, it returns the right operand (which can be any type). The access coalesce operators are used for object member and array element access: if an identifier struct is intended to hold an object, but turns out to be null, a reference to in an expression would throw a runtime evaluation error; using struct?.name will instead result in null with no exception thrown. This is convenient because you can carry it down?.a?.long?.list?.of?.member?.names without crashing if something is undefined. Likewise if beans was intended to be an array but ended up null, the expression beans[2] would throw an error, while beans?[2] would result in null.

The in operator is used to establish if an object contains a specified key (e.g. key in obj) or an array contains an element at the given index (e.g. 15 in arr). It is important to note that this operator works on keys only, not values, and in particular, cannot be used to search an array for a value (i.e. 4 in [ 4, 5, 6 ] is false). To find an array element, use the indexOf() function. The first statement can be used to find a value in an object.

The .. range operator produces an array containing all integers from the left operand to the right, so 3..6 results in [3,4,5,6]. A for-style counting loop can be implemented using each with the range operator as its operand: each i in 0..9: <statement> would execute <statement> 10 times. The increment for the range operator is always 1 (or -1 if the ending value is less than the starting value); if you need a different increment, use the range() function.

Multiple expressions can be chained together by separating them with a comma. The result of a chained expression is the last expression evaluated.

The following is the list of operators supported in order from lowest precedence to highest. Operators on the same line have equal precedence and are evaluated left-associative (from left to right) unless otherwise indicated:

  • = (assignment, right associative)
  • ? (ternary operator first)
  • : (ternary operator second)
  • ?? (coalesce) and ?# (coalesceNaN)
  • || (logical OR)
  • && (logical AND)
  • | (bitwise OR)
  • ^ (bitwise XOR)
  • & (bitwise AND)
  • ==, ===, !=, !== (equality/inequality, non-associative)
  • in (presence of key/index in object/array, non-associative)
  • <, <=, >, >= (comparison, non-associative)
  • .. (range opreator integers
  • <<, >> (bit shift)
  • +, -
  • *, /, % (mod)
  • ** (power, right associative)
  • - (unary minus)
  • ! (not/negation, right-associative)
  • ., ?., ?[ (member access)

Data Types

The data types known to lexpjs are boolean, number, string, array, object, and the following special type/values (both a type and a value):

  • null, which basically is used to mean "no value";
  • NaN, which stands for "Not a Number", which results when a conversion to number fails (e.g. 5 * "hello" or int( 'what is this?' );
  • Infinity, which results from division by zero and other similar math failures.

Arrays and objects can be constructed and used on the fly: [ 5, 99, 23, 17 ] constructs a four-element array, while { name: 'spot', type: 'dog', weight: 33 } constructs an object.


The expression language has a couple of "lightweight statements" that function as a hybrid of a statement and an expression. These are:

each <element-identifier> [, <element-identifier> ] in <array-or-object-expression>: <body-expression>

The each statement will iterate over the given array or object (or expression resulting in an array or object), each time placing an array value or object element in the named variable (and the key or index in the second named variable, if given), and then execute the body expression. The body expression result, if non-null, is pushed to an array that forms the each expression result. For example, each num in [ 4,7,33 ]: num * 2 will return an array [ 8, 14, 66 ], while each v,k in { "alpha": 1, "beta": 2 }: k will return ["alpha", "beta"].

first <element-identifier> [, <element-identifier> ] in <array-or-object> with <test-expression> [ : <result-expression> ]

The first statement will search through the elements of an array or object (top level, no traversal) and return the first value that for which <test-expression> is true (or truthy). The result is the value matched, unless the optional : <result-expression> clause is given, in which case the result will be that of the expression. Example: first val,key in devices with val.type=="window": + ' ' + key will find the first device in an array or map (object) of device objects for which the device object key type is window, and rather than return the device object, return the device name and key as a space-separated string.

do <statement-list> done

Since the limited syntax of each, case, etc. allow only a single statement to be executed, the do...done statement creates a statement block that appears to be a single statement, thus allowing multiple statements/expressions to be executed in that context. The standard multi-statement result rule applies: the result of the statement block is the result produced by the last expression in the block.

if <conditional> then <true-expression> [ elif <conditional> then <true-expression> ]* [ else <false-expression> ] endif

The true- and false-expressions may be any expression, including a do...done block enclosing multiple expressions. To remove the ambiguity that can arise from using else if with a nested if block, any number of elif subconditions may be included. The optional else clause is a catch-all for no other conditions matching.

    t = 2,
    if t === 1 then 'A'
    elif t === 2 then 'B'
    elif t === 3 then 'C'
    else null
    # Result is "B"

The keywords elsif and elseif are synonyms for elif that you may use "if you're not into the whole brevity thing."

case when <conditional-expr-1>: <true-expression-1> [ when <conditional-expr-n>: <true-expression-n> ]* [ else <default-expression> ] end

Sometimes if statements need to make multiple tests, and the if statement and ternary operator can become very difficult to write and follow later. To make things tidier, the case statement evaluates a series of when clauses; the first <conditional-expression> that is true will cause the statement to return the value of its matching <true-expression>. If none is true, the <default-expression> result is returned if an else clause is present, or null otherwise. The <true-expressions> and <default-expression> may be any expression, including assignments or block statements (even another case statement). Example below; lines and spacing for clarity only.

  when tempF < 65: "it's cold in here!"
  when tempF < 76: "we're comfortable"
  when tempF < 85: "it's a bit warm in here!"
  else "we need to cool this place down!"

Note that while the above example shows all when clauses testing the value of tempF, there is no requirement that the conditionals be consistent or related in this way. It is perfectly acceptable to write case when sun.isup: "sun is up" when pool.isfull "pool is full" else "read a book" end, if that is what you need to do.

define <functionName>( <args...> ) <expression>

Defines a function named <functionName> that returns the evaluated <expression>. Arguments passed to the function will be received as <args...>, which must be a comma-separated list of identifiers. Example: define square(a) a*a defines a function that returns the square of a single value passed to it received in the variable a; the function result is the result of the expression (no return statement is required or exists in this syntax). If multiple expressions are required for the implementation of the function, enclose them in a do ... done block.

Scope of Statements

The expression language has some rudimentary scoping like most programming languages. Variables defined in the interior expressions of the above statements will be local to the statement and not available outside the statement.

For example, given this expression (an iterator):

each v in [1,2,3,4,5,6]: a=v

One might assume at first glance that this iterator assigns each value of the array to a, and when the statement ends a will be available to the next expression with the value 6. The former is true, but not the latter: a is not available outside of the each expression. When making assignments to variables, the language will see if the target identifier is defined in any accessible scope, and if it is not defined in any scope, it is created in the current (lowest) scope. So, if you wish to preserve a value computed in the interior of such an expression, define the variable outside the statement first, like this:

a=0, each v in [1,2,3,4,5,6]: a=v

Now a will have a value of 6, because it was defined outside the statement, so assignments made within the statement target the exterior variable.

This behavior can be explicitly controlled through the use of the global and local keywords used as a prefix to an assignment. The global keyword will assure that the named identifier is assigned in global scope, and the local keyword assures that the name identifier is assigned in the current scope (which could be the global scope or a descendent).

a = 1   # this is the global scope, so a is created as a global
b = 0   # this creates b in global scope
    # This "DO" block has its own scope (child of the global scope)
    local a = 2  # this sets a local variable a to 2; the global a is still 1
    global a = a * 4  # this sets global a to local a * 4 (so 8)
    a = a * 2  # since local a exists, local a is now 16
    b = a  # this sets global b to local a's value of 16
# global a is now 8
# local a is now 16

Note that the global and local keywords can only be used as a modifier to the left-side of an assignment. One cannot, for example, say global a = local a * 2 to set global a to twice local a's value. That's invalid syntax.

One more thing to think about... the topmost/outermost scope, the global scope, is local to itself, so in the global scope, the following statements all have the same effect of creating a in global scope:

a = 0        # When in global scope and a is undefined, the default scope is the global scope
local a = 0  # When in global scope, the local scope is the global scope
global a = 0 # Specifying global scope is redundant when in global scope

NOTE: The use of the global keyword in particular will allow you to do things that are considered "bad style." For example, you can define a function or have a deeply nested statement that "communicates" with other expressions by setting variables in global scope. This is regarded as bad style because it can be difficult to figure out why global variables are changing (the changes are buried deep in other statements) and it may reduce the reusability of functions and expressions. There are many treatises on global variables in programming languages available on the web.

Pre-defined Functions

I keep adding things as I need them or people ask, so let me know if I'm missing what you need.

The syntax guides shown below (which are based on a well-known BNF) denotes optional arguments by enclosing them in [] (square brackets). These characters are not be included when writing your expressions. For example, the BNF dateparts( [time] ) denotes that the time argument is optional and may be omitted, so you could write dateparts( someTimeVariableName ) or just dateparts(), but dateparts( [ someTimeVariableName ] ) would not be correct.

Arithmetic Functions

  • abs( number ) — returns the absolute value of its argument;
  • sign( number ) — returns the sign of its argument: -1 if negative, 0 if zero, 1 if positive;
  • floor( number ) — returns the largest integer less than or equal to its argument;
  • ceil( number ) — returns the next integer greater than or equal to its argument;
  • round( number, precision ) — rounds number to precision decimal digits;
  • trunc( number ) — returns the integer portion of its argument (e.g. trunc(-3.4) is -3, where floor(-3.4) is 4);
  • cos/sin/tan/acos/asin/atan/atan2( number ) — trig operations (same as JavaScript functions of same name);
  • log/exp( number ) — natural logarithm and exponential;
  • pow( base, power ) — raises base to the powerth power (e.g. pow(10,3) is 1000);
  • sqrt( number ) — square root (of number > 0);
  • random() — returns a random number greater than or equal to 0 and less than 1;
  • min/max( ... ) — returns the smallest/largest value of its arguments; if an argument is an array, the array is scanned; non-numeric values are ignored, so these functions return null unless at least one number (type) value is found;
  • isNaN( various ) — returns true if the argument is NaN, null, or if an attempted conversion with parseInt()/parseFloat() would result in NaN (e.g. isNaN( 'this is not a number' ) returns true, but isNaN( '123' ) returns false). Note that this is a little different from JavaScript's isNaN(): under JS (as tested in nodejs 16.13.1 and Chrome browser 108.0.5359.98) isNaN(null) returns false while parseInt(null) and parseFloat(null) both return NaN. I think this is a vexing inconsistency, so lexpjs will return true for isNaN(null) to agree with parseInt()/parseFloat().
  • isInfinity( value ) &mash; returns true if the argument is Infinity, as would result in, for example, division by zero.

String Handling Functions

  • len( string ) — returns the length of the string;

  • substr( string, start, length ) — returns the portion of the string from the startth character for length characters;

  • upper/lower( string ) — converts the string to upper/lower-case;

  • match( string, regexp [ , ngroup [ , flags ] ] ) — matches, if possible, the regular expression regexp to the string, and returns the matched string, or null if no match; if ngroup is given and the regexp contains groups, the matched part of that group is returned; if flags is "i", a case-insensitive match is done;

  • find( string, regexp [ , flags ] ) — like match(), but returns the index of the first character of the match, rather than the matched string, or -1 if no match; the meaning of (optional) flags is the same as for match();

  • replace( string, regexp, replacement [ , flags ] ) — replaces the first substring matched by the regular expression regexp with the replacement string and returns the result; the optional flags (a string) may include "i" for case-insensitive search, and "g" for global replacement (all matches in string are replaced; combined would be "ig"); the $ is a special character in the replacement string and follows the JavaScript semantics (for String.replace()).

  • rtrim/ltrim/trim( string ) — removes whitespace from the right/left/both side(s) of the string;

  • split( string, regexp [, max ] ) — splits the string at the matching regular expression and returns an array (e.g. split( "1,5,8", "," ) returns ["1","5","8"]).

  • pad( string, length [, padchar ] ) — returns string padded to length length characters. If length is > 0, padding is done on the right; if length < 0, padding is done on the left. If optional padchar is specified, it is used to pad the string (default padchar is space). If the given string is longer than length, it is returned unmodified. Examples:

      pad("a", 3)     produces    "a  "
      pad("a", -3)                "  a"
      pad("5", -4, "0")           "0005"
      pad("toolong", -4)          "toolong"
  • quote( string ) — escape any interior characters of the string argument so that the result can be embedded in double-quotes safely (i.e. for a lexpjs/JavaScript/JSON-compatible result). For example, the string hello "there" would be returned as hello \\"there\\"; and the string abc<newline>def (where <newline> represents an embedded newline ASCII 10 character) would be abc\\ndef.

Type Handling Functions

  • int( various ) — attempts conversion of its argument to an integer; returns NaN if the argument cannot be converted, otherwise, it returns the integer;
  • float( various ) — attempts conversion to a floating-point value;
  • bool( various ) — attempts conversion to boolean; in this expression language, the strings "0", "no", "off" and "false", the empty string, the number 0, and boolean false all result in false; otherwise, the result is true;
  • str( various ) — converts the argument to a string;
  • isnull( various ) — more a test than a conversion, returns true if the argument is null.
  • typeof( various ) — returns the data type of the argument. While JavaScript reports arrays and null as objects, lexpjs reports them more specifically as array and null respectively.

Date/Time Handling Functions

  • time( [datetime-string] | [ dateparts-obj ] | [ year [, month [, day [, hour [, minute [, second ]]]]]] ) — Since all arguments are optional, time() returns the current time if none are given. If a single string argument is given, the function will attempt to parse it simplistically. Very complex date/time strings may not parse successfully, but simple strings similar to the locale's default format should work, and ISO-8601 is explicitly supported. If the string contains no time component, it will be assumed to be midnight; if it contains no date component, then the date is assumed to be the current day. If an object is given, it is assumed to be of the same form as that returned by dateparts(), and the date is constructed from its available members. If the argument is neither string nor object, time() expects numeric arguments and a date/time is constructed using as many parts as are provided (in the order shown). The result is always a Unix Epoch time in milliseconds. See additional notes below.
  • dateparts( [time] ) — returns an object with keys year, month, day, hour, minute, second, millis, weekday (0-6, where 0=Sunday), yday (day of the year, where 1=Jan 1), isoweek (ISO-8601 week number 1-53), and dst (true if Daylight Saving in effect, false otherwise) for the given time, or the current time if not given.

Important notes with respect to date handling (currently; this will evolve):

  • All time functions operate in the local time and timezone set for the runtime. There are currently no UTC functions.
  • Where month is an argument (e.g. to time()) or return value (e.g. from dateparts()), there is a configuration flag in the code for whether months should numbered 0-11 (like JavaScript) or 1-12 (for hoomans); the lexpjs default is 1 (months 1-12), and all examples and test code assumes this.
  • When passing a dateparts-form object (i.e. an object with keys year, month, etc.) into time(), a missing key is assumed to be 0 except year (which is assumed to be the current year), and month and day which are assumed to be 1. An offset date can be computed by adjusting the values in the object by the offset required. For example, fifteen days before March 1, 2022 can be found with { year: 2022, month: 3, day: -14 } (-14 = 1 - 15). Five hours and 8 minutes before the current time could be written as t=dateparts(), t.hour=t.hour-5, t.minute=t.minute-8, time(t). Using this form of date offset computation (rather than simply subtracting 18,480,000 milliseconds from the current time in milliseconds) accounts for changes in DST, leap seconds, or leap days occurring during the offset interval.

Array/Object Handling Functions

  • len( array ) — returns the number of elements in the array (including any null elements; also see count());
  • keys( object ) — returns, as an array, the keys in the given object;
  • values( object ) — returns, as an array, the values in the given object;
  • clone( various ) — returns a (deep) copy of its argument; this is particularly useful for arrays and objects;
  • join( array, joinstring ) — returns a string with the elements of array converted to strings and joined by joinstring (e.g. join([4,6,8], ":") results in the string "4:6:8", while join([9], ":") would be simply "9");
  • list( ... ) — returns an array of its argument; this is legacy syntax (i.e. list(5,7,9) is the same as writing [5,7,9], so this function is now obsolete and may be removed later);
  • indexOf( array, value ) — if value is present in array, the index (>=0) is returned; otherwise -1 is returned;
  • count( array ) — returns the number of non-null elements of array;
  • sum( array ) — returns the sum of non-null elements of array; note that only a single argument, which must be an array, is accepted;
  • median( array ) — returns the statistical median of the elements of array;
  • slice( array, start, end ) — returns a new array containing the elements of array from start (zero-based) to, but not including, end;
  • insert( array, pos, newElement ) — inserts newElement into array before pos (zero-based); the array is modified in place and is also returned as the function value;
  • remove( array, pos [ , numRemove ] ) — removes elements from array starting at pos; if numRemove is not given, only the one element at pos is removed, otherwise numRemove elements are removed from the array; the array is modified in place and also returned as the function value;
  • push( array, value [ , maxlen ] ) — appends value at the end of array; if maxlen is given, elements are removed from the head of the array to limit its length to maxlen elements; the array is modified in place and also returned as the function value;
  • unshift( array, value [ , maxlen ] ) — insert value at the beginning of array; if maxlen is given, elements are removed from the end of the array to limit its length to maxlen elements; the array is modified in place and also returned as the function value;
  • pop( array ) — removes the last element of array and returns it; returns null if array is empty; the array is modified in place;
  • shift( array ) — removes the first element of array and returns it; returns null if array is empty; the array is modified in place;
  • arrayConcat( a, b ) — returns a new array that is the concatenation of a and b; for example, arrayConcat( [1,2,3], [1,3,5] ) returns [1,2,3,1,3,5];
  • arrayIntersection( a, b ) — returns a new array containing all values in array a that are also in array b; for example, arrayIntersection( [1,2,3], [1,3,5] ) returns [1,3];
  • arrayDifference( a, b ) — returns a new array containing all values of array a that do not appear in array b; for example, arrayDifference( [1,2,3], [1,3,5] ) returns [2];
  • arrayExclusive( a, b ) — returns a new array containing all values of the arrays a and b that appear only in either, but not both (this is often referred to as the symmetric difference); for example, arrayExclusive( [1,2,3], [1,3,5] ) returns [2,5];
  • arrayUnion( a, b ) — returns a new array containing all values of the arrays a and b; for example, arrayUnion( [1,2,3], [1,3,5] ) returns [1,2,3,5];
  • sort( array [, comparison] ) — sort the given array, returning a new array (the given array is not modified). The array to be sorted may contain data of any type. The default sort is a case-sensitive ascending string sort (so the array is assumed to contain strings, and if it contains any other type the values are coerced to strings prior to comparison). To sort differently (e.g. descending, numeric, etc.), comparison can be given as the either the name of a defined function taking two arguments as the values to be compared, or an expression that compares the local variables $1 and $2 (defined by the sort() function as it runs). In either case, the result must be an integer: 0 if the two values are equal; less than 0 (e.g. -1) if the first value sorts before the second; or greater than zero (e.g. 1) if the first value sorts after the second. The comparison must be stable: given two values, it must return the same result every time it runs. Do not apply randomness or other heuristics to the comparison, as this can lead to long runtimes or even infinite loops in the attempt to sort.
  • range( start, end [, increment] ) — returns an array of integers counting up from start to end inclusive. If end < start, the array elements will count down. If increment is not specified, it assumed to be 1 (or -1 if end < start). Examples: range(0,5) returns [0,1,2,3,4,5]; range(5,0) returns [5,4,3,2,1,0]; range(0,5,2) returns [0,2,4]; range(5,0,-2) returns [5,3,1]; range(5,0,2) returns [], an empty array, because a positive increment on a descending range (end < start) is impossible. A common use of this function is to iterate over the even-numbered items of some other array: sum=0, each k in range(0,len(otherArray),2): sum = sum + otherArray[k].
  • isArray( various ) — returns true if the argument is an array (of any length);
  • isObject( various ) — returns true if the argument is an object;

Conversion Functions

  • hex( num ) — returns the hexadecimal (string) representation of the numeric argument (or "NaN" if non-numeric);
  • toJSON( various ) — returns the argument as a JSON-formatted object (string);
  • parseJSON( json ) — returns the data represented the (parsed) JSON string argument;
  • btoa( str ) — returns the Base64-encoded representation of the string argument;
  • atob( str ) — returns a string containing the decoded Base64 argument (string).
  • urlencode( string ) — URL-encodes the given string.
  • urldecode( string ) — URL-decodes the given string.

Other Functions

  • err( message ) — throw an error with the given message. This can be used when some complex expression would return unexpected/invalid results if a "soft" or logical error (as determine by the expression's author) occurs within its interior. For example, in the middle of some expression, it may be an error for a particular result to be zero or null when added to an array. The expression push( arrayName, value ) might be rewritten as push( arrayName, value || err( "invalid value" ) ) to implement this trap.

Reserved Words

As a result of the syntax, the following words are reserved and may not be used as identifiers or function names: true, false, null, each, in, first, of, with, if, then, else, elif, elsif, elseif, endif, case, when, do, done, define, and, or, not, NaN, Infinity, pi. Note that keywords and identifiers are case-sensitive, so while each is not an acceptable identifier, Each or EACH would be. The names of all defined functions are also reserved.

Updated 2023-Oct-24 (for version 23297)


lexpjs is a Lightweight EXPression parser and evaluator for JavaScript







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