essy-evaluator

Javascript Expression Parser and Evaluator

Description

Parses and evaluates Javascript string expressions. The module can be used to replace some functionality provided by Javascript's eval but also allows for custom operator and function definitions to enable a custom pseudo programming language.

The module was created during development of Essy Tree, online decision tree analysis software, where it is used in production.

Installation

npm install essy-evaluator

Basic Usage

var essy      = require('essy-evaluator');
var evaluator = new essy.Evaluator();
var result;

// Use built-in operators.
result = evaluator.evaluate('2 * 3'); // 6

// Use built-in functions.
result = evaluator.evaluate('max(1, 3, 2) + 4'); // 7

// Use custom definitions.
evaluator.defineName('myCustomName', 8);
result = evaluator.evaluate('myCustomName / 4'); // 2

// Equivalently pass custom definitions to evaluate.
result = evaluator.evaluate('myCustomName / 4', { myCustomName: 8 }); // 2

Documentation

Parser

The Parser class provides a method parse() to parse an expression. There is a default set of recognized operators that can be customized.

Parser ([config {Object}])

Constructs a new Parser instance. Accepts an optional configuration object that can be used to disable default operators or add new operators. For example, the following disables the default "+" operator and adds two custom operators, "#" and "$".

var parser = new Parser({
  operators: {

    // Disable default "+" operator.
    '+': false,

    // Add custom "#" and "$" operators.
    '#': true,
    '$': true
  }
});

By default the following operators are defined:

• '('
• ')'
• '['
• ']'
• '+'
• '-'
• '*'
• '/'
• '^'
• '%'
• '?'
• ':'
• ','
• '<'
• '<='
• '>'
• '>='
• '=='
• '!='
• '!'
• '&&'
• '||'

parse (expression {String})

Parses an expression into tokens. parse() returns an array of simple token objects, each with properties type {String} and value {Any}. The token type can be one of the following:

• name All strings not enclosed in quotes are converted to name tokens. The value is equal to the name.
• number Numbers represented in decimal or scientific notiation.
• operator Any operator defined in the operator list.
• string Any string encapsulated in quotes.

parse() will throw a ParseException in the following cases:

• Invalid number
• Invalid trailing operator
• Unrecognized operator
• Unterminated string

updateOperators (operators {Object})

Updates valid operator list. Keys should be the operator (e.g., '+') and values indicate whether the operator is valid. Values can be "truthy" or "falsey".

parser.updateOperators({

  // Disable some built-in operators. false or 0 work just as well.
  '+': false,
  '*': 0,

  // Add some custom operators.
  '$': true,
  '#': 1
});

Evaluator

The Evaluator class provides methods to evaluate an expression or array of tokens returned from Parser.parse() as well as define custom operators, functions, and names. See below the methods section for lists of pre-defined constants, operators, and functions.

Methods

evaluate (exp {String|Object[]} [, names {Object}])

Evaluates expression exp, either a string or array of tokens returned from Parser.parse(). Returns result of evaluation, typically a number.

If a string exp is provided, evaluate() will throw a ParseException if parsing fails. See Parser.parse() for details.

An optional names argument can be provided to specify custom definitions. This is a shortcut to calling Evaluator.defineName().

The evaluate() method will throw an EvaluateException in the following cases:

• Unrecognized token.

• Unexpected token (e.g., missing or misplaced parentheses).

• Division by zero.

• Invalid arguments provided to function (e.g., sqrt(-2)).

  var essy = require('essy-evaluator'); var parser = new essy.Parser(); var evaluator = new essy.Evaluator(); var result;

  // Simple evaluation of string. result = evaluator.evaluate('1 + 2'); // 3

  // Evaluation of tokens. var tokens = parser.parse('1 + 2'); result = evaluator.evaluate(tokens); // 3

  // Defining names. result = evaluator.evaluate('1 + myConstant', { myConstant: 2 }); // 3

defineFunction (name {String}, ev {Function} [, noArgs {Boolean}])

Defines a custom function. The name is the name for the function and ev is the function body. The noArgs flag should be set to true if the function does not accept any arguments; by default this value is false.

var essy      = require('essy-evaluator');
var evaluator = new essy.Evaluator();

evaluator.defineFunction('addTwoNumbers', function () {
  return this.argValue(0) + this.argValue(1);
});

var result = evaluator.evaluate('addTwoNumber(2, 3)'); // 5

As seen above, the ev function has access to provided argument values via the argValue() method, which accepts an argument index. In the above, argValue(0) === 2 and argValue(1) === 3.

You can also define functions that accept an arbitrary number of arguments:

var essy      = require('essy-evaluator');
var evaluator = new essy.Evaluator();

evaluator.defineFunction('addNumbers', function () {

  var sum    = 0,
      values = this.argValues();

  for (var i = 0; i < values.length; i++) {
    sum += values[i];
  }

  return values;
});

var result = evaluator.evaluate('addNumbers(1,2,3,4)'); // 10

The above makes use of the argValues() method, which evaluates and returns all argument values.

defineName (name {String}, value {Any})

Defines a custom name. This can be useful if you want to define custom constant values or include variables in your expressions.

var essy      = require('essy-evaluator');
var evaluator = new essy.Evaluator();

evaluator.defineName('myCustomName', 4);

var result = evaluator.evaluate('3 + myCustomName'); // 7

Note that defineName() will overwrite any existing definition without warning.

Pre-defined Constants

Evaluator defines the following constants by default:

Constant	Description	Evaluates to
E	Base of natural logarithm.	~2.718
FALSE	False	0
LN2	Natural logarithm of 2	~0.693
LN10	Natural logartihm of 10	~2.302
PI	Pi	~3.145
SQRT1_2	1 over the square root of 2	~0.707
SQRT2	Square root of 2	~1.414
TRUE	True	1

Pre-defined Operators

Evaluator defines the following operators by default:

Operator	Example	Description	Returns
+	x + y	Addition	Sum of x and y.
-	x - y	Subtraction	The difference between x and y.
*	x * y	Multiplication	The product of x and y.
/	x / y	Division	x divided by y.
^	x ^ y	Exponentiation	x raised to the y.
!	!x	Logical not	0 if x is greater than 0, else 1.
&&	x && y	Logical and	1 if x and y are greater than 0, else 0.
||	x || y	Logical or	1 if x or y is greater than 0, else 0.
? :	x ? y : z	Ternary	y if x is greater than 0, else z.
()	x * (y + z)	Parentheses	Grouping for operator precedence.

Pre-defined Functions

Evaluator defines the following functions by default:

Function	Description
abs(x)	Returns absolute value of x.
acos(x)	Returns arccosine of x. Throws exception if x is not in range [-1, 1].
and(x0, x1, ... xN])	Returns 1 if all arguments are greater than 0, else returns 0.
asin(x)	Returns arcsine of x. Throws exception if x is not in range [0, 1].
atan(x)	Returns arctangent of x. Throws exception if x is not in range [-1, 1].
ceiling(x)	Returns x rounded up to nearest integer.
choose(x, y0, ... yN)	Returns the argument at index x. x = 1 will return y0.
cos(x)	Returns cosine of x.
exp(x)	Returns e raised to x.
fac(x)	Returns factorial of x.
floor(x)	Returns x rounded down to nearest integer.
if(x, y, z)	Returns y if x is greater than 0, else returns z.
log(x)	Returns the base-10 logarithm of x. Throws exception if x is less than or equal to 0.
ln(x)	Returns the natural logarithm of x. Throws exception if x is less than or equal to 0.
max(x0, x1, ... xN)	Returns the argument with the maximum value.
mean(x0, x1, ... xN)	Returns the mean of the provided arguments.
median(x0, x1, ... xN)	Returns the median of the provided arguments.
min(x0, x1, ... xN)	Returns the argument with the minimum value.
mod(x, y)	Returns x modulo y. Throws exception if y equals 0.
not(x)	Returns 0 if x is greater than 0, else returns 1.
or(x0, x1, ... xN)	Returns 1 if any argument is greater than 0, else returns 0.
pow(x, y)	Returns x raised to y.
product(x0, x1, ... xN)	Returns the product of all arguments.
quotient(x, y)	Returns integer portion of x / y. Throws exception if y equals 0.
rand()	Returns a random number in the range [0, 1].
randInt(x, y)	Returns a random integer in the range [x, y].
randRange(x, y)	Returns a random number in the range [x, y].
round(x)	Returns x rounded to the nearest integer.
sin(x)	Returns the sine of x.
sqrt(x)	Returns the square root of x. Throws exception if x is less than or equal to 0.
sum(x0, x1, ... xN)	Returns the sum of provided arguments.
tan(x)	Returns the tangent of x.