expression-calculator

expression-calculator

A simple expression parser, using LL(1) syntax analyzer.

Useful to parse and calculate expression without using eval(), so as to avoid all the security risks caused by eval().

Installation

npm install --save expression-calculator

Import

ES6/Webpack

import Calc from 'expression-calculator';

Node.js

const Calc=require('expression-calculator');

Script Tag

<script type='text/javascript' src='path/to/exprcalc.js'></script>

Initialization

const calc = new Calc();

Initialize a new instance of Calc() with empty RPN expression.

const calc = new Calc(expression);

Initialize a new instance of Calc(), then compile the given expression into RPN (compile() method is called internally).

const calc = new Calc(RPN);

Initialize a new instance of Calc(), then load the given RPN data (setRPN() method is called internally).

const calc = new Calc(calcInstance);

Get a copy of the given Calc() instance.

Methods

compile(expr)

Compile an expression into an RPN expression.

Parameters

• expr - The string to be compiled.

Returns

A reference to this instance for chaining.

Throws

• SyntaxError - If the given expression contains error.

calc(vars)

Calculate the previously compiled expression.

The compiled expression may contain variables, use a key-value dict to specify values for variables in the expression. Variable names are case sensitive.

Parameters

• vars - A dict contains key-value pairs (Required when the RPN contains variables, and all the variables in the RPN expression must have a value here).

Returns

The calculation result, or null if no RPN is loaded or compiled from expression.

Throws

• TypeError - When the value of a variable is not a number.
• ReferenceError - When the value of a variable is not given in the vars parameter.

Example

var calc = new Calc();
calc.compile('3+4').calc(); //Result: 7
calc.compile('3+4-a').calc({a:-10}); //Result: 17

getRPN()

Get the compiled RPN expression.

Returns

Returns the compiled RPN which can be converted into JSON, or null for empty RPN.

Example

var calc = new Calc();
calc.compile('3*(5+a)').getRPN();

The code above will generate the following JSON data:

[
	{type:1, value:3},
	{type:1, value:5},
	{type:2, value:"a"},
	{type:3, value:"+"},
	{type:3, value:"*"}
]

In the expression above, each token's type value has an alias, you may refer to Aliases section for detail.

You may also use it to check if the input expression is valid:

var calc = new Calc();
function checkExpr(expr) {
	try {
		var rpn = calc.compile(expr).getRPN();
		for (var i = 0; i < rpn.length; i++) {
			if (rpn[i].type == VAR) {
				return false;
			}
		}
	} catch(e) {
		return false;
	}
	return true;
}

setRPN(expr)

Load previously compiled RPN from other source like database or memcache/redis.

Parameters

• expr - The RPN expression, with the same format as data returned by method getRPN().

Returns

A reference to this instance for chaining.

Throws

• TypeError - If the given RPN contains tokens with invalid type or value.
• SyntaxError - If the given RPN contains invalid tokens or errors.

compress()

Reduce the size of RPN by precalculating the numbers.

Aliases

• Calc.TOKEN_NUM - Mark RPN tokens as number type.
• Calc.TOKEN_VAR - Mark RPN tokens as variable type.
• Calc.TOKEN_OPER - Mark RPN tokens as operand type.