# Math expression parser in JavaScript

I've written the second iteration of my math expression parser, utilising what I learned from the first attempt to make a more reliable, maintainable piece of code.

If anybody wants to see the first iteration to see my progress, see here.

With the first version, I tried to evaluate the expression string directly, without tokenising it and running it through several levels of progressive processing. It turned into a hard-to-follow mess of boolean flags and if-else blocks.

With the second version, I split the process up into various stages:

1. I replace the variables with their stored values.
2. I look for parentheses and parse those recursively, replacing the range of tokens the brackets spanned with their evaluated values.
3. I parse for negative values, making the proceeding token's value either positive or negative, and removing the operator token.
4. I parse for function calls, and replace their token ranges with the resulting value.
5. I parse the arithmetic operators in the standard order of operations, deducing the value on each round until there is only one value left. The value is then returned.

const NUMERIC_CHARSET = '01234567890.',
ALPHA_CHARSET = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_',
OPERATOR_CHARSET = '+-/*^%',
WHITE_SPACE_REGEX = /\s/;

const MathFunctions = {
fact: value => {
var iter,
multiplier;

for(multiplier = value - 1; multiplier > 0; --multiplier) {
value *= multiplier;
}

return value;
},
exp: value => Math.exp(value),
sqrt: value => Math.sqrt(value),
ceil: value => Math.ceil(value),
floor: value => Math.floor(value),
abs: value => Math.abs(value),
acos: value => Math.acos(value),
asin: value => Math.asin(value),
atan: value => Math.atan(value),
log: value => Math.log(value),
round: value => Math.round(value)
};

const Helpers = {
isNumeric: char => NUMERIC_CHARSET.indexOf(char) !== -1,
isAlpha: char => ALPHA_CHARSET.indexOf(char) !== -1,
isOperator: char => OPERATOR_CHARSET.indexOf(char) !== -1,
isMathFunction: keyword => typeof MathFunctions[keyword] === 'function',
isWhitespace: char => WHITE_SPACE_REGEX.test(char),
radians: angle => angle * Math.PI / 180
};

const OperatorFunctions = {
'+': (left, right) => left + right,
'-': (left, right) => left - right,
'/': (left, right) => left / right,
'*': (left, right) => left * right,
'%': (left, right) => left % right,
'^': (left, right) => Math.pow(left, right)
};

function ExpressionParser() {
'use strict';

this.variables = {
pi: Math.PI,
PI: Math.PI,
e: Math.E,
E: Math.E,
rand: () => Math.random()
};

this.readOnlyVariables = [ ];

for(var varName in this.variables) {
}
};

/* Sets a variable */
ExpressionParser.prototype.setVariable = function(name, value) {
'use strict';

if(this.readOnlyVariables.indexOf(name) !== -1) {
throw new Error('Error: Cannot set read only variable "' + name + '"');
}

this.variables[name] = value;
};

/* Gets a variable */
ExpressionParser.prototype.getVariable = function(name) {
'use strict';

if(this.isVariable(name)) {
var variable = this.variables[name];

if(typeof variable === 'function') {
return variable();
}

return variable;
}
};

/* Checks if a variable exists */
ExpressionParser.prototype.isVariable = function(name) {
'use strict';

return this.variables.hasOwnProperty(name);
};

/* Parse an expression */
ExpressionParser.prototype.parse = function(expression) {
'use strict';

var tokens = this.tokenize(expression);

tokens = this.parseTokens(tokens);

var tokensLength = tokens.length,
iter,
value = null,
last_number = null,
flag_assignment = false;

for(iter = 0; iter < tokensLength; ++iter) {
// Get the value
if(tokens[iter][0] === 'number') {
value = tokens[iter][1];
}

if(tokens[iter][0] === 'assignment') {
if(
iter - 1 === 0 &&                   // Check there is a keyword previous
iter + 1 < tokensLength &&          // Check there is a value to set next

tokens[iter - 1][0] === 'keyword'
) {
flag_assignment = true;
} else {
throw new Error('Error: Unexpected assignment');
}
}
}

if(flag_assignment) {
this.setVariable(tokens[0][1], value);
}

return value;
};

/* Parse tokens */
ExpressionParser.prototype.parseTokens = function(tokens) {
'use strict';

tokens = this.parseVariables(tokens);
tokens = this.parseBrackets(tokens);
tokens = this.parseNegatives(tokens);
tokens = this.parseFunctions(tokens);
tokens = this.parseOperations(tokens);

};

ExpressionParser.prototype.parseBrackets = function(tokens) {
'use strict';

var tokensLength = tokens.length,
bracketDepth = 0,
bracketIndex = 0,
iter;

for(iter = 0; iter < tokensLength; ++iter) {
if(tokens[iter][0] === 'bracket') {
if(bracketDepth > 0) {
if(tokens[iter][1] === ')') {
--bracketDepth;
}

if(bracketDepth === 0) {
let leftSide = tokens.slice(0, bracketIndex),
parsed = this.parseTokens(tokens.slice(bracketIndex + 1, iter)),
rightSide = tokens.slice(iter + 1);

tokens = leftSide.concat(parsed, rightSide);
iter += tokens.length - tokensLength;
tokensLength = tokens.length;
}
}

if(tokens[iter][1] === '(') {
if(bracketDepth === 0) {
bracketIndex = iter;
}

++bracketDepth;
}
}
}

};

ExpressionParser.prototype.parseNegatives = function(tokens) {
'use strict';

var tokensLength = tokens.length,
iter;

for(iter = 0; iter < tokensLength; ++iter) {
// Logic for a negative number
if(
tokens[iter][0] === 'operator' &&
(
tokens[iter][1] === '-' ||          // Check it's a minus symbol
tokens[iter][1] === '+'             // Or a plus symbold
) &&
(
iter - 1 < 0 ||                     // Either there is no previous token...
tokens[iter - 1][0] !== 'number'    // Or it's not a number
) &&
iter + 1 < tokensLength &&              // Check there is a proceeding token
tokens[iter + 1][0] === 'number'        // And it's a number
) {
// Make the next number a negative
tokens[iter + 1][1] = tokens[iter][1] === '-' ? -tokens[iter + 1][1] : tokens[iter + 1][1];
// Remove this token from stack
tokens.splice(iter, 1);
--tokensLength;
--iter;
continue;
}
}

};

ExpressionParser.prototype.parseVariables = function(tokens) {
'use strict';

var tokensLength = tokens.length,
iter;

for(iter = 0; iter < tokensLength; ++iter) {
if(tokens[iter][0] === 'keyword') {
if(
!Helpers.isMathFunction(tokens[iter][1]) && // Check it's not a function
(
iter === tokensLength - 1 ||            // Either this is the last token
tokens[iter + 1][0] !== 'assignment'    // Or the next token is not an assignment
)
) {
// Check variable exists
if(this.isVariable(tokens[iter][1])) {
tokens[iter][0] = 'number';
tokens[iter][1] = this.getVariable(tokens[iter][1]);
continue;
} else {
throw new Error('Error: Undefined variable "' + tokens[iter][1] + '"');
}
}
}
}

};

ExpressionParser.prototype.parseFunctions = function(tokens) {
'use strict';

var tokensLength = tokens.length,
iter;

for(iter = 0; iter < tokensLength; ++iter) {
if(tokens[iter][0] === 'keyword' && tokens[iter][1] in MathFunctions) {
if(
iter + 1 < tokensLength &&          // Check this is not the last token
tokens[iter + 1][0] === 'number'    // And the last next token is a number
) {
// Apply math function
tokens[iter + 1][1] = MathFunctions[tokens[iter][1]](tokens[iter + 1][1]);
// Remove this token from stack
tokens.splice(iter, 1);
--tokensLength;
--iter;
} else {
throw new Error('Error: unexpected function "' + tokens[iter][1] + '"');
}
}
}

};

ExpressionParser.prototype.parseOperations = function(tokens) {
'use strict';

// Order of operations
var operators = ['^', '*', '/', '+', '-'];

operators.forEach(operator => {
tokens = this.parseOperator(tokens, operator);
});

};

ExpressionParser.prototype.parseOperator = function(tokens, operator) {
'use strict';

var tokensLength = tokens.length,
iter;

for(iter = 0; iter < tokensLength; ++iter) {
var token = tokens[iter],
token_type = token[0],
token_value = token[1];

if(token_type === 'operator' && token_value === operator) {
if(
iter - 1 >= 0 &&                        // Check there is a previous token
iter + 1 < tokensLength &&              // Check there is a next token
tokens[iter - 1][0] === 'number' &&     // Check the previous token is a number
tokens[iter + 1][0] === 'number'        // Check the next token is a number
) {
tokens[iter + 1][1] = OperatorFunctions[token_value](tokens[iter - 1][1], tokens[iter + 1][1]);

let leftSide = tokens.slice(0, iter - 1),
rightSide = tokens.slice(iter + 1);

// Replace sub-expression with the result value
tokens = leftSide.concat(rightSide);
iter += tokens.length - tokensLength;
tokensLength = tokens.length;

continue;
} else {
throw new Error('Error: unexpected operator "' + tokens[iter][1] + '"');
}
}
}

};

/**
* Split expression into tokens
*/
ExpressionParser.prototype.tokenize = function(expression) {
'use strict';

// Append space so that the last character before that space is tokenised
expression += ' ';

// TOKENIZER VARS
var expressionLength = expression.length,
iter,
tokens = [ ],
buffer = '';

// FLAGS
var flag_numeric = false,
flag_keyword = false,
flag_operator = false;

// Iterate through expression
for(iter = 0; iter < expressionLength; ++iter) {
var char = expression.charAt(iter),
char_isNumeric = Helpers.isNumeric(char),
char_isOperator = Helpers.isOperator(char),
char_isAlpha = Helpers.isAlpha(char);

if(flag_keyword) {
// We've reached the end of the keyword
if(!char_isAlpha) {
flag_keyword = false;
tokens.push(['keyword', buffer]);
buffer = '';
}
}

if(flag_numeric) {
// We've reached the end of the number
if(!char_isNumeric) {
// Skip char if comma, so we can allow for formatted numbers
if(char === ',' && iter + 1 < expressionLength && Helpers.isNumeric(expression[iter + 1])) {
continue;
}

let parsingFunction = parseInt;

if(buffer.indexOf('.') !== -1) { // Check for float
parsingFunction = parseFloat;
}

tokens.push(['number', parsingFunction(buffer, 10)]);

flag_numeric = false;
buffer = '';
}
}

if(char_isNumeric) {                     // Check for a number
flag_numeric = true;
buffer += char;
} else if(char_isAlpha) {                // Check for a keyword
flag_keyword = true;
buffer += char;
} else if(char_isOperator) {             // Check for an operator
tokens.push(['operator', char]);
} else if(char === '(') {                // Check for parentheses
tokens.push(['bracket', '(']);
} else if(char === ')') {                // Check for closing parentheses
tokens.push(['bracket', ')']);
} else if(char === '=') {                // Check for assignment
tokens.push(['assignment', char]);
} else if(!Helpers.isWhitespace(char)) { // Check for whitespace
throw new Error('Error: Unexpected char "' + char + '"');
}
}

};

var ep = new ExpressionParser();

process.stdin.resume();
process.stdin.setEncoding('utf8');
process.stdin.on('data', function(expression) {
try {
var result = ep.parse(expression);
if(result !== false) {
console.log(result)
}
} catch(e) {
console.log(e.message);
}

process.stdout.write("> ");
});
console.log("Welcome to math2.js:");
process.stdout.write("> ");


Here is the console log for the tests I did:

\$ node math2.js
Welcome to math2.js:
> value = 11 + (exp(2.010635 + sin(PI/2)*3) + 50) / 2
110.9999779427841
> c = 3*10^8
300000000
> c*value
33299993382.835228
> sin pi
1.2246467991473532e-16
> sin (pi)
1.2246467991473532e-16
> fact 5
120
> fact(5 + 1)
720
> fact(c)
Infinity
> 22*-.5
-11
> 2,000 + 4,000
6000
> ,2000
Error: Unexpected char ","
> pi sin
Error: unexpected function "sin"
> sin pi
1.2246467991473532e-16
> E^2
7.3890560989306495


I would like to know if there are things I could have done differently that would result in a more efficient process.

• This is a lot of a code. If you don't get any answers, consider posting a smaller, self-contained snippet of this code, if that's possible... Jun 18, 2016 at 18:26
• I completely understand that. Buts it's not really the kind of things that can be broken up into smaller snippets. I'm not looking for a complete in-depth review of every line, just for a more experienced programmer to maybe look it over and see if the general concept of how I'm going about the whole things is OK. I won't be surprised if nobody does, but I can at least ask. Jun 18, 2016 at 18:38
• Well, not an answer to your question, since parsers and compilers aren't a subject I know too much about, but you might be interested in reading about "the expression problem". Here is a good lecture on it Jun 18, 2016 at 18:51
• I have rolled back the last edit. Please see what you may and may not do after receiving answers. Jun 18, 2016 at 21:31
• Is there a particular reason you're enabling strict mode only inside some functions? Jun 26, 2016 at 11:16

One common theme I notice in the code is that you alter existing variables. For instance value in this function:

fact: value => {
var iter,
multiplier;

for(multiplier = value - 1; multiplier > 0; --multiplier) {
value *= multiplier;
}

return value;
},


There's a rule in ESLint called no-param-reassign which will consider this an error if enabled. There's a linked article with regards to why this is harmful, why that ESLint rule exists, and why you shouldn't be doing this. In a gist, altering the value of an argument alters the arguments object itself.

this.variables = {
pi: Math.PI,
PI: Math.PI,
e: Math.E,
E: Math.E,
rand: () => Math.random()
};


I wonder why these aren't extracted to its own "enum", unless this is something unique to the expression parser.

this.readOnlyVariables = [ ];

for(var varName in this.variables) {
}


Shorter:

Object.keys(this.variables).map(varName => varName);


/* Gets a variable */
ExpressionParser.prototype.getVariable = function(name) {
'use strict';

if(this.isVariable(name)) {
var variable = this.variables[name];

if(typeof variable === 'function') {
return variable();
}

return variable;
}
};


This code implies that there may be a case where this.isVariable(name) is false however there is no return for a falsy case. This can make this function a bit ambiguous especially when it is named getVariable. It implies that the function returns something. If undefined is a valid value, make it explicit in the code.

I can't speak much about the parser, although I believe parsing in the simplest sense is just exploding a string into tokens and trying to make sense of it. This may be as simple as having a string be exploded into an array and being fed through a loop of operations, with a few storages (stacks, registers, etc.) to keep state and markers.

In this case, array operations like forEach, map, reduce and filter might be handy in place of loops. From a performance perspective, loops are faster, but from a readability standpoint, array methods are often shorter and more concise.

• Thank you for taking the time to take a look at this. I appreciate it. I see what you mean, and yes setting an object property is slower than setting a variable I suppose. All makes sense. I'll implement those suggestions and post an update. Jun 18, 2016 at 20:41
• Sorry, I meant to add: the this.variables object's initial properties are there for a reason, they are the initial variables that are set and are read only, so that the variable pi cannot be set to something other than 3.142..., as for the rand variable, I suppose that was a bit of a hack to avoid adding support for a function that doesn't take the right-hand-side token as it's argument. I should fix that. As for the bit about loops, In a lot of places the loop variables need to be modified, therefore one of those array operations won't do, and of course from a performance aspect. Jun 18, 2016 at 21:05
• I have updated my question now. Jun 18, 2016 at 21:23