Customization #

Besides parsing and evaluating expressions, the expression parser supports a number of features to customize processing and evaluation of expressions and outputting expressions.

On this page:

Function transforms #

It is possible to preprocess function arguments and post process a functions return value by writing a transform for the function. A transform is a function wrapping around a function to be transformed or completely replaces a function.

For example, the functions for math.js use zero-based matrix indices (as is common in programing languages), but the expression parser uses one-based indices. To enable this, all functions dealing with indices have a transform, which changes input from one-based to zero-based, and transforms output (and error message) from zero-based to one-based.

// using plain JavaScript, indices are zero-based:
var a = [[1, 2], [3, 4]]; // a 2x2 matrix
math.subset(a, math.index(0, 1)); // returns 2

// using the expression parser, indices are transformed to one-based:
var a = [[1, 2], [3, 4]]; // a 2x2 matrix
var scope = {
  a: a
math.eval('subset(a, index(1, 2))', scope); // returns 2

To create a transform for a function, the transform function must be attached to the function as property transform:

var math = require('../index');

// create a function
function addIt(a, b) {
  return a + b;

// attach a transform function to the function addIt
addIt.transform = function (a, b) {
  console.log('input: a=' + a + ', b=' + b);
  // we can manipulate input here before executing addIt

  var res = addIt(a, b);

  console.log('result: ' + res);
  // we can manipulate result here before returning

  return res;

// import the function into math.js
  addIt: addIt

// use the function via the expression parser
console.log('Using expression parser:');
console.log('2+4=' + math.eval('addIt(2, 4)'));
// This will output:
//     input: a=2, b=4
//     result: 6
//     2+4=6

// when used via plain JavaScript, the transform is not invoked
console.log('Using plain JavaScript:');
console.log('2+4=' + math.addIt(2, 4));
// This will output:
//     6

Functions with a transform must be imported in the math namespace, as they need to be processed at compile time. They are not supported when passed via a scope at evaluation time.

Custom argument parsing #

The expression parser of math.js has support for letting functions parse and evaluate arguments themselves, instead of calling them with evaluated arguments. This is useful for example when creating a function like plot(f(x), x) or integrate(f(x), x, start, end), where some of the arguments need to be processed in a special way. In these cases, the expression f(x) will be evaluated repeatedly by the function, and x is not evaluated but used to specify the variable looping over the function f(x).

Functions having a property rawArgs with value true are treated in a special way by the expression parser: they will be invoked with unevaluated arguments, allowing the function to process the arguments in a customized way. Raw functions are called as:

rawFunction(args: Node[], math: Object, scope: Object)

Where :

Raw functions must be imported in the math namespace, as they need to be processed at compile time. They are not supported when passed via a scope at evaluation time.

A simple example:

function myFunction(args, math, scope) {
  // get string representation of the arguments
  var str = (arg) {
    return arg.toString();

  // evaluate the arguments
  var res = (arg) {
    return arg.compile().eval(scope);

  return 'arguments: ' + str.join(',') + ', evaluated: ' + res.join(',');

// mark the function as "rawArgs", so it will be called with unevaluated arguments
myFunction.rawArgs = true;

// import the new function in the math namespace
  myFunction: myFunction

// use the function
math.eval('myFunction(2 + 3, sqrt(4))');
// returns 'arguments: 2 + 3, sqrt(4), evaluated: 5, 2'

Custom LaTeX handlers #

You can attach a toTex property to your custom functions before importing them to define their LaTeX output. This toTex property can be a handler in the format described in the next section ‘Custom LaTeX and String conversion’ or a template string similar to ES6 templates.

Template syntax #

Example #

var customFunctions = {
  plus: function (a, b) {
    return a + b;
  minus: function (a, b) {
    return a - b;
  binom: function (n, k) {
    return 1;
}; = '${args[0]}+${args[1]}'; //template string
customFunctions.binom.toTex = '\\mathrm{${name}}\\left(${args}\\right)'; //template string
customFunctions.minus.toTex = function (node, options) { //handler function
  return node.args[0].toTex(options) + + node.args[1].toTex(options);


math.parse('plus(1,2)').toTex();    //'1+2'
math.parse('binom(1,2)').toTex();   // '\\mathrm{binom}\\left(1,2\\right)'
math.parse('minus(1,2)').toTex();   // '1minus2'

Custom LaTeX and string output #

All expression nodes have a method toTex and toString to output an expression respectively in LaTex format or as regular text . The functions toTex and toString accept an options argument to customise output. This object is of the following form:

  parenthesis: 'keep',   // parenthesis option
  handler: someHandler,   // handler to change the output
  implicit: 'hide' // how to treat implicit multiplication

Parenthesis #

The parenthesis option changes the way parentheses are used in the output. There are three options available:

There’s two ways of passing callbacks:

  1. Pass an object that maps function names to callbacks. Those callbacks will be used for FunctionNodes with functions of that name.
  2. Pass a function to toTex. This function will then be used for every node.
var expression = math.parse('(1+1+1)');

expression.toString(); //(1 + 1 + 1)
expression.toString({parenthesis: 'keep'}); //(1 + 1 + 1)
expression.toString({parenthesis: 'auto'}); //1 + 1 + 1
expression.toString({parenthesis: 'all'});  //(1 + 1) + 1

Handler #

You can provide the toTex and toString functions of an expression with your own custom handlers that override the internal behaviour. This is especially useful to provide LaTeX/string output for your own custom functions. This can be done in two ways:

  1. Pass an object that maps function names to callbacks. Those callbacks will be used for FunctionNodes that contain functions with that name.
  2. Pass a callback directly. This callback will run for every node, so you can replace the output of anything you like.

A callback function has the following form:

var callback = function (node, options) {

Where options is the object passed to toTex/toString. Don’t forget to pass this on to the child nodes, and node is a reference to the current node.

If a callback returns nothing, the standard output will be used. If your callback returns a string, this string will be used.

Although the following examples use toTex, it works for toString in the same way

Examples for option 1 #

var customFunctions = {
  binomial: function (n, k) {
    //calculate n choose k
    // (do some stuff)
    return result;

var customLaTeX = {
  'binomial': function (node, options) { //provide toTex for your own custom function
    return '\\binom{' + node.args[0].toTex(options) + '}{' + node.args[1].toTex(options) + '}';
  'factorial': function (node, options) { //override toTex for builtin functions
  	return 'factorial\\left(' + node.args[0] + '\\right)';

You can simply use your custom toTex functions by passing them to toTex:

var expression = math.parse('binomial(factorial(2),1)');
var latex = expression.toTex({handler: customLaTeX});
//latex now contains "\binom{factorial\\left(2\\right)}{1}"

Examples for option 2: #

var customLaTeX = function (node, options) {
  if ((node.type === 'OperatorNode') && (node.fn === 'add')) {
    //don't forget to pass the options to the toTex functions
    return node.args[0].toTex(options) + ' plus ' + node.args[1].toTex(options);
  else if (node.type === 'ConstantNode') {
    if (node.value == 0) {
        return '\\mbox{zero}';
    else if (node.value == 1) {
        return '\\mbox{one}';
    else if (node.value == 2) {
        return '\\mbox{two}';
    else {
        return node.value;

var expression = math.parse('1+2');
var latex = expression.toTex({handler: customLaTeX});
//latex now contains '\mbox{one} plus \mbox{two}'

Another example in conjunction with custom functions:

var customFunctions = {
  binomial: function (n, k) {
    //calculate n choose k
    // (do some stuff)
    return result;

var customLaTeX = function (node, options) {
  if ((node.type === 'FunctionNode') && ( === 'binomial')) {
      return '\\binom{' + node.args[0].toTex(options) + '}{' + node.args[1].toTex(options) + '}';

var expression = math.parse('binomial(2,1)');
var latex = expression.toTex({handler: customLaTeX});
//latex now contains "\binom{2}{1}"

Implicit multiplication #

You can change the way that implicit multiplication is converted to a string or LaTeX. The two options are hide, to not show a multiplication operator for implicit multiplication and show to show it.


var node = math.parse('2a');

node.toString(); //'2 a'
node.toString({implicit: 'hide'}); //'2 a'
node.toString({implicit: 'show'}); //'2 * a'

node.toTex(); //'2~ a'
node.toTex({implicit: 'hide'}); //'2~ a'
node.toTex({implicit: 'show'}); //'2\\cdot a'

Customize supported characters #

It is possible to customize the characters allowed in symbols and digits. The parse function exposes the following test functions:

The exact signature and implementation of these functions can be looked up in the source code of the parser. The allowed alpha characters are described here: Constants and variables.

For example, the $ character is not supported by default. It can be enabled by replacing the isAlpha function:

var isAlphaOriginal = math.expression.parse.isAlpha;
math.expression.parse.isAlpha = function (c, cPrev, cNext) {
  return isAlphaOriginal(c, cPrev, cNext) || (c === '$');

// now we can use the $ character in expressions
var result = math.eval('$foo', {$foo: 42}); // returns 42
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