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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
- Custom argument parsing
- Custom LaTeX handlers
- Custom HTML, LaTeX and string output
- Customize supported characters

### 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.

```javascript
// using plain JavaScript, indices are zero-based:
const 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:
const a = [[1, 2], [3, 4]] // a 2x2 matrix
let scope = {
  a: a
}
math.evaluate('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`:

```javascript
import { create, all } from 'mathjs'
const math = create(all)

// 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

const res = addIt(a, b)

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

return res
}

// import the function into math.js
math.import({
  addIt: addIt
})

// use the function via the expression parser
console.log('Using expression parser:')
console.log('2+4=' + math.evaluate('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('')
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:

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

Where :

- `args` is an Array with nodes of the parsed arguments.
- `math` is the math namespace against which the expression was compiled.
- `scope` is a shallow copy of the `scope` object provided when evaluating the expression, optionally extended with nested variables like a function parameter `x` of in a custom defined function like `f(x) = x^2`.

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:

```javascript
function myFunction(args, math, scope) {
  // get string representation of the arguments
  const str = args.map(function (arg) {
    return arg.toString()
  })

// evaluate the arguments
  const res = args.map(function (arg) {
    return arg.compile().evaluate(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
math.import({
  myFunction: myFunction
})

// use the function
math.evaluate('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
- `${name}`: Gets replaced by the name of the function
- `${args}`: Gets replaced by a comma separated list of the arguments of the function.
- `${args[0]}`: Gets replaced by the first argument of a function
- `$$`: Gets replaced by `$`

Example
```javascript
const customFunctions = {
  plus: function (a, b) {
    return a + b
  },
  minus: function (a, b) {
    return a - b
  },
  binom: function (n, k) {
    return 1
  }
}

customFunctions.plus.toTex = '${args[0]}+${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.name + node.args[1].toTex(options)
}

math.import(customFunctions)

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

```javascript
{
  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:

- `keep` Keep the parentheses from the input and display them as is. This is the default.
- `auto` Only display parentheses that are necessary. Mathjs tries to get rid of as much parentheses as possible.
- `all` Display all parentheses that are given by the structure of the node tree. This makes the output precedence unambiguous.

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.
const expression = math.parse('(1+1+1)')

```javascript
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:

```javascript
function callback (node, options) {
  ...
}
```

Where `options` is the object passed to `toHTML`/`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` and `toHTML` in the same way**

##### Examples for option 1
```javascript
const customFunctions = {
  binomial: function (n, k) {
    //calculate n choose k
    // (do some stuff)
    return result
  }
}

const 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:

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

##### Examples for option 2:
```javascript
function customLaTeX(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
    }
  }
}

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

Another example in conjunction with custom functions:

```javascript
const customFunctions = {
  binomial: function (n, k) {
    //calculate n choose k
    // (do some stuff)
    return result
  }
}

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

math.import(customFunctions)
const expression = math.parse('binomial(2,1)')
const 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.

Example:

```javascript
const 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:

- `math.parse.isAlpha(c, cPrev, cNext)`
- `math.parse.isWhitespace(c, nestingLevel)`
- `math.parse.isDecimalMark(c, cNext)`
- `math.parse.isDigitDot(c)`
- `math.parse.isDigit(c)`

The exact signature and implementation of these functions can be looked up in the [source code of the parser](https://github.com/josdejong/mathjs/blob/master/lib/expression/parse.js "source code of the parser"). The allowed alpha characters are described here: [Constants and variables](http://www.bixiaguangnian.com/manual/mathjs/368.html#Constants and variables "Constants and variables").

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

```javascript
const isAlphaOriginal = math.parse.isAlpha
math.parse.isAlpha = function (c, cPrev, cNext) {
  return isAlphaOriginal(c, cPrev, cNext) || (c === '\u260E')
}

// now we can use the \u260E (phone) character in expressions
const result = math.evaluate('\u260Efoo', {'\u260Efoo': 42}) // returns 42
console.log(result)
```