# Functions
DecisionRules functions are essential for building decision processes of any complexity. These functions can be applied in different rule types, such as [Decision Table](https://docs.decisionrules.io/doc/rules/decision-table), [Decision Tree](https://docs.decisionrules.io/doc/rules/decision-tree) and [Flow](https://docs.decisionrules.io/doc/rules/flow), allowing users to write mathematical and logical expressions in both conditions and results.
how functions can be used in a Decision Table
## Categories of Functions
Functions are divided into several categories based on their area of application:
* **Math functions**
* **Logical functions**
* **Date and time functions**
* **Text functions**
* **Data functions**
* **Array functions**
* **Integration functions**
## General Usage and Principles
Though the functions have broad applications and behaviours, they share several core principles. These apply universally across all functions. Below is a quick overview, supported by basic examples.
### Syntax
The syntax for DecisionRules functions is simple. Functions are written using their names (always uppercase, occasionally containing underscores) followed by parentheses that enclose the function arguments, separated by commas.
```javascript
SUM(1,4,5) --> 10
```
Each function accepts a variable number of arguments, some optional, while others do not need any arguments at all.
Example of no-argument function:
```javascript
NOW() --> returns the current date and time
```
{% hint style="warning" %}
In Decision Tables, functions can be used in condition, result, and calculation columns.
* **Condition columns**: You can choose the function from the operators modal, which opens when you click the operator badge. Functions in these columns must evaluate to boolean values (true or false).
* **Calculation and result columns**: These do not have the operators modal. Simply start typing the function name, and a dropdown whisper will appear, showing all matching functions.
{% endhint %}
### Values
DecisionRules supports several data types:
* **Primitive types**: number, string, boolean, null.
* **Complex types**: arrays, objects.
* **Additional types for functions**: date, regular expressions.
{% hint style="info" %}
Read more in the [Supported Data Types](https://docs.decisionrules.io/doc/rules/data-types-and-functions/data-types) section.
{% endhint %}
Basic syntax examples:
```json
12.1 --> number
"abc" --> string
true --> boolean
false --> boolean
null --> null
```
{% hint style="info" %}
As shown above, strings are typically enclosed in double quotes. For simple string expressions that do not contain special characters \[\*/+-(),%{}], the quotes can be omitted. However, we recommend keeping them for clarity and consistency.
{% endhint %}
Special characters within strings can be handled with either single quotes or escape sequences:
```json
"abcd" --> abcd
'ab"cd' --> ab"cd
"ab\"cd" --> ab"cd
"ab\'cd" --> ab'cd
"ab'cd" --> ab'cd
"ab\\cd" --> ab\cd
```
#### Type Conversion
Functions in DecisionRules often handle type casting automatically. If possible, they will convert values to the required type.
```json
SUM(1,4,"5") --> 10
```
However, type casting does not always work. If values cannot be converted, the function will either fail validation or return an error at runtime.
```json
SUM(1,4,"abc") --> invalid
```
#### Arrays and Objects
Some functions accept arrays or objects as arguments. DecisionRules uses JSON-like syntax for arrays and objects.
```json
SUM([1,4,5]) --> 10
```
```json
[1,4,5] --> array of numbers
[{"id":"A5B52","price":45}] --> array of objects
{"id":"A5B52","price":45} --> object with key-value pairs
```
#### Nesting Functions
Functions can be nested inside each other. For instance, one function's result can be passed as an argument to another function.
```json
SUM(MAX(1,2),1) --> 3
```
Here, `MAX(1,2)` evaluates to `2`, which is then used in the `SUM` function.
#### Variables
There are five types of variables that can be used in functions:
* **Input variables**: represent the rule's input data.
* **Output variables**: represent the rule's output data.
* **Rule variables**: user-defined variables with rule-level scope.
* **Calculation column variables**: variables declared in calculation columns in Decision Tables.
* **Abstract function variables**: special variables used in certain functions.
Variables are referenced using curly brackets (`{}`).
```javascript
// number = 3
SUM({number},6) --> 9
```
In cases where the value from the variable cannot be used, the evaluation will fail and return an empty value:
// number = "a"
SUM({number},6) --> null
#### Functions in Decision Tables
In Decision Tables, functions can be used in condition, result, and calculation columns.
* **Condition columns** must evaluate to boolean values (true or false).
* **Calculation columns** allow users to define variables or intermediary values that can be used in other columns for more complex calculations.
* **Result columns** can contain any type of function.
Example of functions in the condition and calculation columns
When using functions in a decision table, you can turn on the Debug Mode and open the Console to see the detail of how they evaluated.
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