Reflex's Var System

Reflex supports some basic operations in state variables on the frontend. Reflex automatically converts variable operations from Python into a JavaScript equivalent.

Here's an example of a Reflex conditional in Python that returns "Pass" if the threshold is equal to or greater than 50 and "Fail" otherwise:

The conditional to roughly the following in Javascript:

Simply put, a Var in Reflex represents a Javascript expression. If the type is known, it can be any of the following:

• NumberVar represents an expression that evaluates to a Javascript number. NumberVar can support both integers and floating point values
• BooleanVar represents a boolean expression. For example: false, 3 > 2.
• StringVar represents an expression that evaluates to a string. For example: 'hello', (2).toString().
• ArrayVar represents an expression that evaluates to an array object. For example: [1, 2, 3], 'words'.split().
• ObjectVar represents an expression that evaluates to an object. For example: {a: 2, b: 3}, {deeply: \{nested: \{value: false}}}.
• NoneVar represent null values. These can be either undefined or null.

State fields are converted to Var by default. Additionally, you can create a Var from Python values using rx.Var.create():

If you want to explicitly create a Var from a raw Javascript string, you can instantiate rx.Var directly:

In the example above, .guess_type() will attempt to downcast from a generic Var type into NumberVar. For this example, calling the function .to(int) can also be used in place of .guess_type().

The Var system also supports some other basic operations. For example, NumberVar supports basic arithmetic operations like + and -, as in Python. It also supports comparisons that return a BooleanVar.

Custom Var operations can also be defined:

Use js_expression to pass explicit JavaScript expressions; in the multiply_array_values example, we pass in a JavaScript expression that calculates the product of all elements in an array called a by using the reduce method to multiply each element with the accumulated result, starting from an initial value of 1. Later, we leverage rx.cond in the' factorial' function, we instantiate an array using the range function, and pass this array to multiply_array_values.