Expression Basics

Intro to Data Types

In the previous article you learnt that each property expects a value that is of a specific structure, or type. In this article you'll learn what those basics types are and what different properties expect.


In JavaScript, every value is of a specific type. Which data type a value is defines how you interact with that value in your code.

In the same way, in After Effects the value of each property has a specific type, and your expression needs to return that same type in order to run without errors.

You can see what type a value is by writing typeof value in an expression (such as the 'Source Text' property of a text layer).


One of the most common types you will use is Number, such as 8 or 12.653.

typeof 63;

Numbers allow you to do mathematical operations such as add, subtract, divide and multiply them together, in the same way you're used to on a calculator.

60 + 20 / 2;

Properties that expect numbers include:

  • Opacity
  • Slider Controls
  • Separated positions (e.g. X Position, Y Position)
  • Rotation


Strings are pieces of text such as "Jane Doe" or 'After Effects'.

typeof "Hello!";

Strings must be wrapped in single ('') or double ("") quotes so JavaScript knows you're not trying to access a variable.

You can create a String with numbers in it, that isn't of a number type, by wrapping it in quotes, e.g. "12.653"

typeof "12.653";

Properties that expect strings:

  • Source Text


You can think of Booleans like switches, as they can either have the value true or false (notice these are not wrapped in quotes).

typeof true;


You'll notice that some properties expect more than one value, such as Position or Scale. You can think of these properties as expecting a list of values that's a specific length.

The number of values a property has is called the dimension of the property.

2D Properties

For example:

  • Scale has 2 values (the X and Y scale) and is a 2 dimensional property
  • The position of a 3D layer expects 3 values (the XYZ coordinates) and is a 3 dimensional property
  • A color control expects 4 values (the red, green, blue and alpha values) and is a 4 dimensional property

In JavaScript, we can create lists with what's called an Array.

Properties such as Position that have more than one value expect an Array.

You create arrays by enclosing a list of comma seperated values in brackets, for example:

["Item 1", "Item 2", "Item 3"];

Which is a 3 dimensional Array (as it contains 3 items) of 3 Strings.

To set the value of a 2D position property (which has 2 dimensions), you would create an Array of 2 number values.

[960, 540];

Properties that expect arrays include:

  • Position (with un-seperated dimensions)
  • Scale
  • Anchor Position
  • Color Controls

Strictly speaking, Arrays aren't a data type, but we've included them here for simplicities sake. There's also more data types than in this article that we'll explore later in the series.

Type Coercion

You'll notice that while a Source Text property displays text, and expects a string, placing a Number or Array in the expression still works.

[960, 540];
960, 540;

This is due to something called type coercion. When you provide a data type different to what is expected, JavaScript will do it's best to convert it to the expected type.

For example, giving a 1 dimensional Array instead of a number on an Opacity property:


You can convert a value from one data type to another yourself by using the name of the type followed by parenthesis. For example String(54) or Number([80]).

typeof String(54);

Often it's best to do this type conversion yourself rather than relying on coercion.

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