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Page updated, redesigned, & republished: 2 November 2018
Last modified: 20 October 2021

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Welcome to the JavaScript Primer WOTM Tour:

Helping you build a foundation for writing expressions

WOTM: Adobe After Effects® Expressions Graphic

What is the JavaScript Primer WOTM Tour?

This tour is powered by the JavaScript Tour menu in the sidebar (or the bottom of the page if you’re reading this on a small screen), which is a simple tool to help organize your reading of several Word of the Moment (WOTM) entries. If you are new to After Effects or have never written code, following the JavaScript Tour menu in the order given should give you a basic foundation in JavaScript as it applies to writing expressions.

If you’ve been using After Effects, but only dabbled in expressions, then the JavaScript Tour menu allows you to skip topics with which you are familiar. This method of presentation also allows the separation of these concepts in the Word of the Moment glossary, which, in turn, allows them to be a part of the tooltip links throughout the donQmedia website.

What is JavaScript?

JavaScript is  a scripting language, and is one of the primary languages used to build the web, along with HTML and CSS. However, we After Effects users don’t have to learn all the ins and outs of JavaScript because we are concerned only with the code that goes in the expression fields of properties.

Though the expression language is based on a scripting language, a subtle but important difference exists between a script and an expression: Whereas a script tells an application to do something, an expression says that a property is something.

Expression Basics

After Effects uses the core JavaScript 1.2 language, but none of the extensions created specifically for web browsers.  All of the JavaScript information on this website is targeted toward writing code in the After Effects expression field. While it is possible to code scripts to add custom tools to After Effects, that is beyond the scope of this website. Plugins are written in C or C++, and require an extensive knowledge of that programming language and the After Effects Application Programming Interface (API).

Two good sites to visit for more advanced expression coding in After Effects are J. J. Gifford’s and Dan Ebbert’s websites.

If you want to know how to do more extensive coding in JavaScript in general I suggest you try and Also, JavaScript: The Definitive Guide by David Flanagan1 is an excellent resource for the serious JavaScript programmer. The way it’s chapters are structured make it an exceptional learning resource, and its index makes it an invaluable reference book.

Why learn JavaScript?

You don’t have to master JavaScript in order to create useful and time saving scripts, but a little knowledge about coding expressions can go a long way. By going beyond simple pick whip connections, or single lines of code (e.g. wiggle), you can  sometimes save yourself a lot of keyframing (e.g., have cloth movement triggered by a puppet’s movement), or  build powerful puppet controllers. You can also have expression work in conjunction with keyframes combining movements (e.g. keyframing an arm movement, then adding a slight wiggle expression to make the movement look jittery). Understanding just a few of the often used expressions can open a whole new world of possibilities when animating puppets.

The JavaScript Tour should give you a good foundation in writing expressions for After Effects, and, if  you pair it with the Controllers & Expressions Series, you will have a good start on learning to understand and write your own expressions. There is, of course, a lot to learn if you want to get into more complex coding, but you may be surprised what you can do with this basic foundation.

Code as language

Like written language, computer code has vocabulary, syntax, punctuation, and spelling rules. One of the big differences between written speech and computer code is that code can be extremely unforgiving. While a first grade teacher becomes adept at deciphering her young charges first attempts at the written word, a computer usually can’t understand what your intentions are if you don’t follow the rules explicitly. For example, JavaScript is case sensitive, which means if is not the same as If. Like human language, there can be many ways to say the same thing, but with code you have to be precise no matter how you say it.

That said, if when you make a coding error, After Effects will usually let you know; often telling you what line it is on and what kind of error it is. The error notice will pop up at the bottom of the Composition Panel. Unfortunately, not every coding error can be picked up when JavaScript is run. The code may be valid, but doesn’t do the job you thought it would.

Screenshot of JavaScript error message.

An error message pops up if you’ve made a mistake in your JavaScript code. If you have more than one mistake, it addresses the first one it comes across, identifying the line number and giving some information about the type of error. Once an error is detected, After Effects automatically disables the expression field.

Many ancient languages had no spaces or punctuation when written. Similarly, with the exception of text between quotation marks, JavaScript doesn’t care about spaces². It also doesn’t care if the code is written on 1 line or 1000 either. The reason for breaking the code into lines is for human consumption – it’s much easier to read and find errors if you spread the code out. It’s also easier to insert and find comments. There are programs that will condense JavaScript code to 1 line, a process called minifying, and is used for web production to reduce the amount of data that needs to flow through the net.

Compare the two blocks of code below (don’t worry about what the code means, we’re just looking at how it appears visually). The are identical, except one is written on a single line:

Code example 1:

HC=comp("AE Mollie V7.2").layer("C_Head").transform.position;

Code example 2:

HC=comp("AE Mollie V7.2").layer("C_Head").transform.position;SP=[250,250,0];if(HC[1]<=0){Y=linear(HC[1],-50,0,-30,0);}else{Y=linear(HC[1],0,50,0,50);}P=SP+[0,Y,0];P

If I had to figure out what was going on, I would want to work with code example 1 above.

To be verbose, or not to be  verbose

One of the attributes of my expression coding is that I tend to be verbose. Almost all of the code I write could be whittled down. Unlike web code that has to be downloaded before your computer works with it, lengthy expressions in After Effects usually will not have a noticeable affect on the speed of the calculations and response time of your computer.

For example, I usually identify a variable that will be my final value. In the code sample above, the last two lines read:


The first line assigns the letter P the value of the mathematical expression that follows the equal sign (if you aren’t familiar with variables, these are covered later in the tour). The next line just has the variable, so After Effects assigns the value that was given to P to the property in which this expression is placed. However, I could have just omitted the variable and wrote the mathematical expression, so the two lines directly above, become the single line number 8 below:

HC=comp("AE Mollie V7.2").layer("C_Head").transform.position;

I’m verbose for my own sense of clarity, and it makes it easier for me to see the logic and structure of my puppet. Because there is often the possibility that I might return to add more code as I add features to a puppet, or examine the code of a successful puppet feature that I created weeks, months, or years ago, it’s important to make the code as clear and easy to understand as possible. As you learn more about JavaScript and create your own projects, you can, of course, decide how verbose you want to be.

Begin the tour

To begin the tour, simply use the JavaScript Tour menu in the sidebar at near the top of each tour  page, either clicking and reading in order (which I suggest if you’re new to all of this), or jump around to the topics you want to explore. For your convenience I’ve provide a set of buttons below.


I hope the information on the tour clears some of the mystery of writing JavaScript expressions. Combined with my tutorials and the Word Of The Moment, you should have enough tools to start writing some useful code that can make your AfterEffects life easier, and put life into your puppets.

1JavaScript: The Definitive Guide, by David Flanagan (O’Reilly). Copyright © 2011 David Flanagan, 978-0-596-80552-4.

2Actually, JavaScript could not care less if you don’t add spaces between words in a variable containing text, but the humans reading the output most likely will.

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