Spe­cial thanks to Gilles-Phil­ippe of Conic Games for hav­ing cre­ated Ex­po­nen­tial Idle. He has also helped me on nu­mer­ous oc­ca­sions while I was cre­at­ing my the­or­ies.

Thanks to Baldy for host­ing the Ex­po­nen­tial Idle guides web­site that this series is rest­ing on, Mathis for provid­ing a draft for the The­ory Sim­u­lator im­ple­ment­a­tion tu­torial (ap­pendix of Week 1), and vari­ous other play­ers for read­ing and test­ing out the guide.

My fi­nal thank goes out to you, who are read­ing this guide right at this mo­ment.

…Most of them have been scattered, like pocket mon­sters across the vast planes of Joutou, and time. These cre­ations, moul­ded from chalk and sweat. Some ex­pose won­ders of math­em­at­ics, some let their atoms burn, some bloom out of fer­tile earth, while some feel just con­tent enough, to watch the plan­ets turn. Some of them em­body the es­sence of chaos, oth­ers ex­er­cise or­der and con­trol. But within all, one thing ex­ists in com­mon…

- The­ory­mon trailer, 2025

Locked be­hind the fi­nal Con­ver­gence Test, cus­tom the­or­ies are the­or­ies which have been cre­ated by the com­munity us­ing JavaS­cript. Un­like what you’ve seen in the base game, cus­tom the­or­ies have ac­cess to a wide ar­ray of fea­tures. In ad­di­tion to hav­ing up­grades, equa­tions and graphs, they are also cap­able of host­ing their own story, set­ting up cus­tom achieve­ments, dis­play­ing cus­tom UI, re­pla­cing ex­ist­ing UI, or even trig­ger­ing player ac­tions in the base game. Al­though lim­ited in their pro­cessing ca­pa­city (a cer­tain boomer shooter game has yet to be re­cre­ated here), need­less to say, Ex­po­nen­tial Idle’s cus­tom the­ory API provides a whole lot of po­ten­tial for unique and in­ter­est­ing cre­ations.

1. JavaS­cript

Be­fore you en­gage with this guide, a ba­sic un­der­stand­ing of JavaS­cript is re­quired, al­though keep in mind that cus­tom the­ory de­vel­op­ment does not util­ize async/​await, nor does it sup­port mod­ules, so you may skip through these con­cepts. A tu­torial that I can re­com­mend is at javas­cripttutorial.net. For a quick un­der­stand­ing, read from Sec­tion 1 to 5, while Sec­tions 6 to 8 can be read later, and then the rest.

2. Ex­po­nen­tial Idle The­ory SDK

The next im­port­ant tool to ac­quire is the Ex­po­nen­tial Idle The­ory SDK, which provides us with a cus­tom the­ory API, and al­lows us to send a the­ory right to your device in real-time. Without the SDK, the only way to fetch the­or­ies would be to host your file on a web­site, and if it hap­pens to be Git­Hub, it could take a few minutes to re­fresh whenever you make a com­mit.

3. Source con­trol

Speak­ing of com­mits, while the guide does not as­sume prior know­ledge of source con­trol sys­tems such as Git, learn­ing source con­trol is highly re­com­men­ded. By set­ting up a re­pos­it­ory for your the­ory, you can keep track of your its his­tory, split it into al­tern­ate real­it­ies, re­vert cata­strophes that can’t be un­done with any Ctrl+Z, and do so much more.

4. LaTeX

LaTeX is the type­set­ting sys­tem used by Ex­po­nen­tial Idle to render the equa­tions. Keep in mind however, be­cause the game’s LaTeX does not in­clude any pack­ages, any tu­torial that re­quires pack­ages such as ams­math or xfrac is not go­ing to be ap­plic­able. Try to take a look around to see how LaTeX is usu­ally writ­ten.

5. A code ed­itor

I am a scrub. I use Visual Stu­dio Code.

Other ed­it­ors in­clude:

• Sub­lime Text
• Note­pad++
• Vim or something

For a bet­ter ex­per­i­ence, in­stall the JavaS­cript ex­ten­sion/​add-on (if avail­able) for your ed­itor.

Now that you have a grip on the ba­sics and have in­stalled the ne­ces­sary tools, the next step would be to check out what a cus­tom the­ory may look like, and for that, I shall see you to­mor­row.