BASIC-40 Interpreter
A classic 8-bit style BASIC interpreter that runs entirely in the browser. It has a 40×24 character CRT screen with green, amber and white phosphor themes. There are no dependencies and no build step.
Live demo: https://cstroie.github.io/basic/
Usage
Open index.html in a browser. No server or build step is needed.
- Type a numbered line (
10 PRINT "HI") to store it. A bare line number (10) deletes that line.
- Type a line without a number to run it immediately (
PRINT 2+2, FOR I=1 TO 3:PRINT I:NEXT).
- Use the Examples menu to load a sample program, then type
RUN.
- Press
Esc or Ctrl+C, or click STOP, to break a running program. CONT resumes it.
↑/↓ scroll through command history. Pasting a multi-line listing enters every line.- OPEN / DOWNLOAD read and write
.bas files. You can also drop a .bas file on the screen.
- The current program is saved automatically and restored on your next visit.
- SHARE copies a link with the program packed into the address. Opening the link loads that program.
Language
| Area | Supported |
|---|
| Commands | RUN [line], LIST [a-b], NEW, CLS, CONT, CLEAR, SAVE name, LOAD name, KILL name, FILES, HELP |
| Statements | PRINT (; , TAB() SPC(), PRINT AT row,col;), INPUT ["prompt";] a, b$, LET (optional), IF … THEN … ELSE, FOR … TO … STEP / NEXT, WHILE / WEND (or END WHILE), EXIT [FOR|WHILE], GOTO, GOSUB / RETURN, ON x GOTO/GOSUB, DIM, READ / DATA / RESTORE, END, STOP, REM / ', ? (short for PRINT) |
| Functions | ABS SGN INT SQR SIN COS TAN ATN LOG EXP RND PI LEN VAL ASC CHR$ STR$ LEFT$ RIGHT$ MID$ INSTR UCASE$ LCASE$ STRING$ |
| Operators | + - * / ^ MOD, = == <> < > <= >=, AND OR NOT, + joins strings |
| User functions | DEF FNA(X) = …, DEF FNB$(A$, N) = … |
| Keyboard and time | INKEY$ (non-blocking), GET k$ (waits for a key), SLEEP seconds |
| Colour and graphics | COLOR fg[,bg] (0–15, C64 palette; −1 is the default), PLOT x,y, DRAW x,y, CIRCLE x,y,r, GCLS on a 320×240 canvas over the text |
| Sound | SOUND hz,ms, BEEP |
| Editing | RENUM [start[,step]], AUTO [start[,step]] (an empty line ends it), EDIT line |
| Options | OPTION CLASSIC prints numbers with MS BASIC spacing; OPTION COMPACT (default) doesn't |
- Separate statements on one line with
:.
- Variables whose names end in
$ hold strings. Arrays can be indexed with A(i) or A[i], and an array used without DIM gets 0..10.
SAVE/LOAD store programs in the browser's localStorage. The name can be quoted or not.
Examples
The menu on the page has eleven built-in programs, including Graphics & Colour and Keyboard & Sound. The same programs, plus the original primes.bas, are in examples/ as plain .bas files. Load one with OPEN or by dropping it on the screen.
| File | Shows |
|---|
hello.bas | PRINT, TAB, FOR loops |
primes.bas | WHILE … END WHILE, EXIT, arrays with [ ], == |
primes-while.bas | the same algorithm in WEND style with INPUT |
factorial.bas | INPUT loop, END |
guess.bas | RND, IF … THEN … : GOTO |
sine.bas | SIN, STEP, text plotting |
fizzbuzz.bas | MOD, print zones with , |
strings.bas | MID$, UCASE$, LEN, IF … ELSE |
sort.bas | DIM, READ / DATA, nested loops |
graphics.bas | COLOR, PLOT, DRAW, CIRCLE |
keys.bas | GET, SOUND, DEF FN, INSTR |
mandelbrot.bas | PRINT AT, WHILE, AND, MID$ |
Install and offline use
The page is a Progressive Web App. Your browser offers to install it (on a phone, use Add to Home Screen). After the first visit it also works offline: a service worker (sw.js) caches the app and refreshes it in the background, so updates appear on the next visit.
Layout
basic.js holds the interpreter core: tokenizer, expression parser, statement executor, character-grid terminal and example programs. It does not touch the DOM, so it can also run under Node.index.html holds the user interface.examples/ holds sample programs.
To try the core without a browser:
const { Basic, Terminal } = require('./basic.js');
const storage = { get: () => null, set: () => true, remove() {}, keys: () => [] };
const b = new Basic(new Terminal(), storage);
b.enter('FOR I=1 TO 3:PRINT I*I:NEXT');
b.runSlice();
console.log(b.term.lines().join('\n'));
Tests
node --test
The tests run the interpreter core under Node (version 18 or later) with no dependencies. GitHub Actions runs them on every push.
License
MIT © Costin Stroie
