jash – a JavaScript‑ish interpreter in BASH

I didn’t build this because we should. We built it because we could – and because it was fun.

jash.sh is a single Bash script that can execute a very small (and growing!) subset of JavaScript: variable declarations, console.log, arithmetic comparisons, simple string concatenation, if … else blocks, and zero-argument function definitions/calls. That’s it – no promises, no guarantees, definitely no production support.

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Quick Start

chmod +x jash.sh # make it executable
DEBUG=false ./jash.sh test.js # run the first demo (quiet mode)
DEBUG=true ./jash.sh hello2.js # run the second demo with verbose tracing

The DEBUG environment variable (default true) controls the flood of DEBUG: lines that show the script’s internal decision‑making.

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Demo Scripts

1 · test.js

// test.jsletx=10;lety=5;letz="Hello";letw="World";if(x>y){console.log(z+" "+w);console.log(x+y);}functionmyFunc(){leta=20;console.log(a);}myFunc();

Expected output

Hello World
15
20

2 · hello2.js

letgreeting="Hi";letname="Alice";lettimes=3;functionrepeatGreet(){leti=0;while(i<times){// not implemented yet → loop skippedconsole.log(greeting+", "+name+"!");i=i+1;}}if(times>1){console.log(greeting+", "+name+"! (once)");console.log(greeting+", "+name+"! (twice)");}else{console.log(greeting+", "+name+"!");}repeatGreet();

Expected output

Hi, Alice! (once)
Hi, Alice! (twice)

Because while loops aren’t implemented (yet!), repeatGreet() prints nothing – a neat smoke‑test for when loops eventually land.

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Turning the Knobs

• Enable/disable debug

  DEBUG=false ./jash.sh yourScript.js # quiet mode
  DEBUG=true ./jash.sh yourScript.js # very chatty

• Supported features: let/var, basic math/comparison, string concatenation with +, if … else, zero‑arg function calls.

• Unsupported (for now): loops (while, for), else if, arrays, objects, everything fancy.

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Why Bash?

• It was a light-hearted "what if" challenge.
• Bash’s crazy quoting rules make regex parsing surprisingly educational.
• It fits in a single shell script you can drop on almost any Unix box.
• Part of the "in roughly 100 lines of BASH" challange, created for myself, because sometimes, Perl, Python, Java, C, Golang is just not as "fun".

Is it a good idea? Absolutely not. Is it fun? Absolutely yes.

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Contributing / Extending

Pull requests welcome if you’d like to add a tiny new feature (loops are the next obvious milestone). Keep it under 200 lines, keep it POSIX-ish, and keep it playful.

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© 2025 — hack for laughs, not for mission-critical workloads.