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JS Compatibility Gaps

Gaps between JavaScript and Tish that may require workarounds.

This page documents gaps discovered when porting JavaScript patterns to Tish. Resolved gaps are listed for reference.

Test backends / deployment target matrix (2025-03 audit)

Integration tests run per target. Results as of audit:

TargetTestStatusNotes
Native (Rust)test_mvp_programs_interpreter_vs_nativePass36 files; requires cargo (native compile)
Cranelifttest_mvp_programs_interpreter_vs_craneliftPass17 curated files; subset only (see Native backend)
WASItest_mvp_programs_interpreter_vs_wasiPassSame 17 curated; requires wasmtime, wasm32-wasip1
JStest_mvp_programs_interpreter_vs_jsPass36 files; requires Node.js

Cranelift/WASI curated list (17 files): fn_any, strict_equality, switch, do_while, typeof, try_catch, json, math, builtins, uri, inc_dec, exponentiation, void, rest_params, arrow_functions, array_methods, types.

Excluded from Cranelift/WASI (stack-underflow/scope bugs): nested_loops, objects, optional_chaining, spread, template_literals, higher_order_methods, destructuring, and others.

The bytecode VM (used by tish run --backend vm, Cranelift, and WASI) has known gaps. For example, optional_chaining.tish runs correctly with the interpreter (--backend interp) and Node, but with the VM it may error. Use the repo script ./scripts/run_parity_compare.sh (or just parity) in the tish repo to compare outputs across runtimes (interp, vm, rust, cranelift, wasi, node).

Resolved (as of 2025-03)

Number.prototype.toFixed(digits?) ✓

Tish now supports n.toFixed(digits) for formatting numbers with fixed decimal places (0–20). Returns a string.

console.log((3.14159).toFixed(2))  // "3.14"
console.log("$" + total.toFixed(2))  // currency formatting

See Numbers for details.

indexOf(search, fromIndex?) ✓

Tish now supports the optional fromIndex parameter: s.indexOf("x", 5).

String index semantics ✓

All string methods now use character (Unicode scalar) indices consistently: indexOf, slice, substring, charAt, length. No more byte-offset confusion with multi-byte UTF-8.

RegExp exec() result index property ✓

exec() returns an array-like object with [0], [1], ... and "index" (match start as character offset). Access via m["index"].

let m = RegExp("(\\d+)").exec("abc123")
console.log(m[0])       // "123"
console.log(m["index"]) // 3

String.prototype.replace with function replacer ✓

When the search value is a RegExp and the replacement is a function, the function is invoked for each match. Callback receives (match, g1, g2, ..., index, fullString).

let result = "hello 123 world 456".replace(RegExp("(\\d+)", "g"), (match, g1) => "[" + g1 + "]")
// result: "hello [123] world [456]"

Supported in both interpreter and compiled modes.

See also