Wast bootstrap under data:text/html,<textarea%20onkeyup="out.innerText=eval(this.value)"></textarea><pre%20id=out>

code.js

/*
- panes: input | compiler | wast | runtime | output
TODO
- snapshot button that saves json of [date input compiler runtime] to localStorage
- timestamp buttons to cycle through old versions - also sth with diffs?
- <details> on  "foo =\n", w/ collapse back if backspace on beginning
*/
document.body.querySelector('script') || document.body.appendChild(
  document.createRange().createContextualFragment(
    '<script src=https://cdn.rawgit.com/WebAssembly/wabt/aae5a4b7/demo/libwabt.js>'
) )
document.body.querySelector('script').onload = ()=>
  {out.innerText = eval(document.querySelector('textarea').value)}
mnt = document.body.querySelector('div')
mnt && mnt.remove()
document.body.appendChild(mnt = document.createElement('div'))
//out.innerText=''; (new Date)+"\n"+document.body.outerHTML
mnt.innerHTML = `
  <pre id=e_inp contenteditable style=background:#efe></pre>
  <pre id=e_comp contenteditable style=display:inline-block;background:#eff></pre>
  <pre id=e_wast style=display:inline-block;vertical-align:top;background:#eee></pre>
  <pre id=e_werr style=display:inline-block;vertical-align:top;background:#fee></pre>
  <pre id=e_rt contenteditable style=background:#eff></pre>
  <pre id=e_out></pre>
`
e_inp.innerText = "42"
e_comp.innerText = `(a)=>\`(module
  (func $main (result i32)
    i32.const \${a}
  )
  (export "main" (func $main))
)\``

run = () => {}
compile = ()=>{
  var astModule
  e_wast.innerText = ""
  e_werr.innerText = ""
  try {
    this.wasmModule = null
    e_wast.innerText = eval(e_comp.innerText)(e_inp.innerText)
    astModule = wabt.parseWat('compiled.wat',e_wast.innerText)
    astModule.resolveNames();
    astModule.validate();
    var binaryOutput = astModule.toBinary({write_debug_names:true})
    this.wasmModule = new WebAssembly.Module(binaryOutput.buffer)
    run()
  } catch(err) {
    e_werr.innerText = err
  } finally {
    astModule && astModule.destroy()
  }
}
compile()
e_inp.onkeyup = compile
e_comp.onkeyup = compile

e_rt.innerText = `(module)=> {
   const instance = new WebAssembly.Instance(module, {})
   return instance.exports.main()
}`
run = ()=> {
  try {
    e_out.innerText = eval(e_rt.innerText)(this.wasmModule)
    e_out.style.background = "#eee"
  } catch(_res) {
    e_out.innerText = _res
    e_out.style.background = "#fee"
  }
}
e_rt.onkeyup = run
run()