test.html

<script src='https://unpkg.com/simple-statistics@4.1.0/dist/simple-statistics.min.js'></script>
<script>
function errorFunction(x/*: number */)/*: number */ {
    var t = 1 / (1 + 0.5 * Math.abs(x));
    var tau = t * Math.exp(-Math.pow(x, 2) -
        1.26551223 +
        1.00002368 * t +
        0.37409196 * Math.pow(t, 2) +
        0.09678418 * Math.pow(t, 3) -
        0.18628806 * Math.pow(t, 4) +
        0.27886807 * Math.pow(t, 5) -
        1.13520398 * Math.pow(t, 6) +
        1.48851587 * Math.pow(t, 7) -
        0.82215223 * Math.pow(t, 8) +
        0.17087277 * Math.pow(t, 9));
    if (x >= 0) {
        return 1 - tau;
    } else {
        return tau - 1;
    }
}

var	p0	= .2080384346694663001443843411e7;
var	p1	= .3028697169744036299076048876e5;
var	p2	= .6061485330061080841615584556e2;
var	q0	= .6002720360238832528230907598e7;
var	q1	= .3277251518082914423057964422e6;
var	q2	= .1749287689093076403844945335e4;
var	log2e	= 1.4426950408889634073599247;
var	sqrt2	= 1.4142135623730950488016887;
var	maxf	= 10000;

function exp(arg) {
	if(arg == 0.)
		return(1.);
	if(arg < -maxf)
		return(0.);
	if(arg > maxf) {
		errno = ERANGE;
		return(HUGE);
	}
	arg *= log2e;
	ent = Math.floor(arg);
	fract = (arg-ent) - 0.5;
	xsq = fract*fract;
	temp1 = ((p2*xsq+p1)*xsq+p0)*fract;
	temp2 = ((1.0*xsq+q2)*xsq+q1)*xsq + q0;
	return (sqrt2*(temp2+temp1)/(temp2-temp1)) * Math.pow(2, ent);
}

fetch('./build/error_function.wasm').then(response =>
  response.arrayBuffer()
).then(bytes =>
  WebAssembly.instantiate(bytes)
).then(results => {
  for (var i = -1; i <= 1; i += 0.001) {
    var wasmResult = results.instance.exports.errorFunction(i);
    var result = errorFunction(i);
    console.assert(wasmResult.toFixed(7) == result.toFixed(7), `wasm result ${wasmResult} != ${result}`);
  }

  console.time('wasm errorFunction');
  var accum = 0;
  for (var i = -2; i <= 2; i += 0.001) {
    accum += results.instance.exports.errorFunction(i);
  }
  console.timeEnd('wasm errorFunction');

  console.time('errorFunction');
  var accum = 0;
  for (var i = -2; i <= 2; i += 0.001) {
    accum += errorFunction(i);
  }
  console.timeEnd('errorFunction');
});
</script>