Replacing number based html entities with characters, either hex or decimal.

replaceNumericEntities.js

const NORMAL = 0;
const FOUNDAMP = 1;
const FOUNDHASH = 2;
const FOUNDX = 3;
const FOUNDHEX = 4;
const GOTENTITY = 5;
const FOUNDDEC = 6;

function replaceNumericEntities(input)
{
    const hexdigits = '0123456789abcdefABCDEF';
    const digits = '0123456789';
    let output = '';
    let entity = '';
    let isHex = false;
    let state = NORMAL;
    
    for(let c of input+' ')
    {
  	switch(state)
	{
	    case NORMAL:
	    if(c === '&')
	    {
		state = FOUNDAMP;
		entity += c;
	    }
	    else
	    {
		output += c;
	    }
	    break;
	    case FOUNDAMP:
	    if(c === '#')
	    {
		state = FOUNDHASH;
		entity += c
	    }
	    else
	    {
		({output, entity, state} = reset(c, entity, output));
	    }
	    break;
	    case FOUNDHASH:
	    {
		if(c === 'x')
		{
		    state = FOUNDX;
		    isHex = true;
		    entity += c;
		}
		else if(digits.indexOf(c) === -1)
		{
		    ({output, entity, state} = reset(c, entity, output));
		}
		else
		{
		    state = FOUNDDEC;
		    isHex = false;
		    entity += c;
		}
	    }
	    break;
	    case FOUNDX:
	    {
		if(hexdigits.indexOf(c) === -1)
		{
		    ({output, entity, state} = reset(c, entity, output));
		}
		else
		{
		    state = FOUNDHEX;
		    entity += c;
		}
	    }
	    break;
	    case FOUNDHEX:
	    {
		if(c === ';')
		{
		    state = GOTENTITY;
		}
		else if(hexdigits.indexOf(c) === -1)
		{
		    ({output, entity, state} = reset(c, entity, output));
		}
		else
		{
		    state = FOUNDHEX;
		    entity += c;
		}
	    }
	    break;
	    case GOTENTITY:
	    {
		if((isHex && withinHexRange(entity)) || (!isHex && withinDecRange(entity)))
		{
		    output += getCharacter(entity, isHex);
		}
		else
		{
		    output += entity;
		}
		({output, entity, state} = reset(c, '', output));		
	    }
	    break;
	    case FOUNDDEC:
	    {
		if (c === ';')
		{
		    state = GOTENTITY;
		}
		else if(digits.indexOf(c) === -1)
		{
		    ({output, entity, state} = reset(c, entity, output));
		}
		else
		{
		    state = FOUNDDEC;
		    entity += c;
		}
	    }
	    break;
	}
    }

    return output.slice(0, -1);
}

function reset(c, entity, output)
{
    let state = 0;
    output += entity;
    entity = '';
    if(c === '&')
    {
	state = FOUNDAMP;
	entity = c
    }
    else
    {
	state = NORMAL;
	output += c;
    }		    

    return {output: output, entity: entity, state: state};
}

function withinHexRange(entity)
{
    let value = Number.parseInt(entity.slice(3), 16);
    return value <= 0x10ffff;
}

function withinDecRange(entity)
{
    let value = Number.parseInt(entity.slice(2), 10);
    return value <= 0x10ffff;
}

function getCharacter(entity, isHex)
{
    let value = 0;
    if(isHex)
    {
	value = Number.parseInt(entity.slice(3), 16);
    }
    else
    {
	value = Number.parseInt(entity.slice(2), 10);
    }

    let cha = String.fromCodePoint(value);
    return cha;
}