gecko-dev/dom/base/test/unit/test_serializers_entities.js

const encoders = {
    xml: (doc) => {
	let enc = Cu.createDocumentEncoder("text/xml");
	enc.init(doc, "text/xml", Ci.nsIDocumentEncoder.OutputLFLineBreak);
	return enc;
    },
    html: (doc) => {
	let enc = Cu.createDocumentEncoder("text/html");
	enc.init(doc, "text/html", Ci.nsIDocumentEncoder.OutputLFLineBreak);
	return enc;
    },
    htmlBasic: (doc) => {
	let enc = Cu.createDocumentEncoder("text/html");
	enc.init(doc, "text/html",
		 Ci.nsIDocumentEncoder.OutputEncodeBasicEntities |
		 Ci.nsIDocumentEncoder.OutputLFLineBreak);
	return enc;
    },
    xhtml: (doc) => {
	let enc = Cu.createDocumentEncoder("application/xhtml+xml");
	enc.init(doc, "application/xhtml+xml",
		 Ci.nsIDocumentEncoder.OutputLFLineBreak);
	return enc;
    },
};

// Which characters should we encode as entities?  It depends on the serializer.
const encodeAll = { html: true, htmlBasic: true, xhtml: true, xml: true };
const encodeHTMLBasic = { html: false, htmlBasic: true, xhtml: false, xml: false };
const encodeXML = { html: false, htmlBasic: false, xhtml: true, xml: true };
const encodeNone = { html: false, htmlBasic: false, xhtml: false, xml: false };
const encodingInfoMap = new Map([
    // Basic sanity chars '<', '>', '"', '&' get encoded in all cases.
    [ '<',  encodeAll ],
    [ '>',  encodeAll ],
    [ '&',  encodeAll ],
    // nbsp is only encoded with the HTML encoder when encoding basic entities.
    [ '\xA0', encodeHTMLBasic ],
]);

const encodingMap = new Map([
    [ '<', '&lt;' ],
    [ '>', '&gt;' ],
    [ '&', '&amp;' ],
    // nbsp is only encoded with the HTML encoder when encoding basic entities.
    [ '\xA0', '&nbsp;' ],
]);

function encodingInfoForChar(c) {
    var info = encodingInfoMap.get(c);
    if (info) {
	return info;
    }
    return encodeNone;
}

function encodingForChar(c, type) {
    var info = encodingInfoForChar(c);
    if (!info[type]) {
	return c;
    }
    return encodingMap.get(c);
}

const doc = new DOMParser().parseFromString("<root></root>", "text/xml")
const root = doc.documentElement;
for (let i = 0; i < 255; ++i) {
    let el = doc.createElement("span");
    el.textContent = String.fromCharCode(i);
    root.appendChild(el);
}
for (let type of ["xml", "xhtml", "htmlBasic", "html"]) {
    let str = encoders[type](doc).encodeToString();
    const prefix = "<root><span>";
    const suffix = "</span></root>";
    Assert.ok(str.startsWith(prefix), `${type} serialization starts correctly`);
    Assert.ok(str.endsWith(suffix), `${type} serialization ends correctly`);
    str = str.substring(prefix.length, str.length - suffix.length);
    let encodings = str.split("</span><span>");
    for (let i = 0; i < 255; ++i) {
	let c = String.fromCharCode(i);
	Assert.equal(encodingForChar(c, type), encodings[i],
		     `${type} encoding of char ${i} is correct`);
    }
}