Boolean attributes should not be special in xmlMode

[rau8344]

For HTML boolean attributes cheerio's attr() method behavior is both different from jquery and undocumented:

• jquery documented behavior is to get the actual value of attributes (except for values matching the attribute name in version 1.5 and earlier, which gets true)
  • This seems to match actual behavior, as per the following unit test, run against jquery@3.4.1, jquery@1.6, and jquery@1.5
• cheerio documented behavior refers to jquery documentation, without mentioning any edge cases attributes.
  • Actual behavior gets attribute name for boolean attributes, as per the following unit test using xmlMode.

    it("jquery reads boolean attributes as-is", () => {
        const text = `<Item name="a" checked="a" disabled="a" required="a" selected="a" />`;
        const xmlJ = jQuery(jQuery.parseXML(text));
        const rootJ = xmlJ.children();
        expect(rootJ.attr("name")).toBe("a");
        expect(rootJ.attr("checked")).toBe("a");
        expect(rootJ.attr("disabled")).toBe("a");
        expect(rootJ.attr("selected")).toBe("a");
        expect(rootJ.attr("required")).toBe("a");
    });

    it("cheerio gets the name of defined boolean attributes instead of value", () => {
        const options = { xmlMode: true };
        const text = `<Item name="a" checked="a" disabled="a" required="a" selected="a" />`;
        const xml = cheerio.load(text, options);
        const root = xml.root().children();
        expect(root.attr("name")).toBe("a");
        expect(root.attr("checked")).toBe("checked");
        expect(root.attr("disabled")).toBe("disabled");
        expect(root.attr("selected")).toBe("selected");
        expect(root.attr("required")).toBe("required");
    });

This behavior was apparently intentional, and commented in the source for getAttr() https://cheerio.js.org/lib_api_attributes.js.html, and made for made for #570. Stated reason was "jquery behaviour is to return the attrribute name when present", but I cannot find any version of jquery where that is the actual behavior. This may have related behavior for attribute setting, though I have not tested that.

Suggest one of the following:

1. (Breaking change) Change attr() to always match documented jquery behavior.
2. Change attr() to conditionally match documented jquery behavior if certain options are provided to cheerio.load() (e.g. for xmlMode)
3. Change cheerio documentation to match the actual behavior.
4. Provide additional API methods/overloads to allow both old and new behavior. (Could be confusing; would definitely require documentation)

[fb55]

Very interesting case. It seems like this only applies to XML inside of jQuery — HTML actually shows the behavior described in #570.

@rau8344 My proposed solution would be to change the behavior in XML mode. How do you feel about this?

[rau8344]

@fb55 That makes sense to me.