DOMでユニークアクセスできるセレクタ作る

uniqueSelector.js

Object.defineProperty(Element.prototype, "index", {
	get: function(){
		return this.parentElement ?
			[].indexOf.call(this.parentElement.children, this)
		:
			-1
	}
})

function createUniqueSelector(elem){
	var nths = []
	for(;!canUniqueAccess(elem);elem = elem.parentElement){
		nths.push(elem.index)
	}
	return nths.reduceRight((a,b) => a + `>*:nth-child(${b+1})`, getUniqueSelector(elem))
}

function canUniqueAccess(elem){
	return elem.id || elem === document.documentElement
}

function getUniqueSelector(elem){
	return elem.id ?
		"#" + elem.id
	:
		elem === document.documentElement ?
			":root"
		:
			false
}