'functional' {a,b}to{b,a} implementation with no named functions or loops

cursed64.js

const btoa = input =>
	(output =>
		(length =>
			length % 3 ?
			output.slice(0, (length % 3) - 3) + "=".repeat(3 - (length % 3)) :
			output
		)(input.length)
	)(input.replace(/.{1,3}/g, chunk =>
		(f =>
			(x =>
				f((v, w) =>
					x(x)(v, w)
				)
			)(x =>
				f((v, w) =>
					x(x)(v, w)
				)
			)
		)(encodeFunc =>
			(byteTriple, index) => 
			index < 4 ?
			'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'[(byteTriple >> ((3 - index) * 6)) & 63] + encodeFunc(byteTriple, index + 1) :
			""
		)(chunk.split("").reduce((bitstring, byte, index) =>
			bitstring | byte.charCodeAt(0) << ((2 - index) * 8), 0
		), 0)
	));

const atob = encoded =>
	(encoded => {
		if (/^([a-zA-Z0-9+/]+=*)?$/.test(encoded = encoded.replace(/[\t\n\f\r ]+/g, "")) &&
			(encoded = encoded.replace(/=*$/, "")).length !== 1)
			return encoded.replace(/.{1,4}/g, chunk =>
				(output =>
					(length =>
						(length % 4) > 1 ?
						output.slice(0, -[2, 1][(length % 4) - 2]) :
						output
					)(chunk.length)
				)((f =>
					(x =>
						f((v, w) =>
							x(x)(v, w)
						)
					)(x =>
						f((v, w) =>
							x(x)(v, w)
						)
					)
				)(decodeFunc =>
					(byteTriple, index) => 
					index < 3 ?
					String.fromCharCode(byteTriple >> ((2 - index) * 8) & 0xFF) + decodeFunc(byteTriple, index + 1) :
					""
				)(chunk.split("").map(c =>
					'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'.indexOf(c)
				).reduce((bitstring, sextet, index) =>
					bitstring | sextet << ((3 - index) * 6), 0
				), 0)
			))
		else
			throw 'Incorrect encoded string';
	})(encoded);

why

why not