#JavaScript (ES6), <s> 21 </s> 20 bytes Takes input as `(a)(b)`. <!-- language-all: lang-javascript --> a=>b=>[a.length,a]+b [Try it online!](https://tio.run/##bcq7DYAgEADQ3jGoICobwCLG4vhrLpwBouNjaI31eyfcUG05rrZmcr4H1UFpo/QGEn2OLS2wz6ZbypXQS6TIA2fJIxITnD1U0DEhpk@Agcb@krEDB/UX "JavaScript (Node.js) – Try It Online") Returns the length of \$a\$, followed by a comma, followed by the concatenation of \$a\$ and \$b\$.