ProtParamJS is a JavaScript version of ProtParam, and ProtParam is a tool to analyze a protein from several kinds of point of view.
ExPASy defines ProtParam as follows:
ProtParam (References / Documentation) is a tool which allows the computation of various physical and chemical parameters for a given protein stored in Swiss-Prot or TrEMBL or for a user entered protein sequence. The computed parameters include the molecular weight, theoretical pI, amino acid composition, atomic composition, extinction coefficient, estimated half-life, instability index, aliphatic index and grand average of hydropathicity (GRAVY) (Disclaimer).
<script src="https://cdn.jsdelivr.net/npm/protparamjs@1.0.1/protparam.js"></script>
npm install protparamjs
protparam = new ProtParam('AQEEGGGAS')
You can apply ProtParamJS and compare several proteins on the web browser at https://kyu999.github.io/ProtParamJS/
