aes-gcm-webcrypto.js

//https://github.com/diafygi/webcrypto-examples
window.crypto.subtle.encrypt(
    {
        name: "AES-GCM",

        //Don't re-use initialization vectors!
        //Always generate a new iv every time your encrypt!
        //Recommended to use 12 bytes length
        iv: window.crypto.getRandomValues(new Uint8Array(12)),

        //Additional authentication data (optional)
        additionalData: ArrayBuffer,

        //Tag length (optional)
        tagLength: 128, //can be 32, 64, 96, 104, 112, 120 or 128 (default)
    },
    key, //from generateKey or importKey above
    data //ArrayBuffer of data you want to encrypt
)
.then(function(encrypted){
    //returns an ArrayBuffer containing the encrypted data
    console.log(new Uint8Array(encrypted));
})
.catch(function(err){
    console.error(err);
});