DOMString. This should be set to
— the length in bits of the key. If this is omitted the length of the key is equal to the length of the digest generated by the digest function you have chosen. Unless you have a good reason to use a different length, omit this property and use the default.
See the examples for
Web Cryptography API
The definition of 'SubtleCrypto.HmacKeyGenParams' in that specification.
Browsers that support the "HMAC" algorithm for the
method will support this type.