How to properly decode a SCALE-encoded string using polkadot-js?

Question

Using @polkadot-js/api, one can encode a string aabbcc into 0x18616162626363

> u8aToHex(api.createType('String', "aabbcc").toU8a())
"0x18616162626363"

I know that SCALE encoding adds the length info at the beginning of the encoded result, so in total the output is 7 bytes instead of 6.

However, when I try decode the output back to a string, the leading byte is treated as part of the string instead of being stripped out.

> api.createType('String', "0x18616162626363").toString()
"\x18aabbcc"

The expected result should be aabbcc without the initial \x18 byte.

So, how to get rid of the leading byte when decoding a SCALE-encoded string?

Answer 1

You want to follow the same steps you took to generate the hex to get back.

In your example generating the hex you do:

1. String: "aabbcc"
2. toU8a: Uint8Array(7) [ 24, 97, 97, 98, 98, 99, 99 ]
3. toHex: 0x18616162626363

On the way back, just do the same:

1. Hex string: "0x18616162626363"
2. toU8a: hexToU8a("0x18616162626363")
3. createType

So:

api.createType("String", hexToU8a("0x18616162626363")).toString()
> "aabbcc"