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Spectator supports ES2015+ by transpiling files with Babel. Any JavaScript feature included in the latest preset can be used in a toy problem. The tradeoff for this support is that error messages can sometimes be a bit more obscure and/or line numbers in the stack trace don't map precisely to their original line numbers.
If this is not an acceptable tradeoff, you can write only ES5 code create a spectator.json
file in the root of the problem directory that looks like this:
{
"babel": false
}