what feature in this javacript

i want to learn naive bayes classifier from https://github.com/miguelmota/bayes-classifier and i read the code like this

for (var feature in this.features) {
features.push(Number(!!~doc.indexOf(feature)));

} i dont know where come from feature and this.feature, i have try to read data but i cant read what mean feature. and !!~ what mean anybody can help me guys.

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