Easy version of the board game mastermind

Mastermind.js

class Mastermind {
    constructor() {
        this.length = 4
        this.chars = ['0', '1', '2', '3', '4', '5']
    }

    generate() {
        this.key = []
        for (var i = 0; i < this.length; i++)
            this.key.push(this.chars[(Math.random() * this.chars.length) | 0])
    }

    guess(attempt) {
        if (attempt.length != this.length)
            return 'INVALID ATTEMPT'

        let correct = 0, almost = 0, key2 = this.key.slice(0), idx

        for (var i = 0; i < this.length; i++) {
            if (key2[i] === attempt[i])
                ++correct
            else if ((idx = key2.indexOf(attempt[i])) !== -1 
                    && key2[idx] !== attempt[idx]) {
                key2[idx] = ' '
                ++almost
            }
        }

        if (correct === this.length)
            return 'CORRECT'

        return correct + ' CORRECT, ' + almost + ' ALMOST'
    }
}