Kata

7.js

(() => {

    // Add max to the Array prototype chain
    Array.prototype.max = function () {
        // I know they say the array will have at least one element, but that can't be guaranteed IMHO
        const length = this.length;
        if (length === 0) {
            return Number.NaN;
        }

        let max = this[0];
        for (let i = 0; i < length; i++) { // i = 0 is done on purpose and not 1, to check if it's a valid number =)
            let value = this[i];
            if (Object.prototype.toString.call(value) !== '[object Number]') {

                // Lets be explicit with parsing as an integer and not leaving it up to the engine
                value = Number.parseInt(value);

                // Because NaN !== Number.NaN, so we must use isNaN()
                if (Number.isNaN(value)) {
                    return Number.NaN;
                }

            }

            // Strings that are numbers will be coerced by the JavaScript engine. Good 'ol JavaScript, but I parsed above
            // to be explicit
            if (value > max) {
                max = value;
            }
        }

        return max;
    };

    // Tests

    console.log([2,5,1,3].max()); // 5
    console.log([1,2,3,8,4,9,7,42,99].max()); // 99
    console.log([2,'5',1,3].max()); // 5
    console.log([2,5,1,'ab'].max()); // NaN

})(); // IIFE, for creating an anonymous scope and not polluting the global scope