prefix = (str, len = null) => {
	if (len === "undefine" || len === null) {
		return str[0];
	} else {
		return str.substring(0,len);
	}
}
console.log(prefix('this is a test'))
console.log(prefix('this is a test', 3))


extend = (...data) => {
    newData = {};
    data.forEach((val, Index) => {
	    newData = Object.assign({},newData,val);
    })
    return newData;
}

console.log(extend({a:10, b:20}, {a:30, c: 40}, {b: 50, c: 20, d: 100}))
console.log(extend({a:10, b:20}, {c:30, d: 40}))

isEven = function(x) {
	if (x%2 == 0) {
		return true;
	} else {
		return false;
	}
}

isOdd = function(x) {
	if (x%2 != 0) {
		return true;
	} else {
		return false;
	}
}

filter = (data, func) => {
	let newData = Array();
	data.forEach((val, index) => {
		if (func(val)) {
			newData.push(val);
		}
	})
	return newData;
}
console.log(filter([1,2,3,4,5],isEven));
console.log(filter([1,2,3,4,5],isOdd));