Returns the head of a list.

Check if arr is truthy and has a length property, use arr[0] if possible to return the first element, otherwise return undefined.

代码片段

const head = arr => (arr && arr.length ? arr[0] : undefined);

使用样例

head([1, 2, 3]); // 1
head([]); // undefined
head(null); // undefined
head(undefined); // undefined