总结：
    0 '' undefined null属于消极的，|| 中会往后走，二元运算时，就直接取后者了
    [] {} 属于积极的,||时，取其

    2. let a = undefined;  // let a = null;
    4. let m = a || '1';
    5. let n = a || '';
    7. let m1 = '' || '2';
    8. let n1 = '' || a;
    10. let m2 = [] || '2';
    11. let n2 = [] || a;
    13. let m3 = 0 || '2';
    14. let n3 = 0 || a;

    结果：
    d85db6340fe482bee887c7ceb9b222b.png