DOM API
    async/defer
    interpreted, just-in-time compiling
    dynamic, loosely typed
    scripts can be put internal, external, inline (bad).

    1. jquery
      w3 tutorial
    2. underscore

    In fact, there is no “constructor” function, there is only “constructive call”.
    Inheritance is essentially copying.
    In javascript, there are mainly 7 types:

    • number
    • boolean
    • string
    • null
    • undefined
    • symbol
    • object

    copy value vs. copy reference:
    For object (including function, array), assignments pass reference; for others (scalar types), assignments pass values.
    variable vs. value
    javacript中变量没有类型，只有值才有类型。
    给变量赋值只会改变该变量的指向，不会改变原指向的值。
    undefined vs. undeclared (Reference Error) typeof <undeclared>有安全保护

    1. if (typeof DEBUG != 'undefined'){
    2. //...
    3. }

    string type is inmutable.
    undefined 指从未赋值
    null 指曾赋过值，但是当前值为空 null是关键字，不能作为identifier, 而undefined可以
    NaN 是number类型, 唯一非自反的值 （NaN !=NaN)