underscore1.4.4读书笔记

###line:80 数字字符串转成数字的方便方式     
    //in chrome
    +'0.123'//0.123
    +'0.123e4'//1230
    +void 0//NaN
    +null//0
    +false//0
    +true//1
    +NaN//NaN
    
###line:80 判断一个变量是不是数字的hack
    var a = '1';
    a === +a//false
    var arr = ['1'];
    arr[0] === +arr[0];//false
    var b = 1;
    b === +b;//true

###line:143 JavaScript会提升变量声明。这意味着 var 表达式和 function 声明都将会被提升到当前作用域的顶部(无论var是否在代码块里面)
    _.reduceRight = _.foldr = function(obj, iterator, memo, context) {
        ...
        var length = obj.length;
        if (length !== +length) {
          var keys = _.keys(obj);
          length = keys.length;
        }
        each(obj, function(value, index, list) {
            //起初怀疑这里的keys是否没有被声明
          index = keys ? keys[--length] : --length;
          ...
        });
    };
    相当于
    _.reduceRight = _.foldr = function(obj, iterator, memo, context) {
        ...
        var length = obj.length;
        var keys;//提升到作用域顶部
        if (length !== +length) {
          keys = _.keys(obj);
          length = keys.length;
        }
        each(obj, function(value, index, list) {
          index = keys ? keys[--length] : --length;
          ...
        });
    };

###line:616 bind的实现要防止对构造函数做保护，当对一个构造函数使用bind，再调用的时候this是参数中的context，这时候有的初始化就不正常 
    //源代码:
    var ctor = function() {};

    _.bind = function(func, context) {
        var args, bound;
        ...
        args = slice.call(arguments, 2);
        return bound = function() {
        //这里为什么要做这个判断呢
        if (!(this instanceof bound)) 
            return func.apply(context, args.concat(slice.call(arguments)));
        ctor.prototype = func.prototype;
        var self = new ctor;
        ctor.prototype = null;
        var result = func.apply(self, args.concat(slice.call(arguments)));
        if (Object(result) === result) return result;
        return self;
        };
    };
    
    //假设代码变成这样
    _.bind = function(func, context) {
        var args, bound;
        ...
        args = slice.call(arguments, 2);
        return bound = function() {
        //这里为什么要做这个判断呢
        //if (!(this instanceof bound)) 
            return func.apply(context, args.concat(slice.call(arguments)));
        };
    };
    
    //案例代码
    function Person(name) {
        this.name = name;
    }
    var obj={name:'obj'}
    Person = _.bind(Person, obj);
    var p = new Person('the name');
    console.info(p.name);//==> undefined
    console.info(obj.name);//==> 'the name'
    //因为_.bind(Person, obj)之后,Person构造函数里面的this就是obj