代碼如下:
var each = _.each = _.forEach = function(obj, iterator, context) { 
if (obj == null) return; 
if (nativeForEach && obj.forEach === nativeForEach) { 
obj.forEach(iterator, context); 
} else if (obj.length === +obj.length) { 
for (var i = 0, l = obj.length; i < l; i++) { 
if (iterator.call(context, obj[i], i, obj) === breaker) return; 
} 
} else { 
for (var key in obj) { 
if (_.has(obj, key)) { 
if (iterator.call(context, obj[key], key, obj) === breaker) return; 
} 
} 
} 
};

if (obj.length === +obj.length)

if (typeof obj.length === 'number')

代碼如下：
function isNumber1(a){ 
return typeof a === 'number' 
}

代碼如下：
function isNumber2(a) { 
return Object.prototype.toString.call(a) === '[object Number]' 
}

代碼如下：
function isNumber3(a){ 
return a === +a 
}

代碼如下:
var arr = ['1', true, false, undefined, null, {}, [], 1] 
for (var i=0; i<arr.length; i++) { 
console.log(isNumber3(arr[i])) 
}