根据underscore精简的函数（集合篇）
这里将underscore.js内的函数进行精简，指在帮助理解或者快速回忆起underscore.js的原理，更深入的了解函数式编程。
本文基于underscore.js1.8.3版本，参考手册为
http://www.css88.com/doc/underscore/
_.each
function each(arr,iteratee){
for(i=0,length=arr.length;ireturn arr;
}
var a=[1,2,3];
var b=each(a,function(value){
console.log(value);
});
console.log(b); 运行结果
1
2
3_.map
function map(arr,iteratee){
var length=arr.length,
results=Array(length);
for(var index=0;indexreturn results;
}
var array=[12,5,2,4,6,13,12];
var a=map(array,function(value){
return value*2;
}); 运行结果
[ 24, 10, 4, 8, 12, 26, 24 ]_.reduce
var createReduce=function(dir){
var reducer=function(arr,iteratee,memo,initial){
var length=arr.length,
index=dir>0?0:length-1;
if(!initial){
memo=arr[index];
index+=dir;
}
for(;index>=0&&indexreturn memo;
};
return function(arr,iteratee,memo,context){
var initial=arguments.length>=3;
return reducer(arr,iteratee,memo,initial);
};
};
reduce=createReduce(1);
var sum = reduce([1, 2, 3], function(memo, num){ return memo + num; }, 0);
console.log(sum); 运行结果
6_.reduceRight
var createReduce=function(dir){
var reducer=function(arr,iteratee,memo,initial){
var length=arr.length,
index=dir>0?0:length-1;
if(!initial){
memo=arr[index];
index+=dir;
}
for(;index>=0&&indexreturn memo;
};
return function(arr,iteratee,memo,context){
var initial=arguments.length>=3;
return reducer(arr,iteratee,memo,initial);
};
};
reduceRight=createReduce(-1);
var list = [[0, 1], [2, 3], [4, 5]];
var flat = reduceRight(list, function(a, b) { return a.concat(b); }, []);
console.log(flat); 运行结果
[ 4, 5, 2, 3, 0, 1 ]_.filter
function filter(arr,predicate){
var results=[];
each (arr,function(value,index,list){
if (predicate(value,index,list))results.push(value);
});
return results;
}
var array=[12,5,2,4,6,13,12];
var morethan10=filter(array,function(elem){
return elem>10;
});
console.log(morethan10);运行结果
[ 12, 13, 12 ]_.reject
function negate(predicate){
return function(){
return !predicate.apply(this,arguments);
}
}
var reject=function(arr,predicate){
return filter(arr,negate(predicate));
};
var array=[12,5,2,4,6,13,12];
var lessthan10=reject(array,function(elem){
return elem>10;
});
console.log(lessthan10);运行结果
[ 5, 2, 4, 6 ]_.every
function every(arr,predicate){
for(let i=0;iif(!predicate(arr[i],i,arr)) return false;
}
return true;
}
var array=[12,5,2,4,6,13,12];
var lessthan10=every(array,function(elem){
return elem>1;
});
console.log(lessthan10); 运行结果
true_.some
function some(arr,predicate){
for(let i=0;iif(predicate(arr[i],i,arr)) return true;
}
return false;
}
var array = [12,50,2,4,6,13,12];
var b= some(array, function(elem){
return elem<10;
});
console.log(b); 运行结果
true_.indexOf
var createIndexFinder=function(dir){
return function(array,item,idx){
var i=0,length=array.length;
if(dir>0){
idx=0;
i=idx>=0?idx:Math.max(idx+length,i);
}else{
idx=length;
length=idx>=0?Math.min(idx+1,length):idx+length+1;
}
for(idx=dir>0?i:length-1;idx>=0&&idxif(array[idx]===item) return idx;
}
return -1;
};
};
indexOf=createIndexFinder(1);
var a=indexOf([1,6, 2, 3,4,5], 2);
console.log(a); 运行结果
2_.lastIndexOf
var createIndexFinder=function(dir){
return function(array,item,idx){
var i=0,length=array.length;
if(dir>0){
idx=0;
i=idx>=0?idx:Math.max(idx+length,i);
}else{
idx=length;
length=idx>=0?Math.min(idx+1,length):idx+length+1;
}
for(idx=dir>0?i:length-1;idx>=0&&idxif(array[idx]===item) return idx;
}
return -1;
};
};
lastIndexOf=createIndexFinder(-1);
var b=lastIndexOf([1, 3, 1, 2, 3], 2);
console.log(b); 运行结果
3_.sortedIndex
var sortedIndex=function(arr,value){
var low=0,high=arr.length;
while(lowvar mid=Math.floor((low+high)/2);
if(arr[mid]1;
else
high=mid;
}
return low;
};
var a=sortedIndex([10, 20, 30, 40, 50], 35);
console.log(a); 运行结果
3_.findIndex
var createPredicateIndexFinder=function(dir){
return function(arr,predicate){
var length=arr.length;
var index=dir>0?0:length-1;
for(;index>=0&&indexif(predicate(arr[index],index,array)) return index;
}
return -1;
};
};
findIndex=createPredicateIndexFinder(1);
var a=findIndex([4, 6, 8, 12], function(value){
return value===8;
});
console.log(a);
运行结果
2_.findLastIndex
var createPredicateIndexFinder=function(dir){
return function(arr,predicate){
var length=arr.length;
var index=dir>0?0:length-1;
for(;index>=0&&indexif(predicate(arr[index],index,array)) return index;
}
return -1;
};
};
findLastIndex=createPredicateIndexFinder(-1);
var a=findLastIndex([4, 6, 8, 12, 5, 12], function(value){
return value===12;
});
console.log(a); 运行结果
5_.findKey
var findKey=function(obj,predicate){
var keys=Object.keys(obj);
for(var i=0,length=keys.length;iif(predicate(obj[key],key,obj)) return key;
}
};
var student = {
name: 'wxj',
age: 18
};
var a=findKey(student, function(value, key, obj) {
return value === 18;
});
console.log(a); 运行结果
age_.where
function where(table,attrs){
//遍历tale
return table.filter(function (elem) {
//判断当前遍历到的对象是都符合条件
return Object.keys(attrs).every(function(attr){
return elem[attr]===attrs[attr];
});
});
}
var users = [
{name: 'wxj', age: 18, sex: 'male'},
{name: 'zxy', age: 18, sex: 'male'},
{name: 'zhangsan', age: 14, sex: 'famale'}
];
var a = where(users, {age: 18, sex: 'male'});
console.log(a);运行结果
[ { name: 'wxj', age: 18, sex: 'male' },
{ name: 'zxy', age: 18, sex: 'male' } ]_.contains
var contains=function(arr,item,fromIndex){
return arr.indexOf(item,fromIndex)>=0;
};
var a =contains([1, 2, 3, 4, 5], 2);
console.log(a);运行结果
true_.sample
function sample(set,n){
var length=set.length;
var i;
var sample=set.map(function(elem){
return elem;
});
if (n==void 0){
n=1;
}
var last=length-1;
for(i=0;ivar randIndex=Math.floor(Math.random()*(last-i+1)+i);
var tmp=sample[i];
sample[i]=sample[randIndex];
sample[randIndex]=tmp;
}
return sample.slice(0,n);
}
var set=[1,2,3,4,5,6,7,8,9,10];
var a=sample(set,4);
console.log(a); 运行结果
[ 8, 9, 2, 6 ]_.group
var group=function(behavior){
return function(arr,iteratee){
var result={};
each(arr,function(value,index){
var key=iteratee(value,index,arr);
behavior(result,value,key);
});
return result;
};
};_.groupBy
var has=function(obj, key) {
return obj != null && hasOwnProperty.call(obj, key);
};
var groupBy=group(function(result,value,key){
if (has(result, key))
result[key].push(value);
else result[key] = [value];
});
var a=groupBy([1.3, 2.1, 2.4], function(num){ return Math.floor(num); });
console.log(a);运行结果
{ '1': [ 1.3 ], '2': [ 2.1, 2.4 ] }