1、ES6语法篇
//ES6语法
// spread扩展符(...),
2、js简洁写法
//js转换成bool值的六个false
!0 = true, !undefined = true
!'' = true, !NaN = true
! null = true , !false = true
//1.丢弃小数部分,保留整数部分
parseInt(3.5); //3
parseInt(-3.5); //-3
//同上
~~3.5 //3
~~-3.5 //-3
//同上
3.5 | 0 //3
-3.5 | 0 //-3
//2.向上取整,有小数就加1
console.log(Math.ceil(3.5)); //4
console.log(Math.ceil(-3.5)); //-3
//3.四舍五入
console.log(Math.round(3.5) ); //4
console.log(Math.round(-3.5) ); //-3
//4.向下取整
console.log(Math.floor(3.5) ); //3
console.log(Math.floor(-3.5) ); //-4
//5.正常处理
console.log(parseFloat('3.5') ); //3.5
console.log(parseFloat('3.5rf') ); //3.5
console.log(parseFloat('-3.5') ); //-3.5
console.log(parseFloat('-3.5rf') ); //-3.5
//6.if语句写法
var a = 1;
var b = 1;
if(a === 1){
a = 'a等于1';
}else if(a === 2){
a = 'a等于2';
}else if(a === 3){
a = 'a等于3';
}
可以更改为:(a === 1) && (a = 'a等于1') || (a === 2) && (a = 'a等于2') || (a === 3) && (a = 'a等于3')
以上写法效果等同
3、js数组篇
//1、数组元素的新增
var arr = [];
console.log(arr.push(1)); //数组直接push,返回的是数组长度
arr.push(1);
console.log(arr); //push之后,数组已经变成:[1]
//2、数组元素的删除
var arr1 = [1 , 2];
console.log(arr1.splice(1, 1)); //直接删除,返回的是删除掉的元素
arr1.splice(1, 1); //先删除再打印,是删除后剩余的结果splice(index, len)分别为起始下标和要删除的长度
console.log(arr1);
//3、拼接两个数组
//方式一
var a = [1,2,3];
var b = [...a,2]; // b = [1,2,3,2]
//方式二
var b = a.concat(b)
//4、字符串转数组
var c = [...'abc']; // c = ['a','b','c']
//5、深拷贝数组的几种方式
var b = [1,2];
var a = b;
b[0] = 2;
console.log(a,b); //a和b都变成[2,2]
//方式一:
var b = [1,2];
var a = [...b];
b[0] = 2;
console.log(a,b); // a = [1,2]; b = [2,2]
//方式二:
var b = [1,2];
var a = JSON.parse(JSON.stringify(b)); //如果数组元素为对象,且对象有方法会导致方法丢失
console.log(a,b); // a = [1,2]; b = [2,2]
//方式三:
var b = [1,2];
var a = b.concat();
b[0] = 2;
console.log(a,b); // a = [1,2]; b = [2,2]
//方式四
var b = [1,2];
var [...a] = b;
b[0] = 2;
console.log(a,b); // a = [1,2]; b = [2,2]
//6、拷贝一个对象
var a = {
a : 1,
b : true,
c : {
c1 : 'ci'
},
d : [1,2],
e : function () {
console.log('123')
}
};
//方式一:
var b = {...a};
a.a = 3;
console.log(a,b); //改变a.a不会影响b.a的值
//方式二: 注意,次方式只能拷贝数据,如果原对象有方法,会直接丢失,
var b = JSON.parse(JSON.stringify(a));
//7、 数组去重,js filter()方法ES5已存在,ES6之后里面可以嵌套箭头函数了,
// 方式一:
var arrUniqe = [1,2,3,2,1,'a','b','a'];
Array.prototype.unique = function() {
var a = this.concat();
for(var i=0; i<a.length; ++i) {
for(var j=i+1; j<a.length; ++j) {
if(a[i] === a[j])
a.splice(j, 1);
}
}
return a;
};
console.log(arrUniqe.unique()); //去重结果 [1,2,3,'a','b'];执行效率比较高的方式
//方式二: //filter可以传三个值,分别是item,index,self,self是数组arr本身
var arr = [1,2,3,2,1,'a','b','a'];
var arr2 = arr.filter((item,index,self) => self.indexOf(item) === index);
console.log(arr2);
//8、数组过滤:
//1、过滤数组某些特征的值,ES5写法
var list1=[
{id:1,age:9},
{id:2,age:14},
{id:3,age:17},
{id:4,age:1}
];
var list2 = list1.filter(function (item) {
if(item.age > 10) return item
});
console.log(list2);
//2、ES6写法
const list2 = list1.filter(item=>(
item.age>10
));
console.log(list2);
//3、过滤掉空值,如:'',0,undefined
var arr1 = [0,'',undefined,1,'a'];
var arr2 = arr1.filter(item => item);
console.log(arr2); //[1,'a']
//9、js判断两个数组是否相等,包括普通数组和包含对象的数组
//情况一:两个普通数组
var a = [1,true,undefined,3];
var b = [1,true,undefined,3];
function diff(a,b) { //这种方式只能对比两个是值的数组是否相等,如果数组的项是对象{},此方法失效
var arr1 = [...a].sort();
var arr2 = [...b].sort();
return arr1.toString() == arr2.toString()
}
console.log(diff(a,b));
//情况二:包含对象的两个数组
var a = [{id:1,name:'a'},{id:2,name:'b'},{id:2,name:'b'}];
var b = [{id:1,name:'a'},{id:2,name:'b'},{id:2,name:'b'}];
function diff(a,b,key1,key2) { //三个或者四个参数分别为两个数组,key1,key2分别为两个数组必须要相等的属性值,其他可以不等
var arr1 = [...a];
var arr2 = [...b];
var diff = false;
if(arr1.length == arr2.length){
if(arr1.length <= 0){
diff = true
}else {
for(var j=0;j<arr1.length;j++){
for(var k=0;k<arr2.length;k++){
if(arr1[j][key1] == arr2[k][key1] && arr1[j][key2] == arr2[k][key2]){
arr1[j] = '';
arr2[k] = '';
}
}
}
if(arr1.filter(item => item).length <= 0) diff = true;
}
}
return diff
}
console.log(diff(a,b,'id','name'));
//10、两个数组a1(新),a2(旧),a1是重新变化后的数组,新数组a1和旧数组a2对比,key一样时,保留旧数组a2已经设置的其他项,其他用新数组a1的项,
var a1 = [{k:'key1',v:''},{k:'key2',v:''},{k:'key4',v:''}]; //新数组
var a2 = [{k:'key1',v:'name1'},{k:'key2',v:'name2'},{k:'key3',v:'name3'}]; //老数组
function updateArr(a1,a2,key) { //三个参数,分别是新数组、老数组、要对比的必须一样的key,key一样,
if(a1.length != 0 && a2.length != 0){
for(var i=0;i<a1.length;i++){
for(var j=0;j<a2.length;j++){
if(a1[i][key] == a2[j][key]){
a1[i] = a2[j];
break
}
}
}
}
return a1;
}
var arr = updateArr(a1,a2,'k'); //[{k:'key1',v:'name1'},{k:'key2',v:'name2'},{k:'key4',v:''}]
console.log(arr)
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