数组

Math.sqrt(x)

Math.sqrt(9); // 3
Math.sqrt(2); // 1.414213562373095

Math.sqrt(1);  // 1
Math.sqrt(0);  // 0
Math.sqrt(-1); // NaN

Math.abs(x)

Math.abs(-7.25); //7.25

---

var x = 3;
var y = 1.1;
var z = x*y;  //3.3000000000000003
z.toFixed(2);//3.30

---

function curve(arr,amount){
  for(var i = 0;i < arr.length;++i){
    arr[i]+=amount;
  }
}
var grades = [1,2,3];
curve(grades,5);  //6,7,8

Javascript中，函数的参数传递方式都是按值传递，没有引用传递的参数。但是Javascript中有保存引用的对象，如数组，它们是引用传递的。

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滥用全局变量

function showScope(){
  scope = 'local';
  return scope;
}
scope = 'global';

console.log(scope); //global
console.log(showScope);//local
console.log(scope);//local

---

var arr = [];
var arr = new Array();
var arr = new Array(3); //arr.length == 3;

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将数组转化为字符串：join()和toString()

由已有数组创建新数组：concat()和splice()

concat()对原数组无影响

splice()对原数组有影响

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为数组添加元素

添加末尾：

arr.push(1);
arr[arr.length]=1;

添加开头

一个一个向后移，低效

nums = [3,4];
nums.unshift(1,2);//nums == [1,2,3,4]

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从数组中删除

pop()删末尾

shift()删除数组第一个

---

splice()参数

• 起始索引

• 需要删除的元素个数（添加元素时该参数设为0）

• 想要添加进数组的元素

var arr1 = [1,2,3,7,8,9]
arr1.splice(3,0,4,5,6) //[1, 2, 3, 4, 5, 6, 7, 8, 9]

var arr2 = [1,2,3,100,200,300,4,5];
arr2.splice(3,3);//[1,2,3,4,5]

---

数组排序

翻转reverse();

字符串类型排序sort();

var arr1 = [3,1,2,100,4,200]
arr1.sort();  //1,100,2,200,3,4

function(num1,num2){
  return num1-num2;
}
var nums = [3,1,2,100,4,200]
nums.sorts(compare);//[1,2,3,4,100,200]

不生成新数组的迭代器方法

forEach()

every()对于所有的函数均返回true，则该方法返回true

some()只要一个

reduce()接受一个函数

生成新数组的迭代器方法

map()

filter()

function(word){
  return word[0];
}
var words = ['for','your','information']
var acronym = words.map(first); //[f,y,i]
acronym.join('')//fyi

function afterc(str){
  if(str.indexOf('cie')>-1){
    return true;
  }
  return false
}
var words = ['recieve','deceive','deceit','concieve'];
var misspelled = words.filter(afterc) //recieve,percieve,concieve

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参差不齐的数组

var grades = [[89,77],[76,82,81],[91,94,89,99]];
var total = 0;
var average = 0.0;
for(var row = 0;row<grades.length;++row){
  for(var row = 0;row<grades[row].length;++col){
    total +=grades[row][col];
  }
  average = total/grades[row].length;
  console.log(average.toFixed(2));
}

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对象数组

function Point(x,y){
  this.x=x;
  this.y=y;
}
function displayPts(arr){
  for(var i = 0;i < arr.length;++i){
    console.log(arr[i].x+","+arr[i].y);
  }
}
var p1 = new Point(1,2);
var p2 = new Point(2,3);
var points = [p1,p2]
var p3 = new Point(3,6);
points.push(p3);
points.shift();
displayPts(points);

对象中的数组

function weekTemps(){
  this.dataStore = [];
  this.add = add;
  this.average = average;
}
function add(temp){
  this.dataStore.push(temp);
}
function average(){
  var total = 0;
  for(var i = 0;i<this.dataStore.length;++i){
    total+=this.dataStore[i];
  }
  return total/this.dataStore.length;
}
var thisWeek = new weekTemps();
thisWeek.add(1);
thisWeek.add(2);
thisWeek.add(3);
thisWeek.average();//2