JavaScript での配列の作成と使用に関するディスカッション

var empty = []; //没有元素的数组
var primes = [2,3,5,7,11]; //有5个数值的数组

var misc = [1.1,true, "a"]; //3个不同类型的元素

var base = 1024;
var table = [base,base+1,base+2,base+3];

var b = [ [1,{x:1,y:2}],[2,{x:3,y:4}] ];

var a = [[1, 2], [3, 4]];

//该方法创建一个没有任何元素的空数组，等同于数组直接量[]
var a = new Array();

var a = new Array(10);
console.log(a);//[]
console.log(a[0],a.length);//undefined 10

var a = new Array('10');
console.log(a);//['10']
console.log(a[0],a.length);//10 1

var a = new Array(1,2,3);
console.log(a);//[1,2,3]
console.log(a[0],a[1],a[2]);//1 2 3

var a1 = Array();
var a2 = Array(10);
var a3 = Array(1,2,3);
console.log(a1,a2,a3);//[] [] [1,2,3]

typeof [1, 2, 3] // "object"

var arr = ['a', 'b', 'c'];
console.log(Object.keys(arr));// ["0", "1", "2"]
var obj = {
name1: 'a',
name2: 'b',
name3: 'c'
};

o={}; //创建一个普通的对象
o[1]="one"; //用一个整数来索引它
//数值键名被自动转成字符串
var arr = ['a', 'b', 'c'];
arr['0'] // 'a'
arr[0] // 'a'

var a = [];
//索引
a['1000'] = 'abc';
a[1000] // 'abc'
//索引
a[1.00] = 6;
a[1] // 6

var arr = [1, 2, 3];
arr[0];//1
arr.0;//SyntaxError

var arr = ['a', 'b'];
arr.length // 2
arr[2] = 'c';
arr.length // 3
arr[9] = 'd';
arr.length // 10
arr[1000] = 'e';
arr.length // 1001

a=[1,2,3,4,5]; //从5个元素的数组开始
a.length = 3; //现在a为[1,2,3]
a.length = 0; //删除所有的元素。a为[]
a.length = 5; //长度为5，但是没有元素，就像new

var a = ['a'];
a.length = 3;
console.log(a[1]);//undefined
console.log(1 in a);//false

// 设置负值
[].length = -1// RangeError: Invalid array length
// 数组元素个数大于等于2的32次方
[].length = Math.pow(2,32)// RangeError: Invalid array length
// 设置字符串
[].length = 'abc'// RangeError: Invalid array length

var a = [];
a['p'] = 'abc';
console.log(a.length);// 0
a[2.1] = 'abc';
console.log(a.length);// 0

var a = [1, 2, 3];
for(var i = 0; i < a.length; i++) {
console.log(a[i]);
}

var a = [1, 2, 3];
var i = 0;
while (i < a.length) {
console.log(a[i]);
i++;
}
var l = a.length;
while (l--) {
console.log(a[l]);
}

//跳过不存在的元素
var a = [1,,,2];
for(var i = 0; i < a.length; i++){
if(!(i in a)) continue;
console.log(a[i]);
}

var a = [1,,,2];
for(var i in a){
console.log(a[i]);
}

var a = [1,,,2];
a.b = 'b';
for(var i in a){
console.log(a[i]);//1 2 'b'
} 
//跳过不是非负整数的i
var a = [1,,,2];
a.b = 'b';
for(var i in a){
if(String(Math.floor(Math.abs(Number(i)))) !== i) continue;
console.log(a[i]);//1 2
}

// arguments对象
function args() { return arguments }
var arrayLike = args('a', 'b');
arrayLike[0] // 'a'
arrayLike.length // 2
arrayLike instanceof Array // false

// DOM元素
var elts = document.getElementsByTagName('h3');
elts.length // 3
elts instanceof Array // false

// 字符串
'abc'[1] // 'b'
'abc'.length // 3
'abc' instanceof Array // false

var str = 'abc';
Array.prototype.forEach.call(str, function(chr) {
console.log(chr);//a b c
});
Array.prototype.splice.call(str,1);
console.log(str);//TypeError: Cannot delete property '2' of [object String]

var arr = Array.prototype.slice.call(arrayLike);

var a = {'0':'a','1':'b','2':'c',length:3};
Array.prototype.join.call(a,'+');//'a+b+c'
Array.prototype.slice.call(a,0);//['a','b','c']
Array.prototype.map.call(a,function(x){return x.toUpperCase();});//['A','B','C']