concat 版本1.0 只能接收一个参数
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Array.prototype.concat = function (arr) {
var self = this;
var newArr = [];
var len1 = self.length;
var len2 = arr.length;
var len = len1 + len2;
for(var i = 0;i < len;i++) {
if(i < len1) {
newArr[i] = self[i];
}else {
newArr[i] = arr[i - len1];
}
}
return newArr;
}
版本2.0 用了各种数组的方法, 感觉很幸福.. 当然我们还是要试着只用for if 来写, 不过 用方法写,思路真的是比较清楚.
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Array.prototype.concat = function() {
var self = this;
var newArr = [];
var args = [].slice.call(arguments);
self.forEach(function (item) {
newArr.push(item);
});
args.forEach(function (item) {
item.forEach(function (item1) {
newArr.push(item1);
})
})
return newArr;
}
版本3.0 这是翻译2.0而来的,很快 如果不先写2.0 要写3.0可能会很慢.
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Array.prototype.concat = function() {
var self = this;
var newArr = [];
var args = [];
var lastIndex = 0;
for(var i = lastIndex; i < self.length; i++) {
newArr[i] = self[i];
}
lastIndex = self.length;
for(var i = 0; i < arguments.length;i++) {
arguments[i];
for(var j = 0; j < arguments[i].length;j++) {
newArr[lastIndex + j] = arguments[i][j];
}
lastIndex += arguments.length;
}
return newArr;
}
slice 截取是半开半闭[a,b)
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Array.prototype.slice = function (a,b) {
var arr = this;
var newArr = [];
var a = a || 0;
var b = b || arr.length;
for(var i = a; i < b;i++) {
newArr[i - a] = arr[i];
}
return newArr;
}
join
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Array.prototype.join = function (str) {
var arr = this;
var newStr = '';
var str = str || ',';
var len = arr.length;
for(var i = 0; i < len - 1;i++) {
newStr += arr[i] + str;
}
return newStr + arr[len - 1]
}
渡一讲的是,上面这种字符串拼接的方式不好,
因为字符串是存在栈中,先进后出.
用join更好.
======================================== 2018/11/5 重练一遍
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<script type="text/javascript">
// 返回新数组
Array.prototype.myConcat = function () {
var self = this;
var arr = [];
for(var ii = 0; ii < self.length; ii++) {
arr[ii] = self[ii];
}
var len = arguments.length;
for(var i = 0; i < len; i++) {// 取出所有传进来的数组
var iArr = arguments[i];
var iLen = iArr.length;
for(var j = 0; j < iLen; j++) {// 取出所有元素.赋值给arr
// 问题: 如何更改arr 的索引? 很简单, 每次都获取自己的length 在末尾添加即可
// 这样似乎从形式上更简洁.
var aLen = arr.length;
arr[aLen] = iArr[j];
}
}
return arr
}
Array.prototype.mySlice = function (start, end) {
var self = this;
var arr = [];
// 原来的数组不改变.
// 截取的问题// 先考虑两个参数都有的情况.
// 兼容负数
// 默认值 start = 0; end = len
var len = self.length;
var start = start > 0 ? start : len + start;
var end = end > 0 ? end : len + end;
isNaN(start) && (start = 0);
isNaN(end) && (end = len);
for(var i = 0; i < len; i++) {
if (i >= start && i < end) {
var aLen = arr.length;
arr[aLen] = self[i]
}
}
return arr;
}
Array.prototype.myJoin = function (str1) {
var self = this;
if (typeof str1 == "undefined") {
var str1 = ","
} else {
var str1 = str1
}
console.log(str1);
var str = "";
var len = self.length;
for(var i = 0; i < len - 1; i++) {
str += self[i] + str1
}
str += self[len - 1];
return str
}
var arr1 = [1,2,3];
var arr2 = [11,22,33];
var arr3 = [1111,2222,3333];
var arr4 = [1111,2222,3333];
var arr = arr1.myConcat(arr2,arr3,arr4);
console.log(arr);
var newArr = arr.mySlice();
console.log(newArr);
var str = arr.myJoin(",0,")
var str1 = arr.join(",0,")
console.log(str);
console.log(str1);
</script>
