字符串
String 对象和 字符串的值的区别?
就比如 包装箱 和 箱子里装的东西, 在 Java 中有类似装箱和拆箱的概念
- 包装类实例对象(包装箱 + 箱子里装的东西一箱苹果):
new String("apple") - 原始值(箱子里装的东西一箱苹果):
"apple"
js
const str1 = "hello"; // 原始值(string 类型, 存储在栈中)
const str2 = new String("hello"); // String 对象(object 类型, 纯在堆区)
console.log(typeof str1); // "string"
console.log(typeof str2); // "object"字符串常用方法
操作方法
| 函数名 | 作用 | 返回值 |
|---|---|---|
| concat | 拼接一个或多个字符串 | String |
| slice/substring | 截取字符串从 start 位置开始, end位置结束, 但是不包括 end | String |
| trim/trimLeft/trimRight | 去除字符串俩边的空格 | String |
| repeat | 重复一个字符串指定次数 | String |
| padStart/padEnd | 用一个字符串填充一个字符串 | String |
| toUpperCase/toLowerCase | 装换大小写 | String |
| charAt/charCodeAt/codePointAt | 返回字符串对应的 unicode 码 | Number |
| indexOf | 获取字符在字符串中的位置,如果没有返回 -1 | Number |
| startsWith | 判断一个字符串是否是以一个字符串开头的 | Boolean |
| includes | 判断一个字符串是否包含另外一个字符串 | Boolean |
| fromCharCode/fromCodePoint | 根据 unicode 码返回对应的字符 | String |
转换方法
将字符串切割成数组
| 函数名 | 作用 | 返回值 |
|---|---|---|
| split | 将字符串按照分隔符分割为数组 | Array |
模板匹配方法
| 函数名 | 作用 | 返回值 |
|---|---|---|
| match | 按照字符串/正则表达式去匹配 | Array |
| search | 按照字符串/正则表达式搜索,找到返回索引位置, 没找到返回 -1 | Number |
| replace | 按照字符串/正则表达式替换 | String |
手动实现
concat
javascript
String.prototype.$concat = function () {
var o = Object(this);
var str = o.valueOf();
if (typeof str !== "string") {
throw new TypeError("'this' must be a string");
}
if (!arguments.length) {
return str;
}
var args = Array.prototype.slice.call(arguments);
for (var i = 0, l = args.length; i < l; i++) {
str += String(args[i]);
}
return str;
};slice/substring
javascript
// sbustring
String.prototype.$substring = function () {
var o = Object(this);
var str = o.valueOf();
if (typeof str !== "string") {
throw new TypeError("'this' must be a string");
}
// 参数不支持负数(不是不支持负数,是负数不能像slice那样从后往前截取)
// https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/String/substring#description
var len = str.length >>> 0;
var startIndex = arguments[0] >> 0;
startIndex = startIndex < 0 ? 0 : Math.min(len, startIndex);
var endIndex = arguments.length > 1 ? arguments[1] >> 0 : Infinity;
endIndex = endIndex < 0 ? 0 : Math.min(endIndex, len);
var res = "";
if (startIndex === endIndex) {
return res;
}
if (startIndex > endIndex) {
// 交换两变量的值
var temp = startIndex;
startIndex = endIndex;
endIndex = temp;
}
var i = startIndex;
while (i < endIndex) {
res += str[i];
i++;
}
return res;
};
// slice
String.prototype.$slice = function () {
var o = Object(this);
var str = o.valueOf();
if (typeof str !== "string") {
throw new TypeError("'this' must be a string");
}
var len = str.length >>> 0;
// startIndex 和 end 必须是正整数
var startIndex = arguments[0] >> 0;
startIndex =
startIndex < 0 ? Math.max(len + startIndex, 0) : Math.min(len, startIndex);
var endIndex = arguments.length > 1 ? arguments[1] >> 0 : Infinity;
endIndex = endIndex < 0 ? Math.max(len + endIndex) : Math.min(endIndex, len);
var res = "";
var i = startIndex;
while (i < endIndex) {
res += str[i];
i++;
}
return res;
};trim/trimLeft/trimRight
javascript
//trim: /^\s*(.*?)\s*$/gm
String.prototype.$trim = function () {
var o = Object(this);
var str = o.valueOf();
if (typeof str !== "string") {
throw new TypeError("'this' must be a string");
}
return str.replace(/^\s*(.*?)\s*$/gm, (...args) => args[1]);
};
// trimLeft: /^\s*(.*)$/gm
String.prototype.$trimLeft = function () {
var o = Object(this);
var str = o.valueOf();
if (typeof str !== "string") {
throw new TypeError("'this' must be a string");
}
return str.replace(/^\s*(.*)$/gm, (...args) => args[1]);
};
// trimRight: /^(.*?)\s*$/gm
String.prototype.$trimRight = function () {
var o = Object(this);
var str = o.valueOf();
if (typeof str !== "string") {
throw new TypeError("'this' must be a string");
}
return str.replace(/^(.*?)\s*$/gm, (...args) => args[1]);
};repeat/padStart/padEnd
javascript
String.prototype.$repeat = function (count) {
var str = Object(this).toString();
var count = count >> 0;
if (count < 0) {
throw new RangeError("count value is invlid");
}
var res = "";
for (var i = 0; i < count; i++) {
res += str;
}
return res;
};indexOf/lastIndexOf/includes
javascript
// indexOf
String.prototype.$indexOf = function (needle) {
var str = Object(this).toString();
var len = str.length;
var needle = String(needle);
var needleLen = needle.length;
var startIndex = arguments.length > 1 ? arguments[1] >> 0 : 0;
if (startIndex < 0) {
startIndex = 0;
}
for (var i = startIndex; i < len; i++) {
// 每次都 slice 去对比
if (needle === str.substring(i, i + needleLen)) {
return i;
}
}
return -1;
};
// lastIndexOf
String.prototype.$lastIndexOf = function (needle) {
var str = Object(this).toString();
var len = str.length;
var needle = String(needle);
var needleLen = needle.length;
var startIndex = arguments.length > 1 ? arguments[1] >> 0 : 0;
if (startIndex < 0) {
startIndex = 0;
}
var lastIndex = -1;
for (var i = startIndex; i < len; i++) {
// 每次都 slice 去对比, 匹配到就更新 lastIndex
if (needle === str.substring(i, i + needleLen)) {
lastIndex = i;
}
}
return lastIndex;
};
// includes
String.prototype.$lastIndexOf = function (needle) {
var str = Object(this).toString();
var len = str.length;
var needle = String(needle);
var needleLen = needle.length;
var startIndex = arguments.length > 1 ? arguments[1] >> 0 : 0;
if (startIndex < 0) {
startIndex = 0;
}
for (var i = startIndex; i < len; i++) {
// 每次都 slice 去对比
if (needle === str.substring(i, i + needleLen)) {
return true;
}
}
return false;
};startsWith
javascript
String.prototype.$startsWith = function (needle) {
var str = Object(this).toString();
var needle = String(needle);
return str.substring(0, needle.length) === needle;
};split
手动实现 split 方法, 这是一个经典面试题
javascript
/**
* String.prototype.split
* @param {String} spl
* @returns {Array}
*/
String.prototype.$split = function (spl) {
var str = Object(this).toString(); // raw value
var len = str.length;
// 字符串的长度为0
if (len === 0) {
return [""];
}
// 如果没有传入分隔符 spl
var tag = spl || "";
if (tag.length === 0) {
return [str];
}
var res = [];
var tagIndex;
while (str.length) {
tagIndex = str.indexOf(tag);
if (tagIndex === -1 && tag) {
res.push(str);
str = "";
} else {
res.push(str.substring(0, tagIndex));
str = str.substring(tagIndex + 1);
}
}
return res;
};