docs: update syntax-uts.md

docs/tutorial/syntax-uts.md

@@ -1739,6 +1739,250 @@ forEach 方法会根据集合中元素的插入顺序，依次执行提供的回
 
 has() 方法返回一个布尔值来指示对应的值 value 是否存在 Set 对象中。
 
+### String
+
+String 全局对象是一个用于字符串或一个字符序列的构造函数。
+
+字符串字面量采取以下形式：
+
+```ts
+'string text'
+"string text"
+"中文/汉语"
+"español"
+"English "
+"हिन्दी"
+"العربية"
+"português"
+"বাংলা"
+"русский"
+"日本語"
+"ਪੰਜਾਬੀ"
+"한국어"
+```
+
+#### 实例属性
+
+#### length
+length 属性表示一个字符串的长度。
+```ts
+const x = "Mozilla";
+const empty = "";
+
+console.log("Mozilla is " + x.length + " code units long");
+/* "Mozilla is 7 code units long" */
+
+console.log("The empty string is has a length of " + empty.length);
+/* "The empty string is has a length of 0" */
+```
+
+#### 实例方法
+
+#### at
+
+at() 方法接受一个整数值，并返回一个新的 String，该字符串由位于指定偏移量处的单个 UTF-16 码元组成。该方法允许正整数和负整数。负整数从字符串中的最后一个字符开始倒数。
+
+```ts
+const sentence = 'The quick brown fox jumps over the lazy dog.';
+let index = 5;
+console.log(`Using an index of ${index} the character returned is ${sentence.at(index)}`);
+// expected output: "Using an index of 5 the character returned is u"
+index = -4;
+console.log(`Using an index of ${index} the character returned is ${sentence.at(index)}`);
+// expected output: "Using an index of -4 the character returned is d"
+```
+
+#### charAt
+
+charAt() 方法从一个字符串中返回指定的字符。
+
+```ts
+const anyString = "Brave new world";
+
+console.log("The character at index 0   is '" + anyString.charAt(0)   + "'");
+// The character at index 0 is 'B'
+console.log("The character at index 1   is '" + anyString.charAt(1)   + "'");
+// The character at index 1 is 'r'
+console.log("The character at index 2   is '" + anyString.charAt(2)   + "'");
+// The character at index 2 is 'a'
+console.log("The character at index 3   is '" + anyString.charAt(3)   + "'");
+// The character at index 3 is 'v'
+console.log("The character at index 4   is '" + anyString.charAt(4)   + "'");
+// The character at index 4 is 'e'
+console.log("The character at index 999 is '" + anyString.charAt(999) + "'");
+// The character at index 999 is ''
+```
+
+#### charCodeAt
+
+charCodeAt() 方法返回 0 到 65535 之间的整数，表示给定索引处的 UTF-16 代码单元
+
+```ts
+const sentence = 'The quick brown fox jumps over the lazy dog.';
+const index = 4;
+console.log(`The character code ${sentence.charCodeAt(index)} is equal to ${sentence.charAt(index)}`);
+// expected output: "The character code 113 is equal to q"
+```
+
+#### concat
+
+concat() 方法将一个或多个字符串与原字符串连接合并，形成一个新的字符串并返回。
+
+```ts
+let hello = 'Hello, '
+console.log(hello.concat('Kevin', '. Have a nice day.'))
+// Hello, Kevin. Have a nice day.
+```
+
+#### endsWith
+
+endsWith() 方法用来判断当前字符串是否是以另外一个给定的子字符串“结尾”的，根据判断结果返回 true 或 false。
+
+```ts
+const str1 = 'Cats are the best!';
+console.log(str1.endsWith('best!'));
+// expected output: true
+console.log(str1.endsWith('best', 17));
+// expected output: true
+const str2 = 'Is this a question?';
+console.log(str2.endsWith('question'));
+// expected output: false
+```
+
+#### includes
+
+includes() 方法用于判断一个字符串是否包含在另一个字符串中，根据情况返回 true 或 false。
+
+```ts
+const str = 'To be, or not to be, that is the question.';
+
+console.log(str.includes('To be'));       // true
+console.log(str.includes('question'));    // true
+console.log(str.includes('nonexistent')); // false
+console.log(str.includes('To be', 1));    // false
+console.log(str.includes('TO BE'));       // false
+```
+#### indexOf
+
+indexOf() 方法返回调用它的 String 对象中第一次出现的指定值的索引，从 fromIndex 处进行搜索。如果未找到该值，则返回 -1。
+
+```ts
+const paragraph = 'The quick brown fox jumps over the lazy dog. If the dog barked, was it really lazy?';
+
+const searchTerm = 'dog';
+const indexOfFirst = paragraph.indexOf(searchTerm);
+
+console.log(`The index of the first "${searchTerm}" from the beginning is ${indexOfFirst}`);
+// expected output: "The index of the first "dog" from the beginning is 40"
+
+console.log(`The index of the 2nd "${searchTerm}" is ${paragraph.indexOf(searchTerm, (indexOfFirst + 1))}`);
+// expected output: "The index of the 2nd "dog" is 52"
+
+```
+#### padEnd
+
+padEnd() 方法会用一个字符串填充当前字符串（如果需要的话则重复填充），返回填充后达到指定长度的字符串。从当前字符串的末尾（右侧）开始填充。
+
+```ts
+const str1 = 'Breaded Mushrooms';
+console.log(str1.padEnd(25, '.'));
+// expected output: "Breaded Mushrooms........"
+const str2 = '200';
+console.log(str2.padEnd(5));
+// expected output: "200  "
+```
+#### padStart
+
+padStart() 方法用另一个字符串填充当前字符串 (如果需要的话，会重复多次)，以便产生的字符串达到给定的长度。从当前字符串的左侧开始填充。
+
+```ts
+const str1 = '5';
+console.log(str1.padStart(2, '0'));
+// expected output: "05"
+```
+#### repeat
+
+repeat() 构造并返回一个新字符串，该字符串包含被连接在一起的指定数量的字符串的副本。
+
+```ts
+"abc".repeat(0)      // ""
+"abc".repeat(1)      // "abc"
+"abc".repeat(2)      // "abcabc"
+"abc".repeat(3.5)    // "abcabcabc" 参数 count 将会被自动转换成整数。
+```
+
+#### replace
+
+replace() 方法返回一个由替换值（replacement）替换部分或所有的模式（pattern）匹配项后的新字符串。模式可以是一个字符串或者一个正则表达式，替换值可以是一个字符串或者一个每次匹配都要调用的回调函数。如果pattern是字符串，则仅替换第一个匹配项。原字符串不会改变。
+
+```ts
+const p = 'The quick brown fox jumps over the lazy dog. If the dog reacted, was it really lazy?';
+
+console.log(p.replace('dog', 'monkey'));
+// expected output: "The quick brown fox jumps over the lazy monkey. If the dog reacted, was it really lazy?"
+const regex = /Dog/i;
+console.log(p.replace(regex, 'ferret'));
+// expected output: "The quick brown fox jumps over the lazy ferret. If the dog reacted, was it really lazy?"
+
+```
+#### search
+
+search() 方法执行正则表达式和 String 对象之间的一个搜索匹配。
+
+```ts
+const paragraph = 'The quick brown fox jumps over the lazy dog. If the dog barked, was it really lazy?';
+// any character that is not a word character or whitespace
+const regex = /[^\w\s]/g;
+console.log(paragraph.search(regex));
+// expected output: 43
+console.log(paragraph[paragraph.search(regex)]);
+// expected output: "."
+```
+#### slice
+
+slice() 方法提取某个字符串的一部分，并返回一个新的字符串，且不会改动原字符串。
+
+```ts
+const str = 'The quick brown fox jumps over the lazy dog.';
+console.log(str.slice(31));
+// expected output: "the lazy dog."
+console.log(str.slice(4, 19));
+// expected output: "quick brown fox"
+```
+
+#### split
+
+split() 方法使用指定的分隔符字符串将一个String对象分割成子字符串数组，以一个指定的分割字串来决定每个拆分的位置。
+
+```ts
+const str = 'The quick brown fox jumps over the lazy dog.';
+
+const words = str.split(' ');
+console.log(words[3]);
+// expected output: "fox"
+const chars = str.split('');
+console.log(chars[8]);
+// expected output: "k"
+```
+#### toLowerCase
+
+toLowerCase() 会将调用该方法的字符串值转为小写形式，并返回。
+
+```ts
+console.log('中文简体 zh-CN || zh-Hans'.toLowerCase());
+// 中文简体 zh-cn || zh-hans
+​console.log( "ALPHABET".toLowerCase() );
+// "alphabet"
+```
+#### toUpperCase
+
+toUpperCase() 方法将调用该方法的字符串转为大写形式并返回（如果调用该方法的值不是字符串类型会被强制转换）。
+
+```ts
+const sentence = 'The quick brown fox jumps over the lazy dog.';
+console.log(sentence.toUpperCase());
+// expected output: "THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG."
+```
 
 ### 定时器