梳理总结JavaScript的23个String方法

var anyString = "Brave new world";

console.log("The character at index 0   is '" + anyString.charAt(0)   + "'");
console.log("The character at index 1   is '" + anyString.charAt(1)   + "'");
console.log("The character at index 2   is '" + anyString.charAt(2)   + "'");
console.log("The character at index 3   is '" + anyString.charAt(3)   + "'");
console.log("The character at index 4   is '" + anyString.charAt(4)   + "'");
console.log("The character at index 999 is '" + anyString.charAt(999) + "'");

"ABC".charCodeAt(0) // returns 65:"A"

"ABC".charCodeAt(1) // returns 66:"B"

"ABC".charCodeAt(2) // returns 67:"C"

"ABC".charCodeAt(3) // returns NaN

'ABC'.codePointAt(1);          // 66
'\uD800\uDC00'.codePointAt(0); // 65536
'XYZ'.codePointAt(42); // undefined

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"

'canal'.lastIndexOf('a');     // returns 3（没有指明 fromIndex 则从末尾 l 处开始反向检索到的第一个 a 出现在 l 的后面，即 index 为 3 的位置）
'canal'.lastIndexOf('a', 2);  // returns 1（指明 fromIndex 为 2 则从 n 处反向向回检索到其后面就是 a，即 index 为 1 的位置）
'canal'.lastIndexOf('a', 0);  // returns -1(指明 fromIndex 为 0 则从 c 处向左回向检索 a 发现没有，故返回-1)
'canal'.lastIndexOf('x');     // returns -1
'canal'.lastIndexOf('c', -5); // returns 0（指明 fromIndex 为-5 则视同 0，从 c 处向左回向查找发现自己就是，故返回 0）
'canal'.lastIndexOf('c', 0);  // returns 0（指明 fromIndex 为 0 则从 c 处向左回向查找 c 发现自己就是，故返回自己的索引 0）
'canal'.lastIndexOf('');      // returns 5
'canal'.lastIndexOf('', 2);   // returns 2

const str1 = 'Saturday night plans';

console.log(str1.startsWith('Sat'));
// expected output: true

console.log(str1.startsWith('Sat', 3));
// expected output: false

var str = "To be, or not to be, that is the question.";

alert( str.endsWith("question.") );  // true
alert( str.endsWith("to be") );      // false
alert( str.endsWith("to be", 19) );  // true

var 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

let hello = 'Hello, '
console.log(hello.concat('Kevin', '. Have a nice day.'))
// Hello, Kevin. Have a nice day.

let greetList = ['Hello', ' ', 'Venkat', '!']
"".concat(...greetList)  // "Hello Venkat!"

"".concat({})    // [object Object]
"".concat([])    // ""
"".concat(null)  // "null"
"".concat(true)  // "true"
"".concat(4, 5)  // "45"

String.fromCharCode(65, 66, 67);   // 返回 "ABC"
String.fromCharCode(0x2014);       // 返回 "—"
String.fromCharCode(0x12014);      // 也是返回 "—"; 数字 1 被剔除并忽略
String.fromCharCode(8212);         // 也是返回 "—"; 8212 是 0x2014 的十进制表示

String.fromCodePoint(42);       // "*"
String.fromCodePoint(65, 90);   // "AZ"
String.fromCodePoint(0x404);    // "\u0404"
String.fromCodePoint(0x2F804);  // "\uD87E\uDC04"
String.fromCodePoint(194564);   // "\uD87E\uDC04"
String.fromCodePoint(0x1D306, 0x61, 0x1D307) // "\uD834\uDF06a\uD834\uDF07"

String.fromCodePoint('_');      // RangeError
String.fromCodePoint(Infinity); // RangeError
String.fromCodePoint(-1);       // RangeError
String.fromCodePoint(3.14);     // RangeError
String.fromCodePoint(3e-2);     // RangeError
String.fromCodePoint(NaN);      // RangeError

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"

const strCopy = str.split();
console.log(strCopy);
// expected output: Array ["The quick brown fox jumps over the lazy dog."]

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"

console.log(str.slice(-4));
// expected output: "dog."

console.log(str.slice(-9, -5));
// expected output: "lazy"

var anyString = "Mozilla";

// 输出 "Moz"
console.log(anyString.substring(0,3));
console.log(anyString.substring(3,0));
console.log(anyString.substring(3,-3));
console.log(anyString.substring(3,NaN));
console.log(anyString.substring(-2,3));
console.log(anyString.substring(NaN,3));

// 输出 "lla"
console.log(anyString.substring(4,7));
console.log(anyString.substring(7,4));

// 输出 ""
console.log(anyString.substring(4,4));

// 输出 "Mozill"
console.log(anyString.substring(0,6));

// 输出 "Mozilla"
console.log(anyString.substring(0,7));
console.log(anyString.substring(0,10));

var str = "abcdefghij";

console.log("(1,2): "    + str.substr(1,2));   // (1,2): bc
console.log("(-3,2): "   + str.substr(-3,2));  // (-3,2): hi
console.log("(-3): "     + str.substr(-3));    // (-3): hij
console.log("(1): "      + str.substr(1));     // (1): bcdefghij
console.log("(-20, 2): " + str.substr(-20,2)); // (-20, 2): ab
console.log("(20, 2): "  + str.substr(20,2));  // (20, 2):

const paragraph = 'The quick brown fox jumps over the lazy dog. It barked.';
const regex = /[A-Z]/g;
const found = paragraph.match(regex);

console.log(found);
// expected output: Array ["T", "I"]

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?"

var str = "hey JudE";
var re = /[A-Z]/g;
var re2 = /[.]/g;
console.log(str.search(re)); // returns 4, which is the index of the first capital letter "J"
console.log(str.search(re2)); // returns -1 cannot find '.' dot punctuation

console.log('中文简体 zh-CN || zh-Hans'.toLowerCase());
// 中文简体 zh-cn || zh-hans

console.log( "ALPHABET".toLowerCase() );
// "alphabet"

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."

const name1 = '\u0041\u006d\u00e9\u006c\u0069\u0065';
const name2 = '\u0041\u006d\u0065\u0301\u006c\u0069\u0065';

console.log(`${name1}, ${name2}`);
// expected output: "Amélie, Amélie"
console.log(name1 === name2);
// expected output: false
console.log(name1.length === name2.length);
// expected output: false

const name1NFC = name1.normalize('NFC');
const name2NFC = name2.normalize('NFC');
console.log(`${name1NFC}, ${name2NFC}`);
// expected output: "Amélie, Amélie"
console.log(name1NFC === name2NFC);
// expected output: true
console.log(name1NFC.length === name2NFC.length);
// expected output: true

"abc".repeat(-1)     // RangeError: repeat count must be positive and less than inifinity
"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。
"abc".repeat(1/0)    // RangeError: repeat count must be positive and less than inifinity

({toString : () => "abc", repeat : String.prototype.repeat}).repeat(2)
//"abcabc",repeat 是一个通用方法，也就是它的调用者可以不是一个字符串对象。

var orig = '   foo  ';
console.log(orig.trim()); // 'foo'

// 另一个 .trim() 例子，只从一边删除

var orig = 'foo    ';
console.log(orig.trim()); // 'foo'