Chương này giới thiệu cách làm việc với các chuỗi và văn bản trong JavaScript.
Kiểu chuỗi của JavaScript được sử dụng để thể hiện dữ liệu văn bản. Nó là một tập hợp các "phần tử" của các giá trị nguyên không dấu 16 bit (đơn vị mã UTF-16). Mỗi phần tử trong Chuỗi chiếm một vị trí trong Chuỗi. Phần tử đầu tiên là ở chỉ số 0, phần tử tiếp theo ở chỉ số 1, v.v. Độ dài của Chuỗi là số phần tử trong đó. Bạn có thể tạo chuỗi bằng cách sử dụng chuỗi ký tự hoặc chuỗi đối tượng.
Bạn có thể tạo các chuỗi đơn giản bằng cách sử dụng dấu ngoặc đơn hoặc dấu ngoặc kép:
'foo' "quán ba"
Các chuỗi nâng cao hơn có thể được tạo bằng các chuỗi thoát:
The number after \x is interpreted as a hexadecimal number.
'\xA9' // "©"
The Unicode escape sequences require at least four hexadecimal digits following \u.
'\u00A9' // "©"
New in ECMAScript 2015. With Unicode code point escapes, any character can be escaped using hexadecimal numbers so that it is possible to use Unicode code points up to 0x10FFFF. With simple Unicode escapes it is often necessary to write the surrogate halves separately to achieve the same result.
See also {{jsxref("String.fromCodePoint()")}} or {{jsxref("String.prototype.codePointAt()")}}.
'\u{2F804}'
// the same with simple Unicode escapes
'\uD87E\uDC04'
The {{jsxref("String")}} object is a wrapper around the string primitive data type.
const foo = new String('foo'); // Creates a String object
console.log(foo); // Displays: [String: 'foo']
typeof foo; // Returns 'object'
You can call any of the methods of the String object on a string literal value—JavaScript automatically converts the string literal to a temporary String object, calls the method, then discards the temporary String object. You can also use the String.length property with a string literal.
You should use string literals unless you specifically need to use a String object, because String objects can have counterintuitive behavior. For example:
const firstString = '2 + 2'; // Creates a string literal value
const secondString = new String('2 + 2'); // Creates a String object
eval(firstString); // Returns the number 4
eval(secondString); // Returns the string "2 + 2"
A String object has one property, length, that indicates the number of UTF-16 code units in the string. For example, the following code assigns helloLength the value 13, because "Hello, World!" has 13 characters, each represented by one UTF-16 code unit. You can access each code unit using an array bracket style. You can't change individual characters because strings are immutable array-like objects:
const hello = 'Hello, World!'; const helloLength = hello.length; hello[0] = 'L'; // This has no effect, because strings are immutable hello[0]; // This returns "H"
Characters whose Unicode scalar values are greater than U+FFFF (such as some rare Chinese/Japanese/Korean/Vietnamese characters and some emoji) are stored in UTF-16 with two surrogate code units each. For example, a string containing the single character U+1F600 "Emoji grinning face" will have length 2. Accessing the individual code units in such a string using brackets may have undesirable consequences such as the formation of strings with unmatched surrogate code units, in violation of the Unicode standard. (Examples should be added to this page after MDN bug 857438 is fixed.) See also {{jsxref("String.fromCodePoint()")}} or {{jsxref("String.prototype.codePointAt()")}}.
A String object has a variety of methods: for example those that return a variation on the string itself, such as substring and toUpperCase.
The following table summarizes the methods of {{jsxref("String")}} objects.
| Method | Description |
|---|---|
| {{jsxref("String.charAt", "charAt")}}, {{jsxref("String.charCodeAt", "charCodeAt")}}, {{jsxref("String.codePointAt", "codePointAt")}} | Return the character or character code at the specified position in string. |
| {{jsxref("String.indexOf", "indexOf")}}, {{jsxref("String.lastIndexOf", "lastIndexOf")}} | Return the position of specified substring in the string or last position of specified substring, respectively. |
| {{jsxref("String.startsWith", "startsWith")}}, {{jsxref("String.endsWith", "endsWith")}}, {{jsxref("String.includes", "includes")}} | Returns whether or not the string starts, ends or contains a specified string. |
| {{jsxref("String.concat", "concat")}} | Combines the text of two strings and returns a new string. |
| {{jsxref("String.fromCharCode", "fromCharCode")}}, {{jsxref("String.fromCodePoint", "fromCodePoint")}} | Constructs a string from the specified sequence of Unicode values. This is a method of the String class, not a String instance. |
| {{jsxref("String.split", "split")}} | Splits a String object into an array of strings by separating the string into substrings. |
| {{jsxref("String.slice", "slice")}} | Extracts a section of a string and returns a new string. |
| {{jsxref("String.substring", "substring")}}, {{jsxref("String.substr", "substr")}} | Return the specified subset of the string, either by specifying the start and end indexes or the start index and a length. |
| {{jsxref("String.match", "match")}}, {{jsxref("String.matchAll", "matchAll")}}, {{jsxref("String.replace", "replace")}}, {{jsxref("String.search", "search")}} | Work with regular expressions. |
| {{jsxref("String.toLowerCase", "toLowerCase")}}, {{jsxref("String.toUpperCase", "toUpperCase")}} |
Return the string in all lowercase or all uppercase, respectively. |
| {{jsxref("String.normalize", "normalize")}} | Returns the Unicode Normalization Form of the calling string value. |
| {{jsxref("String.repeat", "repeat")}} | Returns a string consisting of the elements of the object repeated the given times. |
| {{jsxref("String.trim", "trim")}} | Trims whitespace from the beginning and end of the string. |
Template literals are string literals allowing embedded expressions. You can use multi-line strings and string interpolation features with them.
Template literals are enclosed by the back-tick (` `) (grave accent) character instead of double or single quotes. Template literals can contain place holders. These are indicated by the Dollar sign and curly braces (${expression}).
Any new line characters inserted in the source are part of the template literal. Using normal strings, you would have to use the following syntax in order to get multi-line strings:
console.log('string text line 1\n\
string text line 2');
// "string text line 1
// string text line 2"
To get the same effect with multi-line strings, you can now write:
console.log(`string text line 1 string text line 2`); // "string text line 1 // string text line 2"
In order to embed expressions within normal strings, you would use the following syntax:
const five = 5;
const ten = 10;
console.log('Fifteen is ' + (five + ten) + ' and not ' + (2 * five + ten) + '.');
// "Fifteen is 15 and not 20."
Now, with template literals, you are able to make use of the syntactic sugar making substitutions like this more readable:
const five = 5;
const ten = 10;
console.log(`Fifteen is ${five + ten} and not ${2 * five + ten}.`);
// "Fifteen is 15 and not 20."
For more information, read about Template literals in the JavaScript reference.
The {{jsxref("Intl")}} object is the namespace for the ECMAScript Internationalization API, which provides language sensitive string comparison, number formatting, and date and time formatting. The constructors for {{jsxref("Collator")}}, {{jsxref("NumberFormat")}}, and {{jsxref("DateTimeFormat")}} objects are properties of the Intl object.
The {{jsxref("DateTimeFormat")}} object is useful for formatting date and time. The following formats a date for English as used in the United States. (The result is different in another time zone.)
const msPerDay = 24 * 60 * 60 * 1000;
// July 17, 2014 00:00:00 UTC.
const july172014 = new Date(msPerDay * (44 * 365 + 11 + 197));
const options = { year: '2-digit', month: '2-digit', day: '2-digit',
hour: '2-digit', minute: '2-digit', timeZoneName: 'short' };
const americanDateTime = new Intl.DateTimeFormat('en-US', options).format;
console.log(americanDateTime(july172014)); // 07/16/14, 5:00 PM PDT
The {{jsxref("NumberFormat")}} object is useful for formatting numbers, for example currencies.
const gasPrice = new Intl.NumberFormat('en-US',
{ style: 'currency', currency: 'USD',
minimumFractionDigits: 3 });
console.log(gasPrice.format(5.259)); // $5.259
const hanDecimalRMBInChina = new Intl.NumberFormat('zh-CN-u-nu-hanidec',
{ style: 'currency', currency: 'CNY' });
console.log(hanDecimalRMBInChina.format(1314.25)); // ￥ 一,三一四.二五
The {{jsxref("Collator")}} object is useful for comparing and sorting strings.
For example, there are actually two different sort orders in German, phonebook and dictionary. Phonebook sort emphasizes sound, and it’s as if “ä”, “ö”, and so on were expanded to “ae”, “oe”, and so on prior to sorting.
const names = ['Hochberg', 'Hönigswald', 'Holzman'];
const germanPhonebook = new Intl.Collator('de-DE-u-co-phonebk');
// as if sorting ["Hochberg", "Hoenigswald", "Holzman"]:
console.log(names.sort(germanPhonebook.compare).join(', '));
// logs "Hochberg, Hönigswald, Holzman"
Some German words conjugate with extra umlauts, so in dictionaries it’s sensible to order ignoring umlauts (except when ordering words differing only by umlauts: schon before schön).
const germanDictionary = new Intl.Collator('de-DE-u-co-dict');
// as if sorting ["Hochberg", "Honigswald", "Holzman"]:
console.log(names.sort(germanDictionary.compare).join(', '));
// logs "Hochberg, Holzman, Hönigswald"
For more information about the {{jsxref("Intl")}} API, see also Introducing the JavaScript Internationalization API.