Strings

In JavaScript, the textual data is stored as strings. There is no separate type for a single character.

The internal format for strings is always UTF-16, it is not tied to the page encoding.

Quotes

Let’s recall the kinds of quotes.

Strings can be enclosed within either single quotes, double quotes or backticks:

let single = 'single-quoted';
let double = "double-quoted";

let backticks = `backticks`;

Single and double quotes are essentially the same. Backticks, however, allow us to embed any expression into the string, including function calls:

function sum(a, b) {
  return a + b;
}

alert(`1 + 2 = ${sum(1, 2)}.`); // 1 + 2 = 3.

Another advantage of using backticks is that they allow a string to span multiple lines:

let guestList = `Guests:
 * John
 * Pete
 * Mary
`;

alert(guestList); // a list of guests, multiple lines

If we try to use single or double quotes in the same way, there will be an error:

let guestList = "Guests:  // Error: Unexpected token ILLEGAL
  * John";

Single and double quotes come from ancient times of language creation when the need for multiline strings was not taken into account. Backticks appeared much later and thus are more versatile.

Backticks also allow us to specify a “template function” before the first backtick. The syntax is: func`string`. The function func is called automatically, receives the string and embedded expressions and can process them. You can read more about it in the docs. This is called “tagged templates”. This feature makes it easier to wrap strings into custom templating or other functionality, but it is rarely used.

Special characters

It is still possible to create multiline strings with single quotes by using a so-called “newline character”, written as \n, which denotes a line break:

let guestList = "Guests:\n * John\n * Pete\n * Mary";

alert(guestList); // a multiline list of guests

For example, these two lines describe the same:

alert( "Hello\nWorld" ); // two lines using a "newline symbol"

// two lines using a normal newline and backticks
alert( `Hello
World` );

There are other, less common “special” characters as well. Here’s the list:

Character Description
\b Backspace
\f Form feed
\n New line
\r Carriage return
\t Tab
\uNNNN A unicode symbol with the hex code NNNN, for instance \u00A9 – is a unicode for the copyright symbol ©. It must be exactly 4 hex digits.
\u{NNNNNNNN} Some rare characters are encoded with two unicode symbols, taking up to 4 bytes. This long unicode requires braces around it.

Examples with unicode:

alert( "\u00A9" ); // ©
alert( "\u{20331}" ); // 佫, a rare chinese hieroglyph (long unicode)
alert( "\u{1F60D}" ); // 😍, a smiling face symbol (another long unicode)

All special characters start with a backslash character \. It is also called an “escape character”.

We would also use it if we want to insert a quote into the string.

For instance:

alert( 'I\'m the Walrus!' ); // I'm the Walrus!

As you can see, we have to prepend the inner quote by the backslash \', because otherwise it would indicate the string end.

Of course, that refers only to the quotes that are same as the enclosing ones. So, as a more elegant solution, we could switch to double quotes or backticks instead:

alert( `I'm the Walrus!` ); // I'm the Walrus!

Note that the backslash \ serves for the correct reading of the string by JavaScript, then disappears. The in-memory string has no \. You can clearly see that in alert from the examples above.

But what if we need to show an actual backslash \ within the string?

That’s possible, but we need to double it like \\:

alert( `The backslash: \\` ); // The backslash: \

String length

The length property has the string length:

alert( `My\n`.length ); // 3

Note that \n is a single “special” character, so the length is indeed 3.

length is a property

People with a background in some other languages sometimes mistype by calling str.length() instead of just str.length. That doesn’t work.

Please note that str.length is a numeric property, not a function. There is no need to add brackets after it.

Accessing characters

To get a character at position pos, use square brackets [pos] or call the method str.charAt(pos). The first character starts from the zero position:

let str = `Hello`;

// the first character
alert( str[0] ); // H
alert( str.charAt(0) ); // H

// the last character
alert( str[str.length - 1] ); // o

The square brackets are a modern way of getting a character, while charAt exists mostly for historical reasons.

The only difference between them is that if no character is found, [] returns undefined, and charAt returns an empty string:

let str = `Hello`;

alert( str[1000] ); // undefined
alert( str.charAt(1000) ); // '' (an empty string)

We can also iterate over characters using for..of:

for (let char of "Hello") {
  alert(char); // H,e,l,l,o (char becomes "H", then "e", then "l" etc)
}

Strings are immutable

Strings can’t be changed in JavaScript. It is impossible to change a character.

Let’s try it to show that it doesn’t work:

let str = 'Hi';

str[0] = 'h'; // error
alert( str[0] ); // doesn't work

The usual workaround is to create a whole new string and assign it to str instead of the old one.

For instance:

let str = 'Hi';

str = 'h' + str[1];  // replace the string

alert( str ); // hi

In the following sections we’ll see more examples of this.

Changing the case

Methods toLowerCase() and toUpperCase() change the case:

alert( 'Interface'.toUpperCase() ); // INTERFACE
alert( 'Interface'.toLowerCase() ); // interface

Or, if we want a single character lowercased:

alert( 'Interface'[0].toLowerCase() ); // 'i'

Searching for a substring

There are multiple ways to look for a substring within a string.

str.indexOf

The first method is str.indexOf(substr, pos).

It looks for the substr in str, starting from the given position pos, and returns the position where the match was found or -1 if nothing can be found.

For instance:

let str = 'Widget with id';

alert( str.indexOf('Widget') ); // 0, because 'Widget' is found at the beginning
alert( str.indexOf('widget') ); // -1, not found, the search is case-sensitive

alert( str.indexOf("id") ); // 1, "id" is found at the position 1 (..idget with id)

The optional second parameter allows us to search starting from the given position.

For instance, the first occurrence of "id" is at position 1. To look for the next occurrence, let’s start the search from position 2:

let str = 'Widget with id';

alert( str.indexOf('id', 2) ) // 12

If we’re interested in all occurrences, we can run indexOf in a loop. Every new call is made with the position after the previous match:

let str = 'As sly as a fox, as strong as an ox';

let target = 'as'; // let's look for it

let pos = 0;
while (true) {
  let foundPos = str.indexOf(target, pos);
  if (foundPos == -1) break;

  alert( `Found at ${foundPos}` );
  pos = foundPos + 1; // continue the search from the next position
}

The same algorithm can be layed out shorter:

let str = "As sly as a fox, as strong as an ox";
let target = "as";

let pos = -1;
while ((pos = str.indexOf(target, pos + 1)) != -1) {
  alert( pos );
}
str.lastIndexOf(pos)

There is also a similar method str.lastIndexOf(pos) that searches from the end of a string to its beginning.

It would list the occurrences in the reverse order.

There is a slight inconvenience with indexOf in the if test. We can’t put it in the if like this:

let str = "Widget with id";

if (str.indexOf("Widget")) {
    alert("We found it"); // doesn't work!
}

The alert in the example above doesn’t show because str.indexOf("Widget") returns 0 (meaning that it found the match at the starting position). Right, but if considers 0 to be false.

So, we should actually check for -1, like this:

let str = "Widget with id";

if (str.indexOf("Widget") != -1) {
    alert("We found it"); // works now!
}
The bitwise NOT trick

One of the old tricks used here is the bitwise NOT ~ operator. It converts the number to a 32-bit integer (removes the decimal part if exists) and then reverses all bits in its binary representation.

For 32-bit integers the call ~n means exactly the same as -(n+1) (due to IEEE-754 format).

For instance:

alert( ~2 ); // -3, the same as -(2+1)
alert( ~1 ); // -2, the same as -(1+1)
alert( ~0 ); // -1, the same as -(0+1)
alert( ~-1 ); // 0, the same as -(-1+1)

As we can see, ~n is zero only if n == -1.

So, the test if ( ~str.indexOf("...") ) is truthy that the result of indexOf is not -1. In other words, when there is a match.

People use it to shorten indexOf checks:

let str = "Widget";

if (~str.indexOf("Widget")) {
  alert( 'Found it!' ); // works
}

It is usually not recommended to use language features in a non-obvious way, but this particular trick is widely used in old code, so we should understand it.

Just remember: if (~str.indexOf(...)) reads as “if found”.

includes, startsWith, endsWith

The more modern method str.includes(substr, pos) returns true/false depending on whether str contains substr within.

It’s the right choice if we need to test for the match, but don’t need its position:

alert( "Widget with id".includes("Widget") ); // true

alert( "Hello".includes("Bye") ); // false

The optional second argument of str.includes is the position to start searching from:

alert( "Midget".includes("id") ); // true
alert( "Midget".includes("id", 3) ); // false, from position 3 there is no "id"

The methods str.startsWith and str.endsWith do exactly what they say:

alert( "Widget".startsWith("Wid") ); // true, "Widget" starts with "Wid"
alert( "Widget".endsWith("get") );   // true, "Widget" ends with "get"

Getting a substring

There are 3 methods in JavaScript to get a substring: substring, substr and slice.

str.slice(start [, end])

Returns the part of the string from start to (but not including) end.

For instance:

let str = "stringify";
alert( str.slice(0, 5) ); // 'strin', the substring from 0 to 5 (not including 5)
alert( str.slice(0, 1) ); // 's', from 0 to 1, but not including 1, so only character at 0

If there is no second argument, then slice goes till the end of the string:

let str = "stringify";
alert( str.slice(2) ); // ringify, from the 2nd position till the end

Negative values for start/end are also possible. They mean the position is counted from the string end:

let str = "stringify";

// start at the 4th position from the right, end at the 1st from the right
alert( str.slice(-4, -1) ); // gif
str.substring(start [, end])

Returns the part of the string between start and end.

This is almost the same as slice, but it allows start to be greater than end.

For instance:

let str = "stringify";

// these are same for substring
alert( str.substring(2, 6) ); // "ring"
alert( str.substring(6, 2) ); // "ring"

// ...but not for slice:
alert( str.slice(2, 6) ); // "ring" (the same)
alert( str.slice(6, 2) ); // "" (an empty string)

Negative arguments are (unlike slice) not supported, they are treated as 0.

str.substr(start [, length])

Returns the part of the string from start, with the given length.

In contrast with the previous methods, this one allows us to specify the length instead of the ending position:

let str = "stringify";
alert( str.substr(2, 4) ); // ring, from the 2nd position get 4 characters

The first argument may be negative, to count from the end:

let str = "stringify";
alert( str.substr(-4, 2) ); // gi, from the 4th position get 2 characters

Let’s recap these methods to avoid any confusion:

method selects… negatives
slice(start, end) from start to end allows negatives
substring(start, end) between start and end negative values mean 0
substr(start, length) from start get length characters allows negative start
Which one to choose?

All of them can do the job. Formally, substr has a minor drawback: it is described not in the core JavaScript specification, but in Annex B, which covers browser-only features that exist mainly for historical reasons. So, non-browser environments may fail to support it. But in practice it works everywhere.

The author finds himself using slice almost all the time.

Comparing strings

As we know from the chapter Comparisons, strings are compared character-by-character in alphabetical order.

Although, there are some oddities.

  1. A lowercase letter is always greater than the uppercase:

    alert( 'a' > 'Z' ); // true
  2. Letters with diacritical marks are “out of order”:

    alert( 'Österreich' > 'Zealand' ); // true

    This may lead to strange results if we sort these country names. Usually people would expect Zealand to come after Österreich in the list.

To understand what happens, let’s review the internal representation of strings in JavaScript.

All strings are encoded using UTF-16. That is: each character has a corresponding numeric code. There are special methods that allow to get the character for the code and back.

str.codePointAt(pos)

Returns the code for the character at position pos:

// different case letters have different codes
alert( "z".codePointAt(0) ); // 122
alert( "Z".codePointAt(0) ); // 90
String.fromCodePoint(code)

Creates a character by its numeric code

alert( String.fromCodePoint(90) ); // Z

We can also add unicode characters by their codes using \u followed by the hex code:

// 90 is 5a in hexadecimal system
alert( '\u005a' ); // Z

Now let’s see the characters with codes 65..220 (the latin alphabet and a little bit extra) by making a string of them:

let str = '';

for (let i = 65; i <= 220; i++) {
  str += String.fromCodePoint(i);
}
alert( str );
// ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~€‚ƒ„
// ¡¢£¤¥¦§¨©ª«¬­®¯°±²³´µ¶·¸¹º»¼½¾¿ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖ×ØÙÚÛÜ

See? Capital characters go first, then a few special ones, then lowercase characters.

Now it becomes obvious why a > Z.

The characters are compared by their numeric code. The greater code means that the character is greater. The code for a (97) is greater than the code for Z (90).

  • All lowercase letters go after uppercase letters because their codes are greater.
  • Some letters like Ö stand apart from the main alphabet. Here, it’s code is greater than anything from a to z.

Correct comparisons

The “right” algorithm to do string comparisons is more complex than it may seem, because alphabets are different for different languages. The same-looking letter may be located differently in different alphabets.

So, the browser needs to know the language to compare.

Luckily, all modern browsers (IE10- requires the additional library Intl.JS) support the internationalization standard ECMA 402.

It provides a special method to compare strings in different languages, following their rules.

The call str.localeCompare(str2):

  • Returns 1 if str is greater than str2 according to the language rules.
  • Returns -1 if str is less than str2.
  • Returns 0 if they are equal.

For instance:

alert( 'Österreich'.localeCompare('Zealand') ); // -1

This method actually has two additional arguments specified in the documentation, which allows it to specify the language (by default taken from the environment) and setup additional rules like case sensitivity or should "a" and "á" be treated as the same etc.

Internals, Unicode

Advanced knowledge

The section goes deeper into string internals. This knowledge will be useful for you if you plan to deal with emoji, rare mathematical of hieroglyphs characters or other rare symbols.

You can skip the section if you don’t plan to support them.

Surrogate pairs

Most symbols have a 2-byte code. Letters in most european languages, numbers, and even most hieroglyphs, have a 2-byte representation.

But 2 bytes only allow 65536 combinations and that’s not enough for every possible symbol. So rare symbols are encoded with a pair of 2-byte characters called “a surrogate pair”.

The length of such symbols is 2:

alert( '𝒳'.length ); // 2, MATHEMATICAL SCRIPT CAPITAL X
alert( '😂'.length ); // 2, FACE WITH TEARS OF JOY
alert( '𩷶'.length ); // 2, a rare chinese hieroglyph

Note that surrogate pairs did not exist at the time when JavaScript was created, and thus are not correctly processed by the language!

We actually have a single symbol in each of the strings above, but the length shows a length of 2.

String.fromCodePoint and str.codePointAt are few rare methods that deal with surrogate pairs right. They recently appeared in the language. Before them, there were only String.fromCharCode and str.charCodeAt. These methods are actually the same as fromCodePoint/codePointAt, but don’t work with surrogate pairs.

But, for instance, getting a symbol can be tricky, because surrogate pairs are treated as two characters:

alert( '𝒳'[0] ); // strange symbols...
alert( '𝒳'[1] ); // ...pieces of the surrogate pair

Note that pieces of the surrogate pair have no meaning without each other. So the alerts in the example above actually display garbage.

Technically, surrogate pairs are also detectable by their codes: if a character has the code in the interval of 0xd800..0xdbff, then it is the first part of the surrogate pair. The next character (second part) must have the code in interval 0xdc00..0xdfff. These intervals are reserved exclusively for surrogate pairs by the standard.

In the case above:

// charCodeAt is not surrogate-pair aware, so it gives codes for parts

alert( '𝒳'.charCodeAt(0).toString(16) ); // d835, between 0xd800 and 0xdbff
alert( '𝒳'.charCodeAt(1).toString(16) ); // dcb3, between 0xdc00 and 0xdfff

You will find more ways to deal with surrogate pairs later in the chapter Iterables. There are probably special libraries for that too, but nothing famous enough to suggest here.

Diacritical marks and normalization

In many languages there are symbols that are composed of the base character with a mark above/under it.

For instance, the letter a can be the base character for: àáâäãåā. Most common “composite” character have their own code in the UTF-16 table. But not all of them, because there are too many possible combinations.

To support arbitrary compositions, UTF-16 allows us to use several unicode characters. The base character and one or many “mark” characters that “decorate” it.

For instance, if we have S followed by the special “dot above” character (code \u0307), it is shown as Ṡ.

alert( 'S\u0307' ); // Ṡ

If we need an additional mark above the letter (or below it) – no problem, just add the necessary mark character.

For instance, if we append a character “dot below” (code \u0323), then we’ll have “S with dots above and below”: Ṩ.

For example:

alert( 'S\u0307\u0323' ); // Ṩ

This provides great flexibility, but also an interesting problem: two characters may visually look the same, but be represented with different unicode compositions.

For instance:

alert( 'S\u0307\u0323' ); // Ṩ, S + dot above + dot below
alert( 'S\u0323\u0307' ); // Ṩ, S + dot below + dot above

alert( 'S\u0307\u0323' == 'S\u0323\u0307' ); // false

To solve this, there exists a “unicode normalization” algorithm that brings each string to the single “normal” form.

It is implemented by str.normalize().

alert( "S\u0307\u0323".normalize() == "S\u0323\u0307".normalize() ); // true

It’s funny that in our situation normalize() actually brings together a sequence of 3 characters to one: \u1e68 (S with two dots).

alert( "S\u0307\u0323".normalize().length ); // 1

alert( "S\u0307\u0323".normalize() == "\u1e68" ); // true

In reality, this is not always the case. The reason being that the symbol Ṩ is “common enough”, so UTF-16 creators included it in the main table and gave it the code.

If you want to learn more about normalization rules and variants – they are described in the appendix of the Unicode standard: Unicode Normalization Forms, but for most practical purposes the information from this section is enough.

Summary

  • There are 3 types of quotes. Backticks allow a string to span multiple lines and embed expressions.
  • Strings in JavaScript are encoded using UTF-16.
  • We can use special characters like \n and insert letters by their unicode using \u....
  • To get a character, use: [].
  • To get a substring, use: slice or substring.
  • To lowercase/uppercase a string, use: toLowerCase/toUpperCase.
  • To look for a substring, use: indexOf, or includes/startsWith/endsWith for simple checks.
  • To compare strings according to the language, use: localeCompare, otherwise they are compared by character codes.

There are several other helpful methods in strings:

  • str.trim() – removes (“trims”) spaces from the beginning and end of the string.
  • str.repeat(n) – repeats the string n times.
  • …and more. See the manual for details.

Strings also have methods for doing search/replace with regular expressions. But that topic deserves a separate chapter, so we’ll return to that later.

Tasks

importance: 5

Write a function ucFirst(str) that returns the string str with the uppercased first character, for instance:

ucFirst("john") == "John";

Open the sandbox with tests.

We can’t “replace” the first character, because strings in JavaScript are immutable.

But we can make a new string based on the existing one, with the uppercased first character:

let newStr = str[0].toUpperCase() + str.slice(1);

There’s a small problem though. If str is empty, then str[0] is undefined, so we’ll get an error.

There are two variants here:

  1. Use str.charAt(0), as it always returns a string (maybe empty).
  2. Add a test for an empty string.

Here’s the 2nd variant:

function ucFirst(str) {
  if (!str) return str;

  return str[0].toUpperCase() + str.slice(1);
}

alert( ucFirst("john") ); // John

Open the solution with tests in the sandbox.

importance: 5

Write a function checkSpam(str) that returns true if str contains ‘viagra’ or ‘XXX’, otherwise `false.

The function must be case-insensitive:

checkSpam('buy ViAgRA now') == true
checkSpam('free xxxxx') == true
checkSpam("innocent rabbit") == false

Open the sandbox with tests.

To make the search case-insensitive, let’s bring the stirng to lower case and then search:

function checkSpam(str) {
  let lowerStr = str.toLowerCase();

  return lowerStr.includes('viagra') || lowerStr.includes('xxx');
}

alert( checkSpam('buy ViAgRA now') );
alert( checkSpam('free xxxxx') );
alert( checkSpam("innocent rabbit") );

Open the solution with tests in the sandbox.

importance: 5

Create a function truncate(str, maxlength) that checks the length of the str and, if it exceeds maxlength – replaces the end of str with the ellipsis character "…", to make its length equal to maxlength.

The result of the function should be the truncated (if needed) string.

For instance:

truncate("What I'd like to tell on this topic is:", 20) = "What I'd like to te…"

truncate("Hi everyone!", 20) = "Hi everyone!"

Open the sandbox with tests.

The maximal length must be maxlength, so we need to cut it a little shorter, to give space for the ellipsis.

Note that there is actually a single unicode character for an ellipsis. That’s not three dots.

function truncate(str, maxlength) {
  return (str.length > maxlength) ?
    str.slice(0, maxlength - 1) + '…' : str;
}

Open the solution with tests in the sandbox.

importance: 4

We have a cost in the form "$120". That is: the dollar sign goes first, and then the number.

Create a function extractCurrencyValue(str) that would extract the numeric value from such string and return it.

The example:

alert( extractCurrencyValue('$120') === 120 ); // true

Open the sandbox with tests.

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