Now that we've looked at the very basics of strings, let's move up a gear and start thinking about what useful operations we can do on strings with built-in methods, such as finding the length of a text string, joining and splitting strings, substituting one character in a string for another, and more.
| Prerequisites: | Basic computer literacy, a basic understanding of HTML and CSS, an understanding of what JavaScript is. |
|---|---|
| Objective: | To understand that strings are objects, and learn how to use some of the basic methods available on those objects to manipulate strings. |
Most things are objects in JavaScript. When you create a string, for example by using
const string = 'This is my string';
your variable becomes a string object instance, and as a result has a large number of properties and methods available to it. You can see this if you go to the
字符串
object page and look down the list on the side of the page!
Now, before your brain starts melting, don't worry! You really don't need to know about most of these early on in your learning journey. But there are a few that you'll potentially use quite often that we'll look at here.
Let's enter some examples into the browser developer console .
This is easy — you use the
length
property. Try entering the following lines:
const browserType = 'mozilla';
browserType.length;
This should return the number 7, because "mozilla" is 7 characters long. This is useful for many reasons; for example, you might want to find the lengths of a series of names so you can display them in order of length, or let a user know that a username they have entered into a form field is too long if it is over a certain length.
On a related note, you can return any character inside a string by using
square bracket notation
— this means you include square brackets (
[]
) on the end of your variable name. Inside the square brackets you include the number of the character you want to return, so for example to retrieve the first letter you'd do this:
browserType[0];
Remember: computers count from 0, not 1!
To retrieve the last character of
any
string, we could use the following line, combining this technique with the
length
property we looked at above:
browserType[browserType.length-1];
The length of the string "mozilla" is 7, but because the count starts at 0, the last character's position is 6; using
length-1
gets us the last character.
Sometimes you'll want to find if a smaller string is present inside a larger one (we generally say
if a substring is present inside a string
). This can be done using the
includes()
method, which takes a single
参数
— the substring you want to search for.
它返回
true
if the string contains the substring, and
false
否则。
const browserType = 'mozilla';
if (browserType.includes('zilla')) {
console.log('Found zilla!');
} else {
console.log('No zilla here!');
}
Often you'll want to know if a string starts or ends with a particular substring. This is a common enough need that there are two special methods for this:
startsWith()
and
endsWith()
:
const browserType = 'mozilla';
if (browserType.startsWith('zilla')) {
console.log('Found zilla!');
} else {
console.log('No zilla here!');
}
const browserType = 'mozilla';
if (browserType.endsWith('zilla')) {
console.log('Found zilla!');
} else {
console.log('No zilla here!');
}
You can extract a substring from a string using the
slice()
method. You pass it:
例如：
const browserType = 'mozilla';
console.log(browserType.slice(1, 4)); // "ozi"
The character at index
1
is
"o"
, and the character at index 4 is
"l"
. So we extract all characters starting at
"o"
and ending just before
"l"
, giving us
"ozi"
.
If you know that you want to extract all of the remaining characters in a string after a certain character, you don't have to include the second parameter. Instead, you only need to include the character position from where you want to extract the remaining characters in a string. Try the following:
browserType.slice(2); // "zilla"
This returns
"zilla"
— this is because the character position of 2 is the letter
"z"
, and because you didn't include a second parameter, the substring that was returned was all of the remaining characters in the string.
注意：
slice()
has other options too; study the
slice()
page to see what else you can find out.
The string methods
toLowerCase()
and
toUpperCase()
take a string and convert all the characters to lower- or uppercase, respectively. This can be useful for example if you want to normalize all user-entered data before storing it in a database.
Let's try entering the following lines to see what happens:
const radData = 'My NaMe Is MuD';
console.log(radData.toLowerCase());
console.log(radData.toUpperCase());
You can replace one substring inside a string with another substring using the
replace()
方法。
In this example, we're providing two parameters — the string we want to replace, and the string we want to replace it with:
const browserType = 'mozilla';
const updated = browserType.replace('moz','van');
console.log(updated); // "vanilla"
console.log(browserType); // "mozilla"
注意，
replace()
, like many string methods, doesn't change the string it was called on, but returns a new string. If you want to update the original
browserType
variable, you would have to do something like this:
let browserType = 'mozilla';
browserType = browserType.replace('moz','van');
console.log(browserType); // "vanilla"
Also note that we now have to declare
browserType
使用
let
, not
const
, because we are reassigning it.
In this section we'll get you to try your hand at writing some string manipulation code. In each exercise below, we have an array of strings, and a loop that processes each value in the array and displays it in a bulleted list. You don't need to understand arrays or loops right now — these will be explained in future articles. All you need to do in each case is write the code that will output the strings in the format that we want them in.
Each example comes with a "Reset" button, which you can use to reset the code if you make a mistake and can't get it working again, and a "Show solution" button you can press to see a potential answer if you get really stuck.
In the first exercise we'll start you off simple — we have an array of greeting card messages, but we want to sort them to list just the Christmas messages. We want you to fill in a conditional test inside the
if( ... )
structure, to test each string and only print it in the list if it is a Christmas message.
Think about how you could test whether the message in each case is a Christmas message. What string is present in all of those messages, and what method could you use to test whether it is present?
In this exercise we have the names of cities in the United Kingdom, but the capitalization is all messed up. We want you to change them so that they are all lower case, except for a capital first letter. A good way to do this is to:
city
variable to lower case and store it in a new variable.
result
variable to equal to the final result, not the
city
.
注意： A hint — the parameters of the string methods don't have to be string literals; they can also be variables, or even variables with a method being invoked on them.
In this last exercise, the array contains a bunch of strings containing information about train stations in the North of England. The strings are data items that contain the three-letter station code, followed by some machine-readable data, followed by a semicolon, followed by the human-readable station name. For example:
MAN675847583748sjt567654;Manchester Piccadilly
We want to extract the station code and name, and put them together in a string with the following structure:
MAN: Manchester Piccadilly
We'd recommend doing it like this:
result
variable to equal to the final string, not the
station
.
You've reached the end of this article, but can you remember the most important information? You can find some further tests to verify that you've retained this information before you move on — see Test your skills: Strings .
You can't escape the fact that being able to handle words and sentences in programming is very important — particularly in JavaScript, as websites are all about communicating with people. This article has given you the basics that you need to know about manipulating strings for now. This should serve you well as you go into more complex topics in the future. Next, we're going to look at the last major type of data we need to focus on in the short term — arrays.
最后修改： , 由 MDN 贡献者