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现在我们学习基本的字符串语法, 让我们开始思考一下我们可以对内置方法的字符串做什么有用的操作,例如查找文本字符串的长度,加入和分割字符串, 将字符串中的一个字符替换为另一个字符。

前提: 基本的电脑知识, 对HTML和CSS有一定的了解,最好是懂一点javascript知识。
目的: 明白字符串这个对象,学会使用字符串的基本方法去处理字符串

字符串是对象

我们可以说出一样东西之前,一定是我们以前看见过的 —在javascript中,一切都是对象。例如我们创建一个字符串。

var string = 'This is my string';

一旦你的变量成为字符串对象实例, 你就可以有大量的原型和方法编辑它. 如果你进入{{ jsxref("String") }}对象页并观察页面旁边的列表你就会明白这一点。

可能现在你的大脑开始迷糊了,不要担心! 在你的学习进程中你真的不需要过早地理解大部分这方面知识,但是接下来我们这儿要看的是你要经常使用的一些知识。

现在我们在控制台中加些示例 ,我们已经提供了以下示例(你可在单独打开控制台标签或窗口,或者选择使用浏览器开发者控制台)

发现字符串的长度

这很简单 — 你可以很轻松的使用 length 属性. 尝试输入以下的两行代码:

var browserType = 'mozilla';
browserType.length;

这个结果应该返回一个数字:7,因为"mozilla"的长度为7个字符. 说字符串的长度有用是有很多原因的, 例如,你可能想算出一连串名字的长度,并用名字长度来作为名字排序的依据,亦或让一个用户知道他输入的用户名太长,已经超出了输入的字符串长度限制。

Retrieving a specific string character

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];

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 "mozilla" is 7, but because the count starts at 0, the character position is 6, hence us needing length-1. You could use this to, for example, find the first letter of a series of strings and order them alphabetically.

Finding a substring inside a string and extracting it

  1. 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 indexOf() method, which takes a single parameter — the substring you want to search for. Try this:
    browserType.indexOf('zilla');
    This gives us a result of 2, because the substring "zilla" starts at position 2 (0, 1, 2  — so 3 characters in) inside "mozilla". Such code could be used to filter strings. For example, say we have a list of web addresses and only want to print out the ones that contain "mozilla".
  1. This can be done in another way, which is possibly even more effective. Try the following:
    browserType.indexOf('vanilla');
    This should give you a result of -1 — this is returned when the substring, in this case 'vanilla', is not found in the main string.

    You could use this to find all instances of strings that don't contain the substring 'mozilla', or do, if you use the negation operator, as shown below. You could do something like this:
    if(browserType.indexOf('mozilla') !== -1) {
      // do stuff with the string
    }
  2. When you know where a substring starts inside a string, and you know at which character you want it to end, slice() can be used to extract it. Try the following:
    browserType.slice(0,3);
    This returns "moz" — the first parameter is the character position to start extracting at, and the second parameter is the character position after the last one to be extracted. So the slice happens from the first position, up to, but not including, the last position.
     
  3. Also, 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, rather, 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);
    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. 

Note: The second parameter of slice() is optional: if you don't include it, the slice ends at the end of the original string. There are other options too; study the slice() page to see what else you can find out.

Changing case

The string methods toLowerCase() and toUpperCase() take a string and convert all the characters to lower or upper case, 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:

var radData = 'My NaMe Is MuD';
radData.toLowerCase();
radData.toUpperCase();

Updating parts of a string

You can replace one substring inside a string with another substring using the replace() method. This works very simply at a basic level, although there are some advanced things you can do with it that we won't go into yet.

It takes two parameters — the string you want to replace, and the string you want to replace it with. Try this example:

browserType.replace('moz','van');

Active learning examples

In this section we'll get you to try your handing 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.

Filtering greeting messages

In the first exercise we'll start you off simple — we have an array of greetings 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.

  1. First 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?
  2. You'll then need to write a conditional test of the form operand1 operator operand2. Is the thing on the left equal to the thing on the right? Or in this case, does the method call on the left return the result on the right?
  3. Hint: In this case it is probably more useful to test whether the method call isn't equal to a certain result.

Fixing capitalization

In this exercise we have the names of UK cities, 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:

  1. Convert the whole of the string contained in the input variable to lower case and store it in a new variable.
  2. Grab the first letter of the string in this new variable and store it in another variable.
  3. Using this latest variable as a substring, replace the first letter of the lowercase string with the first letter of the lowercase string changed to upper case. Store the result of this replace procedure in another new variable.
  4. Change the value of the result variable to equal to the final result, not the input.

Note: 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.

Making new strings from old parts

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 semi-colon, 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:

  1. Extract the three-letter station code and store it in a new variable.
  2. Find the character index number of the semi-colon.
  3. Extract the human-readable station name using the semi-colon character index number as a reference point, and store it in a new variable.
  4. Concatenate the two new variables and a string literal to make the final string.
  5. Change the value of the result variable to equal to the final string, not the input.

Conclusion

You can't escape the fact that being able to handle words and sentences in programming is very important — particularly in JavaScript, as web sites 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.

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