Should JavaScript add a semicolon?

The semicolon in JavaScript is optional. Whether to add a semicolon or not is mainly a matter of coding style, but don’t add it for a while and not add it for a while. Usually, if a statement begins with "(", "[", "/", " ", or "-", it may be interpreted as part of the previous line of statements; in this case, you can defensively add a semicolon at the beginning of the line .

The operating environment of this tutorial: windows7 system, javascript version 1.8.5, Dell G3 computer.

The semicolon in JavaScript is optional Yes, adding semicolons or not is mainly a matter of coding style. One style is to use semicolons to clearly end statements, even if these semicolons are not necessary; the other style is to avoid adding semicolons as much as possible, only when necessary. Only add it.

Note: You can add semicolon or not add it, but don’t add it for a while and then not add it for a while.

The role of the semicolon in the code

The main function of the semicolon is: as an assertion of a statement (EOS), it is used to end a program statement in order to allow parsing The compiler parses the program correctly. In many C-Style languages, using a semicolon to explicitly end a line of statements is mainly to reduce the cost of language compiler development. But modern compilers are smart enough and can handle multi-line statements well. Many languages ​​do not need to explicitly use a semicolon at the end, such as: Go, Scala, Ruby, Python, Swift, Groovy, etc.

Although JavaScript is a C-like language, it is different from C and Java , semicolons are also optional in JavaScript, and it uses Auto Semicolon Insertion (ASI).

JavaScript automatic semicolon insertion mechanism

JavaScript has a mechanism for automatic semicolon insertion (Automatic Semicolon Insertion), referred to as ASI. There is a clear description of Automatic Semicolon Insertion in ECMA-262 - Automatic Semicolon Insertion:

• The program is parsed from left to right. When encountering a token (offending token) that does not conform to any grammatical production, a semicolon will be automatically inserted in front of the offending token as long as one of the following conditions is met.

At least one newline character (LineTerminator) separates the offending token from the previous token.

The offending token is }.

• From the left By the right parser, the tokens input stream has ended. When the parser cannot parse the input token stream into a single complete ECMAScript program, a semicolon is automatically inserted at the end of the input stream.

• Parse the program from left to right and encounter a token that is allowed by some grammar productions, but it is a restricted operation (Restricted Productions). When at least one newline character separates the object restricted token and the previous token, then a semicolon is automatically inserted in front of the restricted token.

  However, the above rule has an additional precedence condition: if the semicolon is inserted and the parsed result is an empty statement, or if the semicolon is inserted and it becomes one of the two semicolons at the head of a for statement, then no A semicolon is automatically inserted.

Note: The translation of the above instructions is awkward. You can read it several times or read the original English text ECMA-262 - Automatic Semicolon Insertion

Semicolon automatic Insertion situation

To briefly summarize, automatic semicolon insertion is based on line replacement. The parser will try to merge the new line into the current line if and only if it meets the ASI rules. Treat new lines as independent statements

The main automatic insertion rules are as follows:

• When a new line is merged into the current line, it will constitute an illegal statement When it cannot be parsed correctly, a semicolon

• will be automatically inserted when a new line begins with }, which is the end of the code block, a semicolon ## will be automatically inserted.

• #When ending with a

  return statement, a semicolon is automatically inserted at the end of the line

• When ending with a

  break When the statement ends, a semicolon is automatically inserted at the end of the line

• When the statement ends with

  throw, a semicolon is automatically inserted at the end of the line

• When ending with the

  continue statement, a semicolon is automatically inserted at the end of the line

• When ending with the ES6 yield statement, a semicolon is automatically inserted at the end of the line

• , -- The suffix expression is used as the beginning of a new line, and a semicolon is automatically inserted at the beginning of the line

• The automatic insertion number at the end of the source code file

As shown above, if there is no semicolon, an error will be reported when running this code.

Case in which the semicolon cannot be omitted

Normally, if the statement starts with

(,[ , /, , - may be interpreted as part of the previous line of statements. In practice, /, # Statements starting with ##, - are rare. However, statements starting with (, [ are very common, and I can usually defend them at the beginning of the line. Add a semicolon if necessary.<p> Earlier, we learned about the automatic insertion of semicolons, now let’s look at some examples </p><pre class="brush:php;toolbar:false">let hey = &amp;#39;hey&amp;#39; [&amp;#39;liu&amp;#39;,&amp;#39;liuxing&amp;#39;].forEach(console.log)</pre><p> Think about the result above for a second? Running this code will throw </p><pre class="brush:php;toolbar:false">Uncaught TypeError: Cannot read property &amp;#39;forEach&amp;#39; of undefined</pre><p>Based on rule 1. The above code will be parsed into the following code</p><pre class="brush:php;toolbar:false">let hey = &amp;#39;hey&amp;#39;[&amp;#39;liu&amp;#39;,&amp;#39;liuxing&amp;#39;].forEach(console.log)</pre><p>It can be seen that using the statement starting with <code>[, the preceding Without a semicolon, an error may occur.

Let’s look at an example code of a statement starting with (

const a = 1
const b = 2
const c = a + b
(a + b).toString()

Do you think the result of the above code is "3"? But in fact it will throw an error b is not a function, because according to ASI rules, it will be parsed into the following code:

const a = 1;const b = 2;const c = a + b(a + b).toString()

We will not introduce them one by one/, , - is used as the beginning of a statement. This situation is relatively rare. You can try it yourself. Just remember that the statement starts with (, [ , /, , - at the beginning, you need to add a semicolon before the statement!

Let’s take a look at using return In this case,

(() => {
  return
  {
    name: 'Liu Xing'
  }
})()

is your expected value to return an object with name, but it returns undefined. This is ASI automatically in A semicolon is added to return. Here you need to have correct line breaks to ensure that the code runs correctly. We can see that in addition to correct semicolons, we also need correct and reasonable line breaks to make the code structure clearer.

Summary

We understand JavaScript’s automatic semicolon insertion mechanism and when JavaScript will automatically add semicolons. In (, [, /, , - requires us to add a semicolon accurately at the beginning. Automatic Semicolon Insertion The mechanism provides us with two options, to add a semicolon or not? It all depends on the preference of you or your team. Now we also have prettier, Eslint and other tools to automatically unify the style.

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