How to prevent JavaScript from automatically inserting semicolons

In JavaScript, there is an automatic insertion mechanism for the semicolon at the end of the line. In this way, some friends can tolerate forgetting to enter the semicolon. Of course, it's best to get into the habit of typing semicolons and understand how JavaScript handles omitting semicolons, because this knowledge will help you understand code without semicolons.

The JavaScript language has a mechanism: during parsing, a semicolon can be automatically inserted after a sentence to modify the missing semicolon separator at the end of the sentence. However, because this automatically inserted semicolon conflicts with another mechanism of the JavaScript language, which is that all whitespace characters are ignored, the program can use whitespace to format the code.

The conflict between these two mechanisms can easily cover up more serious parsing errors. Semicolons are sometimes inserted at inappropriate times. For example, automatically inserting a semicolon in a return statement will have the following consequences: If the return statement is to return a value, the beginning of the expression of this value must be on the same line as return, for example:

 var f = function（）{
 return
 {
 status: true
 };
 }

It seems that an object containing the status member element is returned here. Unfortunately, JavaScript automatically inserts a semicolon so that it returns undefined, causing the object to be returned below to be ignored.

When the automatic insertion of semicolons causes the program to be misunderstood, there will be no warning. This problem can be avoided by placing { at the end of the previous line instead of at the beginning of the next line, for example:

 var f = function（）{
 return {
 status: true
 };
 }

In order to avoid errors caused by omitting semicolons, it is recommended Develop a good habit. Regardless of whether the statement in a line is complete or not, as long as it is a complete statement, a semicolon must be added to indicate the end of the sentence.

In order to facilitate reading, when a long sentence needs to be displayed on separate lines, it should be ensured that no complete logical semantics can be formed in one line. For example, the following code is a continuous assignment statement. You can view their relationship more clearly by displaying it in separate lines. This kind of branching shows that since independent logical semantics cannot be formed in one line, JavaScript will not treat each line as an independent sentence, so there will be no ambiguity.

 var a =
 b =
 c = 4;

The above statement is displayed as follows in one line: var a = b = c = 4;

For the following statement, if it is not correct When displayed in separate lines, ambiguity can easily arise. The meaning of this sentence: Define a variable i, and then assign a value to it. If the variable a is true, the value is assigned to 1. Otherwise, the variable b is judged. If b is true, the value is assigned to 2. Otherwise, the variable c is judged. If c is true, the assigned value is 3, otherwise the assigned value is 4.

 var i = a ? 1 : b ? 2 : c ? 3 : 4;

The following branch display is wrong, because the expression a? 1: b can form independent logical semantics , so JavaScript will automatically add a semicolon after it to represent an independent sentence.

 var i = a ? 1: b
 ? 2 : c
 ? 3 : 4;

Safe methods should be displayed in separate lines as follows, so that each line cannot form independent semantics.

 var i = a ? 1
 : b ? 2
 : c ? 3
 : 4;

In short, when writing code, you should develop a good habit of using semicolons to end sentences. All complete sentences should be separated by semicolons. Sentences displayed on separate lines should ensure that a single line cannot easily form an independent legal logical semantic

PS: Detailed example of javascript automatically adding a semicolon at the end of the line

Semicolon (; ), usually used at the end of a line of statements, the code is as follows:

var webName = "php中文网";
var url = www.jb51.net;

The above code adds a semicolon after each declaration statement, which requires no further explanation.

var webName = "php中文网"
var url = www.jb51.net

In fact, the semicolon can be omitted, but the omitted semicolon will be automatically added during the compilation phase.
Let’s look at a code example:

function done(webName){
 return
 webName
}
console.log(done("php中文网"));

Since the compiler automatically adds a semicolon after each line, the output value is: undefined, not "php Chinese net".

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