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Is this the expected behavior with the return keyword in JavaScript?

I was on the verge of pulling all my hair out today by a stupid mistake. The point is, I'm not sure if this is the expected behavior.

With my C # habits, I wrote the following javascript code and tried to figure out what was going wrong because it didn't return anything and didn't even log any errors to the console:

this.fullName = ko.computed(function () {

    return
        this.firstName() + " " + this.lastName();

}, this);

Then (half an hour later) I changed the code a below and it worked:

this.fullName = ko.computed(function () {

    return this.firstName() + " " + this.lastName();

}, this);

Is this the expected behavior?

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5 answers


Yes. This is an implicit semicolon.

Here's a similar situation with clarifications: http://premii.com/lab/js_implicit_semicolon_and_return.php



In short: your first snippet is interpreted as:

this.fullName = ko.computed(function () {
    return;
    this.firstName() + " " + this.lastName();
}, this);
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Yes The ECMA specification , which is a Javascript specification, defines behavior like this

Certain ECMAScript statements (empty statement, ... return statement and throw statement) must be terminated with a semicolon. Such semicolons can always appear explicitly in the source text. For convenience , however, such semicolons may be omitted from the source text in certain situations. These situations are described that semicolons are automatically inserted into the source token stream in these situations.

Further

When a continue, break, return, or throw marker is encountered and LineTerminator is encountered before the next token, a semicolon is automatically inserted after a continuation, break, return, or token throw.

The return expression or throw statement must begin on the same line as the return or throw token.

And they gave an example of:

Source

return
a + b

is converted by automatic semicolon insertion into the following:



return;
a + b;

So your first code will be interpreted as:

return;
        this.firstName() + " " + this.lastName();

with an automatically added semitone at the end of the return.

Thus, the spec provides practical advice to counter these situations in general javascript:

The postfix ++ or - operator must appear on the same line as its operand.

The expression in a return or throw statement must begin with the same as a return or throw marker.

The identifier in a break or continue statement must be on the same as a break or continuation token.

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Yes it is. Javascript doesn't need a comma at the end of the line to complete the statement. In this case, it just returns.

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Yes, this semicolon in javascript is generally not required, so there is no exact way to distinguish a complete statement using semicolons alone. Check out this book I hear, it's a great resource for JS, good javascript pieces by douglas crockford.

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Javascript has a pen that is Automatic semicolon which is not very good, you shouldn't depend on it and add semicolons when the statement ends.

In your case, javascript adds ;

to the statement return

. so it is the same as:

return;
this.firstName() + " " + this.lastName();
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