I've heard Javascript inserts ";" automatically and that may cause problems.

Question

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What are the rules for Javascript's automatic semicolon insertion?

I've also heard that Go insert them too, but they followed a different approach

How does Javascript insert semicolons while interpreting?

Any reference would be helpful

Answer 1

The example that taught me the pitfalls of this particular feature was this one in the "Strangest language feature?" question. (Which is why I was for reopening that question, by the way - it is a valuable learning resource.)

Good reference material can be found here: What are the rules for Javascript's automatic semicolon insertion?

Answer 2

Javascript assumes end of statement at any line break where it's possible. For example this:

return
true;

is interpreted as:

return;
true;

turning the return and it's argument into two separate statements, which of course means that the function has no return value (returns undefined).

I wrote a blog entry about that a while back.

Answer 3

One of the biggest things I've found is, let's say you have a function that returns coordinates (or any object really), similar to this.

function getCoordinates() {
    return
        {
            x: 10,
            y: 10
        };
}

You would expect to get back an object right? WRONG! You get back undefined. The interpreter converts the code into this

function getCoordinates() {
    return;
        {
            x: 10,
            y: 10
        };
}

Since return by itself is a valid statement. You need to be sure to write the return as follows

function getCoordinates() {
    return {
            x: 10,
            y: 10
        };
}

Answer 4

Nicely illustrated at Javascript the good parts video from Doug Crockford.