为什么添加分号有时会破坏if语句？

Question

Why is it that you aren't supposed to add semicolons after every set of curly braces? I thought this was a rule in JavaScript. In this code snippet, for instance, it turns out that it does not work because of the semicolon after the initial if statement, but then the semicolon after the else statement is fine.

 function a() { if (var >= length) { count = var / length; }; else { count = 0 }; 

Answer 1

Adding the semicolon between the if and else effectively splits them into two separate blocks.

An if block can exist on its own, but not an else block, since the else depends on the condition in the if statement.

Answer 2

Because the entire expression includes the if block. Semi-colons are introduced after the completion of an expression (and not necessary post if curly braces). When you add a semi-colon after the if , you are cutting the entire expression short.

You are, in a sense, saying:

if (foo === bar) {
    //do something
} /** You: Ok stop doing stuff**/;
/**
    Javascript: Ok! I'm done
**/
else { // Javascript: Wait, what is this crap
    //nothing's gonna happen
}

Answer 3

The semicolon after the first } creates an empty statement. It's as if you wrote:

 alert('test');
 ;

This extra "empty" statement separates the if from the else , so the else block becomes meaningless.