为什么函数声明不是变量声明时的声明

Question

It seems like a stupid question but I really can't wrap my head around it. I began to wonder this when I was thinking of why there is a semicolon after variable declaration but not function declaration. So does that mean a function declaration is an expression?

And what is the official meaning for statement? I've been finding the answer saying that a statement is a "command" that tells computer what to do, but isn't a function declaration telling the computer what to do? Does it have something to do with when it's get loaded when executing the code?

Answer 1

A variable declaration can have side-effects (eg, var x = alert(42); ).

A function declaration cannot.

Answer 2

I began to wonder this when I was thinking of why there is a semicolon after variable declaration but not function declaration

Not every statement ends with a semicolon .

Examples of statements with trailing semicolon:

• Variable declaration: var foo = <expression>;
• Expression statement: <expression>;
• Do while loop: do <statement> while (<expression>);

Examples without trailing semicolons:

• If statement: if (<expression>) <statement>
• Try catch statement: try <block> catch (<identifier>) <block>
• For loop: for (...) <statement>
• Block statement: { <statement list> }

So does that mean a function declaration is an expression?

No, it's more complicated than that. Function declarations are not statements. They are not expressions either, they are source elements . Statements and function declarations are both source elements. **

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And what is the official meaning for statement?

I can't tell you the official definition, but in the context of JS I would say something like "it's an instruction that does not produce an assignable result/value". Expressions on the other hand produce a result that can be used in other expressions.

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** Good to know, but a bit off-topic: Since function declarations are not statements, they are technically not allowed to be used inside blocks. This becomes even more apparent if we also consider hoisting . This example should throw a syntax error in every browser, but unfortunately it doesn't.

if (true) {
  function foo() { alert('foo'); }
} else {
  function foo() { alert('bar'); }
}
foo();

This leads to different behaviors in different browser. While Chrome will show bar , Firefox will show foo . Chrome just hoists both function declarations, and the second overrides the first one. Firefox interprets both declarations as something like a function expression .

Try it yourself in different browsers.