“function xy（）”和“xy = function（）”之间有什么区别？

Question

The only difference between these two pieces of code are in line 1 in both blockquotes.

Why do these two pieces of code behave different?

function cat.meow() {
    console.log("Meow");
};

versus

cat.meow = function () {
    console.log("Meow");
};

Answer 1

The first one is invalid JavaScript, the second is a better idea, more details in the following...

Why?

Variable / Function names can only have Letters, Underscores, $, Numbers (if they're not the first character) along with a few other ACII, and unicode characters (check here or here as recommended by Kaiido and Felix King ). It would be the same as var cat.meow; which is invalid JavaScript and will throw an error. More specifically

SyntaxError: Unexpected token '.'.

The compiler is expecting a parenthesis or a space but it sees a period not assuming it is an object. This also happens with bracket syntax for an object :( which means you can't define a function for an object 'item' the first way

Explanation

When you do: foo = function () {} it set the value of foo to the function. When you do it as a function function foo () {} it 'creates' foo .

So...

The second one is referring to meow in cat . So it is setting meow in cat to the function which is valid. That is equivalent to this:

var cat = {
    meow: function () {
        console.log("Meow!");
    }
};

Obviously is cat isn't defined, this will still throw an error. If you really really want to have your function to be called cat.meow (I would strongly advise against it) , you would do the following

window['cat.meow'] = function () {
    console.log("Meow");
};

This clutters the global scope unless you have a different object and is extremely bad practice; so if you do use it, do it in secret otherwise almost every programmer will be bashing you on your coding practices.

Answer 2

Have you at least tried it? The first one isn't a valid javascript. It should produce

Uncaught SyntaxError: Unexpected token .

The second one is valid. In that context, cat is an object and you're creating a meow method for it.

The error suggests what that . is not a valid identifier . In javascript, and most languages, anything which matches the regex \\w+ , meaning alphanumerical characters + underscore, is a valid identifier.

And in most languages, including javascript, it is also mandatory to have the identifier begin with an alphabet or underscore. In javascript however, $ is also a valid identifier.