JS中的以下定义方式有什么区别？

Question

code block 1:

var a = (function(){
    var obj = {a:'1',b:'2'};
    return obj;
})();
console.log(a);  

code block 2:

var a = function(){
    var obj = {a:'1',b:'2'};
    return obj;
}();
console.log(a);  

I know the results of console.log are the same.But is there any difference during the execution procedure?

Answer 1

The only effect that putting parenthesis around a function has is to ensure it is treated as a function expression.

Putting it as the RHS of an assignment also has that effect.

Consequently, there is no difference between the two.

Answer 2

var a = (function(){
    var obj = {a:'1',b:'2'};
    return obj;
})();
console.log(a);  

The function is an IIFE (Immediately invoked function expression).

The function will be invoked and the returned value will be assigned to a .

In the second expression, Only the syntax differs, but the functionality is same. But as a good practice, it's good to wrap the function within () . If you use just below code, it will give error.

function(){
    var obj = {a:'1',b:'2'};
    return obj;
}();

Whereas

(function(){
        var obj = {a:'1',b:'2'};
        return obj;
    }());

and

(function(){
        var obj = {a:'1',b:'2'};
        return obj;
    })();

both won't give error, because we are explicitly defining it to be treated as a expression.