javascript中的函数参数-local或global

Question

I am a new learner, learning to code front end development. I want to know about the global and local variables in JavaScript functions. I understand the local and global, but how about the function parameters, are they local or global?

Should I always define a function as function add(a,b) or function add(var a, var b) ?

Somewhere I read that any variables defined without the var keyword inside a function becomes global. This point is confusing me.

Answer 1

The first one. function add(a,b)

Using the var keyword up there is actually a syntax error.

somewhere I read that any variables defined without the var keyword inside a function becomes global

That's true, but only when you do c = 'something' . Function parameters are always locally scoped.

Answer 2

The parameters are local to the function, but this is only half the truth, since objects are passed by reference. So if your function takes an object and modifies any member of this object, then the change is also seen outside of the function (because it's the same object).

Try the following in a node shell to demonstrate:

var obj = { a:0 };
obj;
function f(o) { o.a++; }
f(obj);
obj;

$ node

var obj = { a:0 };

undefined

obj;

{ a: 0 }

function f(o) { o.a++};

undefined

f(obj);

undefined

obj;

{ a: 1 }

Answer 3

In function add(a,b) a and b are only accessible within the function so they are local to that function.

function add(var a, var b) is incorrect syntax. You can't put var there.

Answer 4

You must declare a function such as function add(a, b). a and b are the parameters you'll give to the function when you will call it.

They don't exist at the moment of declaration, and are local to the variable. They are more or less a reference to the variables you'll pass to your function when calling it.

 var num1 = 4; var num2 = 3; function add(a, b){ return a + b; } var result = add(num1, num2); // result : 7 

Variables declared inside a bracket's function without the keyword var become global, such as result in the following example :

 function add(a, b){ result = a + b ; }