String passed as function parameter is undefined

Question

When I pass a string to a function as a parameter, it returns undefined. Why is that?

let a = 'b';

let test = (a) => console.log(a);

test(); // undefined

The `a` from the parameter list shadows the higher scoped `let a = 'b'`. Read how scoping works in JavaScript in the linked duplicate. — Madara's Ghost, Jan 07 '17 at 21:27

score 2 · Answer 1 · answered Jan 07 '17 at 21:25

Why is that?

Because you don't pass any argument. Try the following:

test(a);

The following definition:

let test = (a) => console.log(a);

is like the following:

function test(a){
    console.log(a);
}

So when you call test, without passing any argument the value of a would be undefined.

score 2 · Accepted Answer · answered Jan 07 '17 at 21:26

When you call test(); you aren't putting anything between ( and ), so you aren't passing any parameters.

When you defined test (with (a) =>) you created a local variable a that masks the global variable with the same name.

To pass a you have to actually pass it: test(a).

2 Answers2