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Tux9000
Asked: 2020-07-08 16:55:59 +0800 CST 2020-07-08 16:55:59 +0800 CST

Why when creating a function does it return undefined when defined?

  • 772

Why is it that after defining a function through the browser console , " undefined " is returned ? Why happens? What would be missing in a function declaration for this not to happen?

For example, when creating a function with the following syntax, " undefined " is returned:

function miFuncion(algo){
    return algo;
}
//salida: undefined

Also if I use a syntax like the following, " undefined " is returned:

var miFuncion = function(algo){
    return algo;
}
//Salida: undefined

but, when you try something like:

miFuncion = function(algo){
    return algo;
}

change the return after the function is created, does anything change using this syntax?

I add a screenshot to make the idea that the question raises more objective

javascript

2 Answers

  1. Best Answer

    The browser console is designed to take the last expression, look up its return value, and then display that value.

    When you make an assignment statement JavaScript assigns the assigned value, but the console will take one more step and that is to return the return value of the last statement.

    In the console declaring a variable using or not using the keyword varactually does the same thing: assign the property to the object window:

    enter image description here enter image description here enter image description here

    The difference is that the declaration does not have vara return value, which is why the console returns undefined, while using the declaration without the keyword, varthe console immediately returns the assigned value.

    It should be noted that when declaring functions (regardless of the way it is done) it also returns, undefinedsince the function declaration does not have a return value either.

    enter image description here

    I hope I was clear with the explanation.

    • 5

  2. The function is created, but the argument algohas no value assigned, so it returns undefined.

    Try the following:

    For your first function being like this:

    function miFuncion(algo = 10){
    return algo;
}

console.log(miFuncion())

    We assign a default value to the parameter and then print on the screen.

    Resulting:

    //10

    For example, you can check the type of that parameter like so:

    function miFuncion(algo){
    return typeof(algo);
}

console.log(miFuncion())

    Which returns:

    //undefined

    Otherwise, if you assign a value to it as I explained at the beginning and repeat the code using it, typeof()it will return:

    //number

    The above should be applicable for the 3 cases you show.

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