I don't know exactly what you are trying to do, but here are few things you are doing wrong.

1. circumeference and diameter are defined inside the function scope and you are trying to access it outside the function, which is causing this error.
2. You are trying to return multiple values from a function using multiple return statements which is not possible since the first return will end the function. For your purpose you can simply return an object from the function, containing your desired values.

See the code below,

let radius = [3, 2, 1, 4];

function area(radius) {
    let tArea = [];
    let circumference = [];
    let diameter = [];

    for (let i = 0; i < radius.length; i++) {
        tArea.push(Math.PI * radius[i] * radius[i]);
        circumference.push(2 * Math.PI * radius[i]);
        diameter.push(2 * radius[i]);
    }
    return {area: tArea, circumference, diameter};
}

const {area: calculatedArea, circumference, diameter} = area(radius);

console.log(calculatedArea);
console.log(circumference);
console.log(diameter);

Because you can return only a single value from a function while you are trying to return 3 . Declare these 3 arrays globally and you won't need the return statements.