Doing Math in Multiple Javascript Functions

Question

I'm new to the Javascript world and trying to figure out this assignment my teacher assigned. Here is his description of what is expected:

1. Build a function that will start the program. Please call it start()

2. From the start() function, call a function called getValue()

3. The getValue() function will get a number from the user that will be squared.

4. Also from the start function, call a function called makeSquare()

5. The makeSquare() function will square the number that was received by the user in the getValue() function.

6. Make sure that you display the results of squaring the number inside of the makeSquare() function.

Here is what I have so far:

function start() {

    getValue();
    getSquare();
}

function getValue() {

    var a = prompt("Number please")
}

function getSquare() {

    var b = Math.pow(a)
    document.write(b)
}

start()

This assignment doesn't have to be working with any HTML tags. I've only got the prompt box to work, nothing else does though. Am I using variables in a way that can't be used?

Answer 1

You can try this.

<script type="type/javascript">
function start(){
makeSquare(getvalue());
}

function getvalue(){
return prompt("enter a number");
}

function makeSquare(a){
var result=Math.pow(a,2);
alert(result);
}
start();
</script>

Answer 2

1. Your getValue() needs to return the value, so that you then can pass it to the getSquare() function.

2. In my opinion, you should always end each line with ;

3. You will probably need to parse the user input into a number. For that you can use parseFloat(string).

4. Math.pow takes two arguments, so to get the square, you would have to pass 2 as a second argument when calling it.

I edited your code with some clarifying comments:

function start() {
    // Catch the value returned by the function
    var value = getValue();
    // Pass the returned value to the square-function
    getSquare(value);
}

function getValue() {
    // Parse the user input into a number, and return it    
    return parseFloat(prompt("Number please"));
}

// Let the square-function take the user input as an argument
function getSquare(a) {  
    // Math.pow takes a second argument, which is a number that specifies a power
    var b = Math.pow(a, 2);
    document.write(b);
}

start();

Another, less good way

In JavaScript, variable-scoping is based on functions. If a variable is declared using the var keyword, it is only available to that function, and its child-functions. If it is declared without the var keyword, or declared outside any function, it becomes a global variable, which will be accessible by any code run on that page.

That said, you could get rid of the var keyword inside the getValue() function, which would make the variable a global. You could then access it from within getSquare() the way you tried in your example.

This is generally not a good idea though, since you would "pollute" the global namespace, and you would be running the risk that you accidentally have another script using a global variable with the same name, which would cause all kinds of trouble, when the scripts start to work with the same variable.

Answer 3

You were close. But it seems that you don't understand scoping and how exactly to use the pow function.

Math.pow:

Math.pow takes two parameters, the base and the exponent. In your example, you only provide the base. That will cause problems as the function will return the value undefined and set it to b. This is how it should have looked (if you wanted to square it):

Math.pow(a, 2);

Scoping:

Every function has it's own scope. You can access other variables and functions created outside the function from within the function. But you cannot access functions and variables created inside another function. Take the following example:

var c = 5;

function foo() { // we have our own scope
   var a = c; // Okay
}

var b = a; // NOT okay. a is gone after the function exits.

We could say that a function is private. The only exception is that we can return a value from a function. We return values using the return keyword. The expression next to it is the return-value of the function:

function foo() {
    return 5;
}

var a = foo(); // a === 5

foo() not only calls the function, but returns its return-value. A function with no return-value specified has a return value of undefined. Anyway, in your example you do this:

function getValue() {
    var a = prompt("Number please")
}

and access it like this:

// ...
var b = Math.pow(a)

Do you see the error now? a is defined in the function, so it can't be accessed outside of it.

This should be the revised code (Note: always use semicolons. I included them in for you where necessary):

function start() {
    getSquare();
}

function getValue() {

    var a = prompt("Number please");
    return a;
}

function getSquare() {

    var b = Math.pow(getValue(), 2); // getValue() -> a -> prompt(...)
    document.write(b);
}

start();

Answer 4

Am I using variables in a way that can't be used?

Yes, that's where your problem lies. Variables in most programming languages have a scope that determines where they're available. In your case, a and b are local variables of the functions getValue() and makeSquare() respectively. This means they're not available outside the function they're declared in.

Generally speaking, this is a good thing. You should use restricted scopes for your variables to make the "flow" of data through your program clearer. Use return values and parameters to pass data between functions instead of making your variables global:

function start() {
    var a = getValue();
    makeSquare(a);
}

// Return a value entered by the user
function getValue() {
    return prompt("Number please")
}

// Write the square of the `a` parameter into the document
function makeSquare(a) {
    var b = Math.pow(a)
    document.write(b)
}

Answer 5

As this is an homerwork, I won't give you direct answer, but here's some clue.

In javascript variables are functions scoped. That mean that var a inside getValue is only available in there.

You can return a value from a function.

Functions are first class object in javascript, so you can pass them as parameter to other function and finally call them inside that function.