如何与[if]一起使用[getElementById]？

Question

I'm trying to make a projectile calculator, so it really need to change the way to calculate again & again until it is done.

but now i'm facing with a problem:

var Height = Number(document.getElementById("Height").value);
var Velocity = Number(document.getElementById("Velocity").value);
var Range = Number(document.getElementById("Range").value);
document.getElementById("First").innerHTML = ((((Math.SQRT2 * Velocity) +     (Math.sqrt(19.6) * Velocity * Height)) * Math.cos(0.785398163)) / 9.8);
if (First < Range) 
    document.getElementById("show").innerHTML = "Need more velocity to success on this range";

This is my big problem >> on if (First < Range) << it won't work how many times I try it

Seems like the if only work with var object, but i cant create a var object with these much complex calculation. This make me can't continue to next calculation.

Answer 1

Problem :

• You have not declared the variable First . So it is undefined. If you need to compare two variables, you first need to assign values to them. And for that you need to declare them first.

Solution :

Change this code:

document.getElementById("First").innerHTML  = ...

to this:

var First = ((((Math.SQRT2 * Velocity) + ......
document.getElementById("First").innerHTML = First;

Then you can safely compare the two values:

if (First < Range)
    ....

Answer 2

Save your calculation to a variable, then you can refer to it in the rest of the script:

var FIRST = ((((Math.SQRT2 * Velocity) + (Math.sqrt(19.6) * Velocity * Height)) * Math.cos(0.785398163)) / 9.8);

document.getElementById("First").innerHTML = FIRST;

if (First < Range) { etc...

Answer 3

Try this

HTML

<div id="Height">100</div>
<div id="Velocity">25</div>
<div id="Range">2</div>
<div id="First"></div>
<div id="show"></div>

Javascript:

var Height = Number(document.getElementById("Height").innerHTML);
var Velocity = Number(document.getElementById("Velocity").innerHTML);
var Range = Number(document.getElementById("Range").innerHTML);
document.getElementById("First").innerHTML = ((((Math.SQRT2 * Velocity) +     (Math.sqrt(19.6) *    Velocity * Height)) * Math.cos(0.785398163)) / 9.8);
var First=Number(document.getElementById("First").innerHTML);
if(First < Range)
document.getElementById("show").innerHTML = "Need more velocity to success on this range";

Fiddle:

Fiddle