If-else在JavaScript中不起作用

Question

var num1 = prompt("Enter first number: ", "number");
var num2 = prompt("Enter second number: ", "number");

if (num1 > num2) {

    document.write("<p>" + num1 + " is greater than " + num2 + "</p>");
} else {

    document.write("<p>" + num2 + " is greater than " + num1 + "</p>");
}

When I execute this code at that time it executes else part everytime so what is solution for it? Thank you so much in advance.

Answer 1

You are comparing strings (which is what prompt returns, even if the user types in digits). Convert the values to numbers first:

if(+num1 > +num2){

If the user enters any non-digits into the prompt s, a NaN will be produced and you may see weird results. I would set it up this way:

var num1 = +prompt("Enter first number: ", "number"),
    num2 = +prompt("Enter second number: ", "number");

if (!isNan(num1) && !isNaN(num2)) {
    if (num1 > num2) {

    } else if (num2 > num1) {

    } else {
        // Numbers equal
    }
} else {
    // At least one of them isn't a number
}

Answer 2

Javascript has a funky definition of true/false when you compare arbitrary variables. You are going to want to make 100% sure that the input of the two parameters you are prompting for has been cast to a number like you would expect it to be:

var num1 = parseInt(prompt("Enter first number: ", "number"));
var num2 = parseInt(prompt("Enter second number: ", "number"));

Using the + operator is great shorthand, but aside from my quick answer above you are probably going to want to add some checking to make sure they aren't giving you garbage.

Also, be sure to check out Ian's answer - it contains a great explanation of the + operator and how it can be used for coercion.

Answer 3

You need to parse the input (a string) into a number (an integer/float) before comparing.

Use parseInt or parseFloat like: (respectively)

var num1 = parseInt(prompt("Enter first number: ", "number"));

var num2 = parseFloat(prompt("Enter second number: ", "number"));

Answer 4

prompt returns a string, not number. You'll need to convert the results of prompt to a number before comparing.

Answer 5

var num1 = parseFloat(prompt("Enter first number: ", "number"));
var num2 = parseFloat(prompt("Enter second number: ", "number"));

prompt returns a String value, which needs to be parsed to a number.

Answer 6

In your case variables num1 and num2 are STRING. Before compare need to convert to INT type:

 var num1 = parseInt(prompt("Enter first number: ", "number")); var num2 = parseInt(prompt("Enter second number: ", "number")); 

Tip on future: you can will get type of your's variable by typeof()