JavaScript Logic

MMP 350 > Week 6

Logic is an essential part of computer programming which affords for decisions to be made by the computer based on data or input.

JavaScript uses Boolean logic, which, like the Boolean type itself, is named after George Boole, an English mathemtician who invented boolean algebra, in which all values can be reduced to true or false. This type of logic is useful in computer science because it is based on a binary number system, in which each bit has a value of 0 or 1.

Booleans

A boolean is a datatype that can be either true or false.

In JavaScript other values will evaluate to false, such as 0, "" (empty string), NaN, null, and undefined. Everything else defaults to true, such as "Hello World", 1, 99, or -99. Try evaluating some booleans in the console with this syntax:

Boolean(true);
Boolean(false);
Boolean("");
Boolean(null);
Boolean(0);
Boolean(4);

Logical operators

Using comparison operators, we can test the values of variables to detect if they are equivalent.

Let's declare some variables:

var x = 1;
var y = 0;
var z = "1";
var a = 5;

And test to see if their value is equivalent:

x == y;
x != y;
x == z;

We can evaluate numeric values:

x > y;
x < y;
x >= z;
x <= a;

We can also test if their type is equivalent:

x === z;
x !== z;

We can compare strings as well as numbers.

var x = "hello";
var y = "world";
x == y;
x != y;

Conditionals

Using booleans, we can make decision in our program with conditional statements starting with the keyword if:

var answer = "hello world";
if ("hello world" == answer) {
	console.log("Correct!");
}
var response = 10;
if (response != answer) {
	console.log("That's incorrect.");
}

We can evaluate statements using numbers and strings as well:

var x = 0;
var y = 1;
if (x == y) { 
	console.log("x is equal to y"); 
}
if (x < y) { 
	console.log("x is less than y"); 
}

var a = "hello";
var b = "world";
if (a.length = b.length) { 
	console.log("hello and world have the same number of characters"); 
}
if (a == b) { 
	console.log("hello and world are the same string"); 
}

else

We can use else to tell our program what to do if the conditional is false:

if (a == b) { 
	console.log("hello and world are the same string"); 
} else {
	console.log("hello and world are NOT the same string")
}

else if

else if is to test further conditions that are not covered by a simple else.

if (x == y) {
	console.log("The values are equal.");
} else if (x < y) {
	console.log(x + " is less than " + y);
} else {
	console.log(x + " is greater than " + y);
}