Computers have a very particular idea of when things are true and false.

True or False?

Try the following in node:


Comparison operators let you compare two values. JavaScript has all the usual suspects...

Operator Comparison
< less than
> greater than
<= less than or equal to
>= greater than or equal to
== equal to
!= not equal
=== really equal to
!== really not equal to

MDN: comparison operators

These are also called "Boolean operators" after George Boole, a 19th-century mathematician who invented Boolean algebra.)


The magic word if is a conditional.

The phrase immediately after if is a condition.

if (age < 18) {
  console.log("Sorry, you can't vote yet.");
phrase meaning
if ( ... ) if this condition's value is truthy
{ ... } then run this block of code

Wait a second. "Truthy?"

MDN: if...else

What is truthiness?


What is falsiness?

false, null, undefined, 0, NaN, and the empty string ("") are all falsy.

Fortunately, true is truthy and false is falsy.

Unfortunately, the string "false" is truthy, and the string "0" is truthy, even though the number 0 is falsy. This is because the string contains a character, and, even though the character is 0, any string with at least one character is truthy.

MDN: Falsy

if... then... else...

The magic word else allows BRANCHING.

if (age >= 18) {
} else {

Like a fork in the road, the program chooses one path or the other.

It takes the first path if the condition is truthy, and takes the second path if the condition is falsy.

2 + 2 = ?

Sadly, this mathematical expression:

2 + 2 = 4

causes an error. You need to do

2 + 2 == 4

instead. Why?

The Tragedy of the Equal Sign

This is confusing! (More about it on the next slide.)

A Notorious Bad Idea

"A notorious example for a bad idea was the choice of the equal sign to denote assignment. It goes back to Fortran in 1957 and has blindly been copied by armies of language designers. Why is it a bad idea? Because it overthrows a century old tradition to let "=" denote a comparison for equality, a predicate which is either true or false. But Fortran made it to mean assignment, the enforcing of equality... x = y does not mean the same thing as y = x."

Niklaus Wirth, Good Ideas, Through the Looking Glass (2005)

see also

Condition or Assignment?

BEWARE of using a single equal sign inside an if condition!

The Tragedy of the Threequal Sign

In addition to = and ==, JavaScript also has ===.

That's three equal signs in a row.

Operator Operation Example Meaning
= assignment X = Y let X equal Y
== comparison (fuzzy) X == Y does X mostly equal Y?
=== comparison (exact) X === Y does X really equal Y?

== means "does X equal Y, or if not, can Y be converted into something that equals X?"

Since the rules for type conversion are confusing, most JavaScript experts recommend:

always use ===, never use ==

Using == can have some very interesting side effects, see Stackoverflow

Conjunction Junction

You can make more complicated logical expressions using conjunctions:

Conjunction Operator Example Meaning
AND && X && Y "are both X and Y true?"
OR || X || Y "is either X or Y (or both) true?"
NOT ! !X "is X false?"

For example:

if (age >= 18 || hasPermissionSlip()) {
} else {

MDN: logical operators

LAB: Good Friend, Bad Friend

console.log("What is your name?");
process.stdin.on('data', (chunk) => {
    let name = chunk.toString();
    console.log("Hello, " + name + "!");

Lab: Infinite Names

LAB: Enemies List

Lab: exercises about logic

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