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

True or False?

Try the following in node:

  • 1 < 2
  • 2 + 2 < 4
  • 2 + 2 <= 4
  • "anonymous".endsWith("us")
  • "every journey".startsWith("a step")


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?


  • in the Colbert Report, truthiness means things we feel to be true, even though we know they're probably not

  • In JavaScript, all values have truthiness unless they are defined as falsy.

  • MDN: Truthy

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.

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

  • a single equal sign means ASSIGNMENT
    • name = "Alice" -- "assign the value 'Alice' to the variable 'name'"
  • two equal signs means COMPARISON
    • name == "Alice" -- "does the variable 'name' contain the string 'Alice'?"

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 value of a comparison is either true or false

    • so if (x == 2) means if x is 2 which changes based on x
  • the value of an assignment is the value being assigned

    • so if (x = 2) means if 2 which is always truthy
    • also, the value of x will be 2 afterwards, no matter what it was before

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 ==

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 + "!");
  • Now change hello.js so that it doesn't always say hello!
    • If the user's name is "Darth" then say "Noooooo! That's impossible!"

Lab: Infinite Names

  • Change hello.js so it keeps asking for names forever...
    • ...unless and until someone says their name is "bye!"
    • then it stops and exits back to the terminal

LAB: Enemies List

  • Change hello.js so that it says "Go away!" if the user's name is any one of a number of evil names
  • For instance, Voldemort, Satan, Lex Luthor...
  • Bonus: don't let enemies sneak in even if they spell their names with capital letters, like VolDeMort

Lab: exercises about logic