if (condition is true) {
    // then do this ....
}

if(2 + 2 == 4) {
    console.log("You are a mathmatician!");
}

if(prompt("Your name?") != "Estelle"){
    console.log("You are an imposter!");
}

true
false

var truthy = true,
    falsey = false;

Everything value can return a Boolean Value.

False

false
undefined
null
0 (-0)
""
NaN

Everything else is truthy

Everything that is not falsy is truthy

Truthy

true
42
0.000001
"Estelle"

Objects, arrays, functions & regular expressions

function foo(){}
[1,2,3]
[]
{firstName: "Estelle", age: 29, isAwesome: true}
{}
/\w{6,9}/

Does foo exist? Is it truthy?

if(foo) { } 

true or false?

if(x > y) { } 

Is x truthy? If not, is y truthy?

if(x || y) { } 

are x and y BOTH truthy

if(x && y) { } 

Strict Equality Operator: ===

console.log(5 ==  "5");
console.log(5 === "5") 

console.log(5 !=  "5");
console.log(5 !== "5") 

When you're looking for a truthy or falsy value, pass the value into your if statement, negating with ! if necessary. When you're looking specifically for true or false, use === or !==.

Use == or != only when you NEED what it does, like to compare null and undefined.

if(5 > 4)

if("Clinton" > "Bush")

if(5 == '5')

if(5 === '5')

if(0 < x && x < 10)

if(!firstName)

if(heroes.length === 1)

if(isNaN('fruitcake'))

If a data entry point is not case sensitive (like username rather than password), realize that your user's capitalization may not match what you have stored:

var firstName = prompt('First Name?');
  if(firstName.toUpperCase() === 'ESTELLE') {
    alert(firstName);
  }

var errMsg,
     email1 = prompt('Your email address:');
     email2 = prompt('Re-enter email address:');

  if(email1.toLowerCase() !== email2.toLowerCase()) {
  	errMsg += "Email addresses do not match\n";
  }

var age = parseInt(prompt('Lie about your age?'));

if(isNaN(age)) {
  // not a number!! 
  errMsg += "At least make it a number so it's plausible, silly"
} 

var msg,
    age = +prompt("how old are you?");
    
if(!age || isNaN(age)) {
  msg = "You're too old to tell us your age?";
} else if (age <= 18 || age >= 65) {
  msg = "Kudos to you for getting mad skillz";
} else if (age > 45) {
  msg = "You should have lied about your age.";
} else if (age < 25) {
  msg = "You're too young to rent a car,\
  but old enough to rock JS";
} else {
  msg = "There's nothing wrong with being average.";
}

while (condition) {
    statements;      
}

while (condition is true) {
    // do these things
    // alter the condition       
}

var count = 0, 
    iterations = 4;
while (count < iterations) {
    // do these things
    counter++;
}

Declared in one line

for (initialization; condition; increment) {
    statement;
}

for(count = 0; count < iterations; count++) {
    // do these things
}

var pets, i;

pets = ['Spazzo', 'Poopers', 'Sassafrass'];
for (i = 0; i < pets.length; i++){
    console.log('Pet ' + (i + 1) + ": " + pets[i]);
}

var num;
do {
    num = window.prompt('Number?');
    console.log('Guess? ' + num)
} while (num !== '5')

Will execute code block at least once, even if condition is never met.

• Is your user old enough to vote? Drink? Rent a car? Run for president? Retire?
• Multiplication Table: pick a number? How many multiplication values do you want?
  Example input: 9, 3.
  Example output: 9 x 1 = 9, 9 x 2 = 18, 9 x 3 = 27
• If you used the while loop for the above, do it with the for loop. If you used the for loop, rewrite it as a while loop.
• Have the user guess your number. Tell them they're warm if close or cold if far off.