Expressions & Operators

Expressions & Operators are fairly simple in JavaScript. Expressions are any valid unit of code that resolves to a value, while Operators are symbols that perform operations on operands.

// An expression with a side effect (assigning a value)
x = 7;

// An expression that purely evaluates
3 + 4;

An expression that purely evaluates and is not part of a bigger construct will be immediately discarded. So it does not produce any effects.

All complex expressions are joined by operators. There is a lot of different operators in JavaScript, but we will only cover the most important ones:

// Assignment Operators, there are also +=, -=, *=, /=, %=, ...
let rubyAge = 18;

// Comparison Operators, like ==, ===, !=, !==, >, <, >=, <=
rubyAge === 18;
rubyAge == "18";
rubyAge >= 17;
rubyAge > 18;
rubyAge !== 18;

// Arithmetic Operators, like +, -, *, /, %, **, ++, --
rubyAge + 1;
rubyAge - 1;
rubyAge * 2;
rubyAge / 2;
rubyAge % 2;
rubyAge++;
rubyAge--;

// Logical Operators, like &&, ||, !
rubyAge > 17 && rubyAge < 19;
rubyAge > 17 || rubyAge < 19;
!rubyAge > 17;

// Ternary Operator
rubyAge > 17 ? "You are an adult" : "You are a child";

// String Operators, like +, +=
"Ruby" + " " + "is" + " " + "awesome";

// Unary Operators, like delete, typeof, void, +, -, ~, !
delete rubyAge;
typeof rubyAge;
void rubyAge; // evaluates to undefined

To read more about Assignement Operators, check out MDN.

Conditions

Probably a lot of you heard of the programmers joke “Please go to the store and buy milk and if they have eggs, get six --- I came home with 6 cartons of milk”.

This is a perfect example of a condition. A condition is a statement that evaluates to either true or false. If the condition is true, the code inside the block will be executed, otherwise it will be skipped.

// if...else
if (rubyAge > 17) {
  console.log("You are an adult");
} else {
  console.log("You are a child");
}

// switch
switch (rubyAge) {
  case 18:
    console.log("You are an adult");
    break;
  case 17:
    console.log("You are a child");
    break;
  default:
    console.log("You are a baby");
}

Loops

There are a couple of Loops available in JavaScript. Loops are used to execute the same block of code a specified number of times or when a condition is met. The most common ones are:

// for
for (let i = 0; i < 10; i++) {
  console.log(i);
}

// while
let i = 0;
while (i < 10) {
  console.log(i);
  i++;
}

// do...while
let i = 0;
do {
  console.log(i);
  i++;
} while (i < 10);

There are also for…in or for…of loops, but we most of the time, there are methods available that are more suitable for the task of for example iterating over Arrays.