HOME

Read: 05 - Operators and Loops

Comparison and logical operators

• Comparison operators — operators that compare values and return true or false. The operators include: >, <, >=, <=, ===, and !==.

• Logical operators — operators that combine multiple boolean expressions or values and provide a single boolean output. The operators include: &&,

  , and !.

Comparison Operators

Less than (<) — returns true if the value on the left is less than the value on the right, otherwise it returns false. Greater than (>) — returns true if the value on the left is greater than the value on the right, otherwise it returns false. Less than or equal to (<=) — returns true if the value on the left is less than or equal to the value on the right, otherwise it returns false. Greater than or equal to (>=) — returns true if the value on the left is greater than or equal to the value on the right, otherwise it returns false. Equal to (===) — returns true if the value on the left is equal to the value on the right, otherwise it returns false. Not equal to (!==) — returns true if the value on the left is not equal to the value on the right, otherwise it returns false.

Logical Operators

Comparison operators allow us to assert the equality of a statement with JavaScript. For example, we can assert whether two values or expressions are equal with ===, or, whether one value is greater than another with >.

There are scenarios, however, in which we must assert whether multiple values or expressions are true. In JavaScript, we can use logical operators to make these assertions.

&& (and) — This operator will be truthy (act like true) if and only if the expressions on both sides of it are true. || (or) — This operator will be truthy if the expression on either side of it is true. Otherwise, it will be falsy (act like false).

Loops: while and for

We often need to repeat actions.

The “while” loop

The while loop has the following syntax:

while (condition) { // code // so-called “loop body” }

While the condition is truthy, the code from the loop body is executed.

For instance, the loop below outputs i while i < 3:

let i = 0; while (i < 3) { // shows 0, then 1, then 2 alert( i ); i++; } A single execution of the loop body is called an iteration. The loop in the example above makes three iterations.

If i++ was missing from the example above, the loop would repeat (in theory) forever. In practice, the browser provides ways to stop such loops, and in server-side JavaScript, we can kill the process.

Any expression or variable can be a loop condition, not just comparisons: the condition is evaluated and converted to a boolean by while.

For instance, a shorter way to write while (i != 0) is while (i):

let i = 3; while (i) { // when i becomes 0, the condition becomes falsy, and the loop stops alert( i ); i–; }

The “for” loop

The for loop is more complex, but it’s also the most commonly used loop.

It looks like this:

for (begin; condition; step) { // … loop body … } Let’s learn the meaning of these parts by example. The loop below runs alert(i) for i from 0 up to (but not including) 3:

for (let i = 0; i < 3; i++) { // shows 0, then 1, then 2 alert(i); } Let’s examine the for statement part-by-part:

part begin i = 0 Executes once upon entering the loop. condition i < 3 Checked before every loop iteration. If false, the loop stops. body alert(i) Runs again and again while the condition is truthy. step i++ Executes after the body on each iteration. The general loop algorithm works like this:

Run begin → (if condition → run body and run step) → (if condition → run body and run step) → (if condition → run body and run step) → … That is, begin executes once, and then it iterates: after each condition test, body and step are executed.

If you are new to loops, it could help to go back to the example and reproduce how it runs step-by-step on a piece of paper.

Here’s exactly what happens in our case:

// for (let i = 0; i < 3; i++) alert(i)

// run begin let i = 0 // if condition → run body and run step if (i < 3) { alert(i); i++ } // if condition → run body and run step if (i < 3) { alert(i); i++ } // if condition → run body and run step if (i < 3) { alert(i); i++ } // …finish, because now i == 3

Summary

We covered 2 types of loops:

while – The condition is checked before each iteration. for (;;) – The condition is checked before each iteration, additional settings available.