Javascript: Loops

Intro

A loop is an essential method for repeating an action several time very rapidly until a set of conditions are met.

DRY code is an acronym for Don’t Repeat Yourself. It describes the philosophy of writing code intelligently with loops or helper functions instead of typing out repetitive code over and over.

While

while (someCondition) {
  //run some code
}

Adding to the count

For example if we want to run a simple loop that counts to ten we could do the following:

var count = 1
while (count < 11) {
  console.log("Current Number is: " + count)
  count++ //add one to the count
}

What if we wanted to add more than 1 to the count, what if we wanted to add 2?

We set count+=2

var count = 1
while (count < 11) {
  console.log("Current Number is: " + count)
  count += 2 //add two to the count
}

Printing each character in a string

This takes advantage of the .length property and another big concept which is the [ index ] which will select whichever character or object from an array that is defined by the brackets.

Some additional examples:

//print all numbers between -10 & 19
var count = -10
while (count >= -10 && count <= 19) {
  console.log("The Current Number Is " + count)
  count++
}

// refactored
var count = -10
while (count < 20) {
  //techinally we dont need the count is greater than -10 part because we are starting above that
  console.log("The Current Number Is " + count)
  count++
}

//print all even numbers between 10 & 40

var count = 10
while (count % 2 == 0 && count >= 10 && count <= 40) {
  console.log("The Current Number Is " + count)
  count += 2
}

//print all odd numbers bewtween 300 and 333

var count = 301
while (count % 2 !== 0 && count >= 300 && count <= 333) {
  console.log("The Current Number Is " + count)
  count += 2
}

/* print all numbers divisible by 5 AND 3 between 5-50
 Note that the if statement here is a big difference between this one and the previous. 
 This IF statement allows us to continue adding to the count even if a number is not printed.
 In the previous examples the loop would have broken. */

var count = 5
while (count < 51) {
  if (count % 5 === 0 && count % 3 === 0) {
    console.log(count)
  }
  count++
}

For

for (init; condition; step) {
  // run some code
}

Here are some samples of basic for loops:

//Print all numbers between -10 and 19

for(var i = -10; i < 20; i++){
  console.log(i);
}

//Print all even numbers between 10 and 40

for(var i=10; i<41; i+=2){
  console.log(i);
}

//Print all odd numbers between 300 and 333

for(var i=300; i<334; i++){
    if(i%2 !==0){  //if number is not even
    console.log(i);
  }
}

//Print all numbers divisible by 5 AND 3, between 5 and 50

for(var i=5; i<51; i++){
  if((i%5 ===0) && (i%3 ===0)){
    console.log(i);
  }
}

.forEach

forEach loops are a method of arrays and are therefore covered in the Arrays Post

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