JavaScript Loops #

for #

iterator variable is initialized, changed with each loop. Loop ends when some condition met.

Minimal form includes (separated by ;):

initialization
stopping condition - as long as the condition is true, loops will keep running
interation statement / action / thing that happens on each loop

A simple example:

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

// this was loop and print out a value until 10 is reached

Arrays #

.length is handy for looping through arrays.

const someArr = ['first', 'second', 'third']

for (let i = 0; i < someArr.length; i++) {
    console.log(`look! ${someArr[i]})
}

// one by one, prints out elements of the array

Don’t forget the let

while #

While a condition is true, loop continues.

This:

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

// this was loop and print out a value until 10 is reached

becomes this while loop:

let iterVar = 0;
while (iterVar < 10>) {
    console.log(iterVar);
    iterVar++;
}

do-while #

Run the block at least once and then go into a while loop.

If the while condition is true, then the block gets looped.

In its bare form:

do {
    ...;
} while (some condition);

break #

exit a loop when some condition is met.

for (let i = 0; some condition; i++) {
    if (some other condition) {
        break;
    }
}

nesting #

I generally don’t like this, but sometimes it’s necessary :/

Often used for array comparisons.

const arrFirst = ['A', 'B', 'C'];
const arrSecond = ['B', 'C', 'D'];
for (let i = 0; i < arrFirst.length; i++) {
  for (let j = 0; j < arrSecond.length; j++) {
    if (arrFirst[i] === arrSecond[j]) {
      console.log('This letter is in both arrays: ' + arrSecond[j]);
    }
  }
}

for-in #

When it comes down to unordered key-value pairs (unlike arrays), we can use the for...in construct.

This runs a block of code in each key-value pair in an object.

let someObj = {
    key1: {
        keyA: {
            ...
        },
        keyB: {
            ...
        }, 
    ...,
    }
}

for (let someNewVar in someObj.key1) {
    ...;
}

Where someNewVar is one of the keys under key1.