Angelo Geant Gaviola

Posted on Jan 24, 2022

Ways to loop in an array using the different types of "FOR LOOP" in Javascript.

#javascript #programming #beginners #tutorial

Introduction

As a developer, there are times you are going to encounter handling a set of data.

That means there are times that you have to retrieve them and manipulate them for your program.

Here in this article, I'm going to show you four different ways to loop in an array using the different types of for loops.

For loop (generic)

For loop is a loop that keeps executing itself until the condition gets filled. It also stops when a "break" statement gets executed.

This is what the syntax looks like:

for([variable declaration]; [condition]; [incrementation of the variable]);

The first statement is about the declaration of the variable that you want as a counter --- something that you use to keep count of the iteration.
The second statement is about a condition that you need to get filled before stopping the loop.
The third statement is about adding value to the counter.

Example:

for(counter = 0; counter <3; counter++){
    console.log(counter); // writes to output
};

Output:
0
1
2

Reference:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for

For in loop

For in loop is a type of "FOR LOOP" that iterates over the indices in a array. It is similar to the normal "FOR LOOP" when executed, however, this is faster to write since it does the sizing of the length and incremental for us.

This is what the syntax looks like:

for(variable in array)

Example:

const grades = [80,85,85,90];

for(let index in grades){
    console.log("Index: "+ index + " -- " + grades[index])
};

Output:
Index: 0 -- 80 
Index: 1 -- 85 
Index: 2 -- 85 
Index: 3 -- 90

Reference:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...in

For of loop

For of loop is a type of "FOR loop" that iterates over the values in the array.

This is what the syntax looks like:

for(variable of array)

Example:

const grades = [80,86,89,90];
for(let element of grades){
    console.log(element);
}

Output:
80
86
89
90

Reference:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...of

Foreach function

Foreach is a method for array structures that takes a callback function for execution.

This is what the syntax looks like:

array.forEach(callbackfn);

array - is the variable that contains your array.
callbackfn - is the callback function taken as an input.

Example:

let array = [1,2,3]
array.forEach((element) => {
    console.log(element);
})

Output:
1
2
3

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach

Conclusion

As one of my professors said "There are many ways to kill a chicken" depending on what you want to accomplish.

This is my first article, let me know if there's any mistakes or any improvements I should make, Thank you!

Top comments (7)

peerreynders • Jan 25 '22

Being an array method .forEach() is really the only one that is limited to arrays.

Note that eslint no-constant-condition won't tolerate while(true) but will accept for(;;).

let count = 0;
for(;;) {
  if (count < 3) {
    console.log(count);
    count += 1;
    continue;
  }

  break;
}

As the reference linked to states:

The for...in statement iterates over all enumerable properties of an object that are keyed by strings (ignoring ones keyed by Symbols), including inherited enumerable properties.

(… and arrays are "special objects")

const special = Symbol('special');
const grades = [];
grades.a = 1;
grades[0] = 80;
grades.c = 3;
grades[1] = 85;
grades.b = 2;
grades[2] = 90;
grades[special] = 42;

console.log(grades[special]);
console.log(grades.hasOwnProperty('1')); // as a string
console.log(grades.hasOwnProperty(1)); // as a number
console.log(grades.hasOwnProperty(special)); // as a symbol

for (const key in grades) {
  console.log(`${keyToString(key)}: ${grades[key]}`);
}

function keyToString(key) {
  switch (typeof key) {
    case 'symbol':
      return 'some symbol';
    case 'string':
      return `'${key}'`;
  }

  return key.toString();
}

console.log('Just keys()');
for (const key of grades.keys()) {
  console.log(`${keyToString(key)}: ${grades[key]}`);
}

/*
42
true
true
true
"'0': 80"
"'1': 85"
"'2': 90"
"'a': 1"
"'c': 3"
"'b': 2"

"Just keys()"
"0: 80"
"1: 85"
"2: 90"

Note how 'in' always produces "string"s
While '.keys()' used with 'of' actually returns "number"s
 */

As the reference linked to states:

The for...of statement creates a loop iterating over iterable objects, including: built-in String, Array, array-like objects (e.g., arguments or NodeList), TypedArray, Map, Set, and user-defined iterables.

const special = Symbol('special');
const entries = [
  ['0', 1],
  [0, 80],
  ['2', 3],
  [1, 85],
  ['1', 2],
  [2, 90],
  [special, 42],
];
const someMap = new Map(entries);

for (const [key, value] of someMap) {
  console.log(`${keyToString(key)}: ${value}`);
}

function keyToString(key) {
  switch (typeof key) {
    case 'symbol':
      return 'some symbol';
    case 'string':
      return `'${key}'`;
  }

  return key.toString();
}

/*
"'0': 1"
"0: 80"
"'2': 3"
"1: 85"
"'1': 2"
"2: 90"
"some symbol: 42"
 */

Bonus: Both for...in and .forEach() will skip "holes" ("empty values") in arrays while for...of , .keys() and .entries() will not.

const grades = [80, undefined, , 95]; // hole/empty at index 2

console.log(grades.hasOwnProperty(1)); // true - value undefined
console.log(grades.hasOwnProperty(2)); // false - hole
console.log(grades.hasOwnProperty(3)); // true - number 95

const show = (value, key) => console.log(`${keyToString(key)}: ${value}`);

for (let key = 0; key < grades.length; key += 1) show(grades[key], key);
/*
"0: 80"
"1: undefined"
"2: undefined"
"3: 95"
*/

for (const key in grades) show(grades[key], key);
/*
"'0': 80"
"'1': undefined"
"'3': 95"

i.e. key '2' was skipped
*/

for (const value of grades) console.log(value);
/*
80
undefined
undefined
95
*/

for (const [key, value] of grades.entries()) show(value, key);
/*
"0: 80"
"1: undefined"
"2: undefined"
"3: 95"
*/

grades.forEach(show);
/*
"0: 80"
"1: undefined"
"3: 95"

i.e. index 2 was skipped
*/

function keyToString(key) {
  switch (typeof key) {
    case 'symbol':
      return 'some symbol';
    case 'string':
      return `'${key}'`;
  }

  return key.toString();
}