Javascript Loops & Iterations Cheat Sheet (Simple Examples)

Welcome to a beginner’s tutorial on Javascript loops and iterations. Want to loop through arrays and objects, or do things repetitively? The common mechanisms to loop and iterate in Javascript are:

For for (let i=0; i<10; i++) { ... }
For-In for (let KEY in OBJECT/ARRAY/STRING) { ... }
For-Of for (let VALUE of ARRAY/STRING) { ... }
Foreach ARRAY.foreach((VALUE, KEY) => { ... });
For-Object entries for (let [KEY, VALUE] of Object.entries(OBJECT)) { ... }
While while (i<10) { i++; }
Do-While do { i++; } while (i<10);
Break while (i<10) { if (i<0) { break; } i++; }
Continue while (i<10) { if (i==2) { i+=2; continue; } i++; }

That covers the quick basics at a glance, but let us walk through examples of each – Read on!

 

 

TLDR – QUICK SLIDES

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TABLE OF CONTENTS

 

 

JAVASCRIPT LOOPS & ITERATIONS

All right, let us now get into the examples of loops and iterations in Javascript.

 

1) FOR

1-for.js
// (A) INCREMENT
for (let i=1; i<=3; i++) {
  console.log(i); // 1, 2, 3
}
 
// (B) DECREMENT
for (let i=3; i>=1; i--) {
  console.log(i); // 3, 2, 1
}

// (C) LOOP ARRAY
var arr = ["a", "b", "c"];
for (let i=0; i<arr.length; i++) {
  console.log([i]); // 0, 1, 2
  console.log(arr[i]); // a, b, c
}

// (D) WE CAN ACTUALLY OMIT ALL INIT & CONDITIONS
var i = 0;
for (;;) {
  console.log(i); // 0, 1
  i++;
  if (i==2) { break; }
}

The for loop is one of the most commonly used, and some books don’t even explain it right. Take note that there are “3 segments” – for (INITIALIZE; END CONDITION; STEP).

  • INITIALIZE This runs before the loop starts.
  • END CONDITION The condition to stop looping, this statement must resolve to a boolean true/false. Runs after INITIALIZE and after every cycle.
  • STEP Run this after every cycle.

Yes, we can leave all 3 empty… But that will not make any sense.

 

2) FOR-IN

2-for-in.js
// (A) ITERATE OBJECT
var person = {
  name : "Jon Doe",
  email : "jon@doe.com"
};
for (let key in person) {
  console.log(key); // name, email
  console.log(person[key]); // jon, jon@doe.com
}

// (B) ITERATE ARRAY
var animals = ["Birb", "Cate", "Doge"];
for (let i in animals) {
  console.log(i); // 0, 1, 2
  console.log(animals[i]); // birb, cate, doge
}

// (C) ITERATE STRING
var hello = "world";
for (let i in hello) {
  console.log(i); // 0, 1, 2, 3, 4
}

The for-in loop will run through the keys:

  • Properties of an object.
  • The index of arrays.
  • Character position of a string.

 

 

3) FOR-OF

3-for-of.js
/* (A) ITERATE OBJECT - ERROR, NOT ITERABLE
var obj = { "foo" : "bar" };
for (let value of obj) {
  console.log(value);
}*/
 
// (B) ITERATE ARRAY
var animals = ["Birb", "Cate", "Doge"];
for (let value of animals) {
  console.log(value); // birb, cate, doge
}

// (C) ITERATE STRING
var hello = "world";
for (let char of hello) {
  console.log(char); // w, o, r, l, d
}

This should be pretty self-explanatory by now, for-in iterates the properties, for-of iterates the values (data) themselves.

 

4) FOREACH

4-foreach.js
var animals = ["Birb", "Cate", "Doge"];
animals.forEach((value, key) => {
  console.log(value); // birb, cate, doge
  console.log(key); // 0, 1, 2
});

The forEach() function only applies to arrays – It loops through all the elements within the array.

 

 

5) FOR & OBJECT ENTRIES

5-while.js
// (A) PERSON OBJECT
var person = {
  "Name" : "Jon",
  "Email" : "jon@doe.com"
};
 
// (B) LOOP OBJECT ENTRIES
for (let [key, value] of Object.entries(person)) {
  console.log(key); // name, email
  console.log(value); // jon, jon@doe.com
}

This is the “smart and convenient” way to loop through both the key and values of an object within a single loop.

 

6) WHILE

6-while.js
var animals = ["Birb", "Cate", "Doge"], i = 0;
while (i < animals.length) {
  console.log(i); // 0, 1, 2
  console.log(animals[i]); // birb, cate, doge
  i++;
}

A while loop is pretty simple – It will continue to run until the provided condition (i < animals.length) turns false… But beware, a common mistake that many newbies make is an end condition that will never be met, ending up in an infinite loop.

 

7) DO-WHILE

7-do-while.js
var animals = ["Birb", "Cate", "Doge"], i = 0;
do {
  console.log(i); // 0, 1, 2
  console.log(animals[i]); // birb, cate, doge
  i++;
} while (i < animals.length);

Look no further, this does almost the exact same thing as while. What is the difference here?

  • while will not even start if the condition is not met.
  • do-while will run at least once, even when the condition is not met.

 

8) BREAK & CONTINUE

8-break-continue.js
let i = 0;
while (i>=0) {
  // (A) CONTINUE - SKIP TO NEXT CYCLE
  if (i==2) {
    i+=2;
    continue;
  }
  i++;
  console.log(i); // 1, 2, 5, 6

  // (B) BREAK - STOP CYCLE
  if (i==6) { break; }
}

Lastly, we have another 2 Captain Obvious mechanisms:

  • continue Skip the current cycle.
  • break Stop the entire loop. Works in for for-in for-of foreach too.

 

 

DOWNLOAD & NOTES

Here is the download link to the example code, so you don’t have to copy-paste everything.

 

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EXAMPLE CODE DOWNLOAD

Click here for the source code on GitHub gist, just click on “download zip” or do a git clone. I have released it under the MIT license, so feel free to build on top of it or use it in your own project.

 

EXTRA BITS & LINKS

That’s all for the tutorial, and here is a small section on some extras and links that may be useful to you.

 

LINKS & REFERENCES

 

INFOGRAPHIC CHEAT SHEET

Javascript Loop Cheatsheet (click to enlarge)

 

THE END

Thank you for reading, and we have come to the end. I hope that it has helped you to better understand, and if you want to share anything with this guide, please feel free to comment below. Good luck and happy coding!

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