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?

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

ⓘ I have included a zip file with all the source code at the start of this tutorial, so you don’t have to copy-paste everything… Or if you just want to dive straight in.

 

 

TLDR – QUICK SLIDES

Fullscreen Mode – Click Here

 

TABLE OF CONTENTS

 

 

QUICK NOTES

If you spot a bug, feel free to comment below. I try to answer short questions too, but it is one person versus the entire world… If you need answers urgently, please check out my list of websites to get help with programming.

 

EXAMPLE CODE DOWNLOAD

Click here to download all the example source code, I have released it under the MIT license, so feel free to build on top of it or use it in your own project.

 

 

 

1) FOR

// (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

// (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

/* (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

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

// (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

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

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

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.

 

 

 

LINKS & REFERENCES

• For loop – MDN
• While loop – MDN
• For-in – MDN
• For-of – MDN
• Break – MDN
• Continue – MDN
• Object Entries – MDN

 

INFOGRAPHIC CHEAT SHEET