For VS While Loop in Javascript

288 Comments Javascript

When we need to perform similar actions for certain number of times, that is when we get to “Looping”.

For example, when we need to output goods from a list one after another. Or just run the same code for each number from 1 to 10.

Loops are a way to repeat the same part of code multiple times.

In Javascript, there are while loop, for loop and do-while loop. In this post, the while and for loop will be compared.

The syntax for while loop is :

while (condition) {
  // code
  // so-called "loop body"
}

the syntax for for loop is :

for (begin; condition; step) {

  // ... loop body ...

}

So what is the deal with for loop?

For Loop allows you to run through the loop when you are certain as to how many times you would like it to run through the data structure such as for (var i; i < 9; i++); this will continuously increment i until the condition returns false (i is no longer less than 9). Any number can replace the 9 even a different type of variable. But the loop will terminate once the condition is no longer met.

How about a while loop?

A while loop can be used when you need your loop to run an unknown number of times until a specific condition is met.

Let’s use a contrived example. If you are making a tax calculator that allows a human resource manager to see how much tax that each employee out of ALL employees has to pay. The person who uses the calculator will have to input the number of employee that the loop will iterate through, like so :

let totalEmployee = prompt("Please input the total employee");

let i = 0; // the iterator

let tax = {}; // key-value pair for calculated tax for each employee

while(i <= totalEmployee){ ... assign the value of the tax for each employee here ... } 


// execution stops after counting the tax of the last employee

So how are they different from each other?

The key differences between the two loops lie in the intention “why do we need looping for?” and “do we know the condition at which the loop will stop?”

Use for loop if you know the condition or when to end or break the iteration.

Use while loop if you were to use an existing variable (purposefully using VAR keyword that is not block-scoped) in your program; or when your condition uses a new value that is obtained through the interface (like prompt / asking a user to input a number in the calculator example above).

Special to JS though, the iterating variable i for instance can be bound within the block of the for loop if it is defined by the let keyword which makes the for loop safer for some inexperienced JS programmers.

Long story short : The for loop is mostly preferred for the simple iteration jobs while the while loop is more applicable when you need to implement something at a frequency of iteration that is not yet certain or is a dynamic value that will depend on the interaction with other aspects/variables in your program.

Why let for the for-loop?

It has something to do with block-scoping for variables.

Say you have a program with one for-loop in it.

If your for loop has i instance, when you also have i in the outer scope like so :

// outermost scope of the program

var i = "Hello World";

console.log("what is i : i is", i); // prints "What is i : i is Hello World"

// but then you have for-loop below

for (var i = 0; i < 9; i++) { 
   
  // block-scope of for loop here 

}

// let's use the same variable i below :

console.log("what is i : i is", i) // what will be printed to the console? what will be the value of i?

In the example, we are touching the issue of “block scoping”. Read more about it here.

After the closing curly brace, the execution goes back to the outer scope. If you need the same i instance, that was assigned to be “Hello World” before the for-loop begins, the i instance will no longer have the same value. The i in the for loop will override / reassign the value of i, resulting in the console.log to display :

What is i : i is 9 // not "Hello World"

This problem is caused by the fact that var is not block-scoped, so the value spills over all over places, in and outside for-loop. This can be mitigated with the use of LET in replacement of VAR. It is all due to LET having the power to turn variables only applicable within its block.

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