for

The for loop is a fundamental looping structure in JavaScript, though it can initially be challenging to understand. It uses the for keyword and requires three parameters enclosed in parentheses:

Here's a breakdown of these parameters:

Initialization: This is where you initialize a new counter used by the for loop. It's executed only once;
Condition: An expression checked before each iteration, similar to the while loop;
Increment/Decrement: Operations performed on the counter at the end of each loop iteration.

Note

Iteration in loops refers to repeating a block of code a certain number of times or until a specific condition is met. Each time the block of code is executed, it's considered one iteration.

Let's illustrate this with an example:


              123
            
for (let i = 1; i < 5; i++) {
  console.log("Loop iteration:", i);
};

In this example:

let i = 1: Initialization, where we create the variable i inside the for loop. This operation executes once;
i < 5: Condition, checked before each iteration;
i++: Increment expression, executed after each iteration;
console.log("Loop iteration:", i);: Body of the for loop.

Each step in the loop can be described as follows:

Step 2 repeats until the condition becomes false.

It can be beneficial to consider a diagram to gain a clearer understanding of how the loop operates.

You can also use decrement in the for loop, as shown here:


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for (let i = 15; i > 10; i--) {
  console.log("i =", i);
}

The for loop counter is unique to its scope, so you don't need to worry about the counter name conflicting with other variables:


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let i = 2077;
console.log("(global) i =", i);

for (let i = 0; i < 4; i++) {
  console.log("(for) i =", i);
}

console.log("(global) i =", i);

Different expressions for Increment/Decrement operations can be used as well:


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for (let i = 0; i < 40; i += 7) {
  console.log("i =", i);
};

Comparing the for and while loops

When comparing for and while loops, the for loop is often simpler and more concise. Here's an example of equivalent loops:


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// `while`
let a = 1;
while (a <= 3) {
  console.log("While:", a);
  a++;
}

// `for`
for (let i = 1; i <= 3; i++) {
  console.log("For:", i);
}

In this comparison, the for loop is more straightforward and occupies less code space. Additionally, the for loop automatically clears the counter variable (in this case, i) after execution.

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Seção 5. Capítulo 4