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for Keyword

Site: Saylor Academy
Course: PRDV401: Introduction to JavaScript I
Book: for Keyword
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Date: Sunday, May 19, 2024, 1:17 AM

Table of contents

for Keyword

The for statement creates a loop that consists of three optional expressions, enclosed in parentheses and separated by semicolons, followed by a statement (usually a block statement) to be executed in the loop.

Try It

Click here to try an example using Code Runner.

Syntax

for ([initialization]; [condition]; [final-expression])
  statement

initialization

An expression (including assignment expressions) or variable declaration evaluated once before the loop begins. Typically used to initialize a counter variable. This expression may optionally declare new variables with var or let keywords. Variables declared with var are not local to the loop, i.e. they are in the same scope the for loop is in. Variables declared with let are local to the statement.

The result of this expression is discarded.


condition

An expression to be evaluated before each loop iteration. If this expression evaluates to true, statement is executed. If the expression evaluates to false, execution exits the loop and goes to the first statement after the for construct.

This conditional test is optional. If omitted, the condition always evaluates to true.


final-expression

An expression to be evaluated at the end of each loop iteration. This occurs before the next evaluation of condition. Generally used to update or increment the counter variable.


statement

A statement that is executed as long as the condition evaluates to true. To execute multiple statements within the loop, use a block statement ({ /* ... */ }) to group those statements. To execute no statement within the loop, use an empty statement (;).

Examples

Using for

The following for statement starts by declaring the variable i and initializing it to 0. It checks that i is less than nine, performs the two succeeding statements, and increments i by 1 after each pass through the loop. 

for (let i = 0; i < 9; i++) {
  console.log(i);
  // more statements
}


Optional for expressions

All three expressions in the head of the for loop are optional.

For example, in the initialization block it is not required to initialize variables: 

let i = 0;
for (; i < 9; i++) {
  console.log(i);
  // more statements
}

Like the initialization block, the condition block is also optional. If you are omitting this expression, you must make sure to break the loop in the body in order to not create an infinite loop. 

for (let i = 0;; i++) {
  console.log(i);
  if (i > 3) break;
  // more statements
}

You can also omit all three blocks. Again, make sure to use a break statement to end the loop and also modify (increase) a variable, so that the condition for the break statement is true at some point. 

let i = 0;

for (;;) {
  if (i > 3) break;
  console.log(i);
  i++;
}

However, in the case where you are not fully using all three expression positions - especially if you are not declaring variables with the first expression but mutating something in the upper scope - consider using a while loop instead, which makes the intention clearer.

let i = 0;

while (i <= 3) {
  console.log(i);
  i++;
}


Lexical declarations in the initialization block is scoped to the for loop

Declaring a variable within the initialization block has important differences from declaring it in the upper scope, especially when creating a closure within the loop body. For example, for the code below:

for (let i = 0; i < 3; i++) {
  setTimeout(() => {
    console.log(i);
  }, 1000);
}

…it logs 0, 1, and 2, as expected. However, if the variable is defined in the upper scope:

let i = 0;
for (; i < 3; i++) {
  setTimeout(() => {
    console.log(i);
  }, 1000);
}

…it logs 3, 3, and 3. The reason is that each setTimeout creates a new closure that closes over the i variable, but if the i is not scoped to the loop body, all closures will reference the same variable when they eventually get called - and due to the asynchronous nature of setTimeout, it will happen after the loop has already exited, causing the value of i in all queued callbacks' bodies to have the value of 3.

This also happens if you use a var statement as the initialization, because variables declared with var are only function-scoped, but not lexically scoped (i.e. they can't be scoped to the loop body).

for (var i = 0; i < 3; i++) {
  setTimeout(() => {
    console.log(i);
  }, 1000);
}
// Logs 3, 3, 3

The scoping effect of the initialization block can be understood as if the declaration happens within the loop body, but just happens to be accessible within the condition and final-expression parts.


Using for without a statement

The following for cycle calculates the offset position of a node in the final-expression section, and therefore it does not require the use of a statement section, a semicolon is used instead. 

function showOffsetPos(id) {
  let left = 0;
  let top = 0;
  for (
    let itNode = document.getElementById(id); /* initialization */
    itNode; /* condition */
    left += itNode.offsetLeft, top += itNode.offsetTop, itNode = itNode.offsetParent /* final-expression */
  ); /* semicolon */

  console.log(`Offset position of '${id}' element:\n left: ${left}px;\n top: ${top}px;`);

}

/* Example call: */

showOffsetPos('content');

// Output:
// "Offset position of "content" element:
// left: 0px;
// top: 153px;"

Note: This is one of the few cases in JavaScript where the semicolon is mandatory. Indeed, without the semicolon the line that follows the cycle declaration will be considered a statement.


Using for with two iterating variables

You can create two counters that are updated simultaneously in a for loop using the comma operator.

const arr = [1, 2, 3, 4, 5, 6];
for (let l = 0, r = arr.length - 1; l < r; l++, r--) {
  console.log(arr[l], arr[r]);
}
// 1 6
// 2 5
// 3 4