for
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
Syntax
js
for (initialization; condition; afterthought)
statement
initialization
Optional-
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
orlet
keywords. Variables declared withvar
are not local to the loop, i.e. they are in the same scope thefor
loop is in. Variables declared withlet
are local to the statement.The result of this expression is discarded.
condition
Optional-
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 thefor
construct.This conditional test is optional. If omitted, the condition always evaluates to true.
afterthought
Optional-
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. You can use a block statement to execute multiple 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.
js
for (let i = 0; i < 9; i++) {
console.log(i);
// more statements
}
Initialization block syntax
The initialization block accepts both expressions and variable declarations. However, expressions cannot use the in
operator unparenthesized, because that is ambiguous with a for...in
loop.
js
for (let i = "start" in window ? window.start : 0; i < 9; i++) {
console.log(i);
}
// SyntaxError: 'for-in' loop variable declaration may not have an initializer.
js
// Parenthesize the whole initializer
for (let i = ("start" in window ? window.start : 0); i < 9; i++) {
console.log(i);
}
// Parenthesize the `in` expression
for (let i = ("start" in window) ? window.start : 0; i < 9; i++) {
console.log(i);
}
Optional for expressions
All three expressions in the head of the for
loop are optional. For example, it is not required to use the initialization
block to initialize variables:
js
let i = 0;
for (; i < 9; i++) {
console.log(i);
// more statements
}
Like the initialization
block, the condition
part 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.
js
for (let i = 0; ; i++) {
console.log(i);
if (i > 3) break;
// more statements
}
You can also omit all three expressions. 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.
js
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.
js
let i = 0;
while (i <= 3) {
console.log(i);
i++;
}
Lexical declarations in the initialization block
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:
js
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:
js
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).
js
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 afterthought
parts. More precisely, let
declarations are special-cased by for
loops — if initialization
is a let
declaration, then every time, after the loop body is evaluated, the following happens:
- A new lexical scope is created with new
let
-declared variables. - The binding values from the last iteration are used to re-initialize the new variables.
afterthought
is evaluated in the new scope.
So re-assigning the new variables within afterthought
does not affect the bindings from the previous iteration.
Creating closures allows you to get hold of a binding during any particular iteration. This explains why closures created within the initialization
section do not get updated by re-assignments of i
in the afterthought
.
js
for (let i = 0, getI = () => i; i < 3; i++) {
console.log(getI());
}
// Logs 0, 0, 0
This does not log "0, 1, 2", like what would happen if getI
is declared in the loop body. This is because getI
is not re-evaluated on each iteration — rather, the function is created once and closes over the i
variable, which refers to the variable declared when the loop was first initialized. Subsequent updates to the value of i
actually create new variables called i
, which getI
does not see. A way to fix this is to re-compute getI
every time i
updates:
js
for (let i = 0, getI = () => i; i < 3; i++, getI = () => i) {
console.log(getI());
}
// Logs 0, 1, 2
In fact, you can capture the initial binding of the i
variable and re-assign it later, and this updated value will not be visible to the loop body, which sees the next new binding of i
.
js
for (
let i = 0, getI = () => i, incrementI = () => i++;
getI() < 3;
incrementI()
) {
console.log(i);
}
// Logs 0, 0, 0
This logs "0, 0, 0", because the i
variable in each loop evaluation is actually a separate variable, but getI
and incrementI
both read and write the initial binding of i
, not what was subsequently declared.
Using for without a body
The following for
cycle calculates the offset position of a node in the afterthought
section, and therefore it does not require the use of a statement
section, a semicolon is used instead.
js
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 // afterthought
); // semicolon
console.log(
`Offset position of "${id}" element:
left: ${left}px;
top: ${top}px;`,
);
}
showOffsetPos("content");
// Logs:
// Offset position of "content" element:
// left: 0px;
// top: 153px;
Note that the semicolon after the for
statement is mandatory, because it stands as an empty statement. Otherwise, the for
statement acquires the following console.log
line as its statement
section, which makes the log
execute multiple times.
Using for with two iterating variables
You can create two counters that are updated simultaneously in a for loop using the comma operator. Multiple let
and var
declarations can also be joined with commas.
js
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
Specifications
Specification |
---|
ECMAScript Language Specification # sec-for-statement |
Browser compatibility
BCD tables only load in the browser