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目录

JavaScript for Loop

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Example

Loop (iterate over) a code block five times:

for (let i = 0; i < 5; i++) {
  text += i + "<br>";
}
Try it Yourself »

Loop (iterate over) an array to collect car names:

const cars = ["BMW", "Volvo", "Saab", "Ford"];
for (let i = 0; i < cars.length; i++) {
  text += cars[i] + "<br>";
}
Try it Yourself »
  • The loop starts in position 0 (let i = 0).
  • The loop automatically increments i for each run.
  • The loop runs as long as i < cars.length.

More examples below.

Description

The for statement defines a code block that is executed as long as a condition is true.

Note

If you omit statement 2, you must provide a break inside the loop.

Otherwise the loop will never end. This will crash your browser.

See Also:

The JavaScript for Tutorial

Syntax

for (statement 1; statement 2; statement 3) {
  code block to be executed
}

Parameters

Parameter Description
statement 1 Optional.
Executed before the code block starts.
Normally used to initialize a counter variable.
To initiate multiple values, separate each value with a comma.

This parameter can be omitted, but not the semicolon ";"
statement 2 Optional.
The condition for running the code block.
If it returns true the loop will start over again, otherwise the loop will end.

This parameter can be omitted, but not the semicolon ";"
statement 3 Optional.
Executed after the code block.
Normally used to increment the counter variable.

This parameter can be omitted (e.g. to increase/decrease values inside the loop)

JavaScript Loop Statements

Statement Description
break Breaks out of a loop
continue Skips a value in a loop
while Loops a code block while a condition is true
do...while Loops a code block once, and then while a condition is true
for Loops a code block while a condition is true
for...of Loops the values of any iterable
for...in Loops the properties of an object

More Examples

Initiate multiple values in the first parameter:

const cars = ["BMW", "Volvo", "Saab", "Ford"];
for (let i = 0, len = cars.length, text = ""; i < len; i++) {
  text += cars[i] + "<br>";
}
Try it Yourself »

Omit the first parameters (set the values before the loop starts):

const cars = ["BMW", "Volvo", "Saab", "Ford"];
let i = 2;
let len = cars.length;
let text = "";
for (; i < len; i++) {
  text += cars[i] + "<br>";
}
Try it Yourself »

Use continue - Loop through a block of code, but skip the value 3:

let text = "";
for (let i = 0; i < 5; i++) {
  if (i == 3) continue;
  text += i + "<br>";
}
Try it Yourself »

Use break - Loop a code block, but exit the loop when i == 3:

let text = "";
for (let i = 0; i < 5; i++) {
  if (i == 3) break;
  text += i + "<br>";
}
Try it Yourself »

Omit the second parameter.

Use break to exit the loop, otherwise the loop will never end, and your browser will crash:

const cars = ["BMW", "Volvo", "Saab", "Ford"];
let text = "";
for (let i = 0; ; i++) {
  if (i == cars-length) break;
  text += cars[i] + "<br>";
}
Try it Yourself »

Loop over an array in descending order (negative increment):

const cars = ["BMW", "Volvo", "Saab", "Ford"];
let text = "";
for (let i = cars.length - 1; i >= 0; i--) {
  text += cars[i] + "<br>";
}
Try it Yourself »

Omit the last parameter, and increment the values inside the loop:

const cars = ["BMW", "Volvo", "Saab", "Ford"];
let i = 0;
let len = cars.length;
for (; i < len;) {
  text += cars[i] + "<br>";
  i++;
}
Try it Yourself »

Loop a NodeList and change the color of all p elements in the list:

const myNodelist = document.getElementsByTagName("P");
for (let i = 0; i < myNodelist.length; i++) {
  myNodelist[i].style.color = "blue";
}
Try it Yourself »

A nested loop (a loop inside a loop):

let text = "";
for (let = 0; i < 3; i++) {
  text += i + "<br>";
  for (let j = 10; j < 15; j++) {
    text += j + "<br>";
  }
}
Try it Yourself »

Browser Support

for is an ECMAScript1 (ES1) feature.

ES1 (JavaScript 1997) is fully supported in all browsers:

Chrome Edge Firefox Safari Opera IE
Yes Yes Yes Yes Yes Yes
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