for - JavaScript 编辑

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.

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The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

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. This conditional test is optional. If omitted, the condition always evaluates to true. If the expression evaluates to false, execution skips to the first expression following the for construct.

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:

var 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.

var i = 0;

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

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(sId) {

  var nLeft = 0, nTop = 0;

  for (

    var oItNode = document.getElementById(sId); /* initialization */

    oItNode; /* condition */

    nLeft += oItNode.offsetLeft, nTop += oItNode.offsetTop, oItNode = oItNode.offsetParent /* final-expression */

  ); /* semicolon */

  console.log('Offset position of \'' + sId + '\' element:\n left: ' + nLeft + 'px;\n top: ' + nTop + '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.

Specifications

Specification
ECMAScript (ECMA-262) The definition of 'for statement' in that specification.

Browser compatibility

BCD tables only load in the browser

See also

• empty statement
• break
• continue
• while
• do...while
• for...in
• for...of