for

Введение

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

Синтаксис

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

initialization

An expression (including assignment expressions) or variable declaration. Typically used to initialize a counter variable. This expression may optionally declare new variables with the var keyword. These variables are not local to the loop, i.e. they are in the same scope the for loop is in. 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 (;).

Примеры

Пример: Использование 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 (var i = 0; i < 9; i++) {
   console.log(i);
   // more statements
}

Пример: Необязательные аргументы инициализации в  for

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 (var 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++;
}

Пример: Использование for без блока выражений

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 or block statement section, an empty statement 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
       /* empty statement */ ;
  
  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;"

Спецификации

Поддержка в браузерах

Так же можете просмотреть

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