for

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.

句法

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 ( ; ).

范例

使用 for

下列 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
}
					

可选 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++;
}
					

使用 for without a statement

下列 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;"
					
注意: 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.

规范

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

浏览器兼容性

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图例

完整支持
完整支持

另请参阅

Metadata

  1. JavaScript
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    33. SyntaxError: missing ; before statement
    34. SyntaxError: missing = in const declaration
    35. SyntaxError: missing ] after element list
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    49. TypeError: "x" is not a non-null object
    50. TypeError: "x" is read-only
    51. TypeError: 'x' is not iterable
    52. TypeError: More arguments needed
    53. TypeError: Reduce of empty array with no initial value
    54. TypeError: X.prototype.y called on incompatible type
    55. TypeError: can't access dead object
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    57. TypeError: can't assign to property "x" on "y": not an object
    58. TypeError: can't define property "x": "obj" is not extensible
    59. TypeError: can't delete non-configurable array element
    60. TypeError: can't redefine non-configurable property "x"
    61. TypeError: cannot use 'in' operator to search for 'x' in 'y'
    62. TypeError: cyclic object value
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    64. TypeError: invalid Array.prototype.sort argument
    65. TypeError: invalid arguments
    66. TypeError: invalid assignment to const "x"
    67. TypeError: property "x" is non-configurable and can't be deleted
    68. TypeError: setting getter-only property "x"
    69. TypeError: variable "x" redeclares argument
    70. URIError: malformed URI sequence
    71. Warning: -file- is being assigned a //# sourceMappingURL, but already has one
    72. Warning: 08/09 is not a legal ECMA-262 octal constant
    73. Warning: Date.prototype.toLocaleFormat is deprecated
    74. Warning: JavaScript 1.6's for-each-in loops are deprecated
    75. Warning: String.x is deprecated; use String.prototype.x instead
    76. Warning: expression closures are deprecated
    77. Warning: unreachable code after return statement
  13. 杂项
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    4. Enumerability and ownership of properties
    5. Iteration protocols
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    7. Transitioning to strict mode
    8. Template literals
    9. 弃用特征