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Loops offer a quick and easy way to do something repeatedly. This chapter of the JavaScript Guide introduces the different iteration statements available to JavaScript.
You can think of a loop as a computerized version of the game where you tell someone to take X steps in one direction then Y steps in another; for example, the idea "Go five steps to the east" could be expressed this way as a loop:
var step; for (step = 0; step < 5; step++) { // Runs 5 times, with values of step 0 through 4. console.log('Walking east one step'); }
There are many different kinds of loops, but they all essentially do the same thing: they repeat an action some number of times (and it's actually possible that number could be zero). The various loop mechanisms offer different ways to determine the start and end points of the loop. There are various situations that are more easily served by one type of loop over the others.
The statements for loops provided in JavaScript are:
- for statement
- do...while statement
- while statement
- labeled statement
- break statement
- continue statement
- for...in statement
- for...of statement
for
statement
A for loop
repeats until a specified condition evaluates to false. The JavaScript for loop is similar to the Java and C for
loop. A for
statement looks as follows:
for ([initialExpression]; [condition]; [incrementExpression]) statement
When a for
loop executes, the following occurs:
- The initializing expression
initialExpression
, if any, is executed. This expression usually initializes one or more loop counters, but the syntax allows an expression of any degree of complexity. This expression can also declare variables. - The
condition
expression is evaluated. If the value ofcondition
is true, the loop statements execute. If the value ofcondition
is false, thefor
loop terminates. If thecondition
expression is omitted entirely, the condition is assumed to be true. - The
statement
executes. To execute multiple statements, use a block statement ({ ... }
) to group those statements. - The update expression
incrementExpression
, if there is one, executes, and control returns to step 2.
Example
The following function contains a for
statement that counts the number of selected options in a scrolling list (a <select>
element that allows multiple selections). The for
statement declares the variable i
and initializes it to zero. It checks that i
is less than the number of options in the <select>
element, performs the succeeding if
statement, and increments i
by one after each pass through the loop.
<form name="selectForm"> <p> <label for="musicTypes">Choose some music types, then click the button below:</label> <select id="musicTypes" name="musicTypes" multiple="multiple"> <option selected="selected">R&B</option> <option>Jazz</option> <option>Blues</option> <option>New Age</option> <option>Classical</option> <option>Opera</option> </select> </p> <p><input id="btn" type="button" value="How many are selected?" /></p> </form> <script> function howMany(selectObject) { var numberSelected = 0; for (var i = 0; i < selectObject.options.length; i++) { if (selectObject.options[i].selected) { numberSelected++; } } return numberSelected; } var btn = document.getElementById("btn"); btn.addEventListener("click", function(){ alert('Number of options selected: ' + howMany(document.selectForm.musicTypes)) }); </script>
do...while
statement
The do...while
statement repeats until a specified condition evaluates to false. A do...while
statement looks as follows:
do statement while (condition);
statement
executes once before the condition is checked. To execute multiple statements, use a block statement ({ ... }
) to group those statements. If condition
is true, the statement executes again. At the end of every execution, the condition is checked. When the condition is false, execution stops and control passes to the statement following do...while
.
Example
In the following example, the do
loop iterates at least once and reiterates until i is no longer less than 5.
var i = 0; do { i += 1; console.log(i); } while (i < 5);
while
statement
A
while
statement executes its statements as long as a specified condition evaluates to true. A while
statement looks as follows:
while (condition) statement
If the condition becomes false, statement
within the loop stops executing and control passes to the statement following the loop.
The condition test occurs before statement
in the loop is executed. If the condition returns true, statement
is executed and the condition is tested again. If the condition returns false, execution stops and control is passed to the statement following while
.
To execute multiple statements, use a block statement ({ ... }) to group those statements.
Example 1
The following while
loop iterates as long as n
is less than three:
var n = 0; var x = 0; while (n < 3) { n++; x += n; }
With each iteration, the loop increments n
and adds that value to x
. Therefore, x
and n
take on the following values:
- After the first pass:
n
= 1 andx
= 1 - After the second pass:
n
= 2 andx
= 3 - After the third pass:
n
= 3 andx
= 6
After completing the third pass, the condition n < 3
is no longer true, so the loop terminates.
Example 2
Avoid infinite loops. Make sure the condition in a loop eventually becomes false; otherwise, the loop will never terminate. The statements in the following while
loop execute forever because the condition never becomes false:
while (true) { console.log("Hello, world"); }
labeled
statement
A label
provides a statement with an identifier that lets you refer to it elsewhere in your program. For example, you can use a label to identify a loop, and then use the break
or continue
statements to indicate whether a program should interrupt the loop or continue its execution.
The syntax of the labeled statement looks like the following:
label : statement
The value of label
may be any JavaScript identifier that is not a reserved word. The statement
that you identify with a label may be any statement.
Example
In this example, the label markLoop
identifies a while
loop.
markLoop: while (theMark == true) { doSomething(); }
break
statement
Use the break
statement to terminate a loop, switch
, or in conjunction with a labeled statement.
- When you use
break
without a label, it terminates the innermost enclosingwhile
,do-while
,for
, orswitch
immediately and transfers control to the following statement. - When you use
break
with a label, it terminates the specified labeled statement.
The syntax of the break
statement looks like this:
break [label];
The first form of the syntax terminates the innermost enclosing loop or switch
; the second form of the syntax terminates the specified enclosing labeled statement.
Example 1
The following example iterates through the elements in an array until it finds the index of an element whose value is theValue
:
for (var i = 0; i < a.length; i++) { if (a[i] == theValue) { break; } }
Example 2: Breaking to a label
var x = 0; var z = 0; labelCancelLoops: while (true) { console.log("Outer loops: " + x); x += 1; z = 1; while (true) { console.log("Inner loops: " + z); z += 1; if (z === 10 && x === 10) { break labelCancelLoops; } else if (z === 10) { break; } } }
continue
statement
The continue
statement can be used to restart a while
, do-while
, for
, or label
statement.
- When you use
continue
without a label, it terminates the current iteration of the innermost enclosingwhile
,do-while
, orfor
statement and continues execution of the loop with the next iteration. In contrast to thebreak
statement,continue
does not terminate the execution of the loop entirely. In awhile
loop, it jumps back to the condition. In afor
loop, it jumps to theincrement-expression
. - When you use
continue
with a label, it applies to the looping statement identified with that label.
The syntax of the continue
statement looks like the following:
continue
[label];
Example 1
The following example shows a while
loop with a continue
statement that executes when the value of i
is three. Thus, n
takes on the values one, three, seven, and twelve.
var i = 0; var n = 0; while (i < 5) { i++; if (i == 3) { continue; } n += i; }
Example 2
A statement labeled checkiandj
contains a statement labeled checkj
. If continue
is encountered, the program terminates the current iteration of checkj
and begins the next iteration. Each time continue
is encountered, checkj
reiterates until its condition returns false
. When false
is returned, the remainder of the checkiandj
statement is completed, and checkiandj
reiterates until its condition returns false
. When false
is returned, the program continues at the statement following checkiandj
.
If continue
had a label of checkiandj
, the program would continue at the top of the checkiandj
statement.
checkiandj: while (i < 4) { console.log(i); i += 1; checkj: while (j > 4) { console.log(j); j -= 1; if ((j % 2) == 0) { continue checkj; } console.log(j + " is odd."); } console.log("i = " + i); console.log("j = " + j); }
for...in
statement
The for...in
statement iterates a specified variable over all the enumerable properties of an object. For each distinct property, JavaScript executes the specified statements. A for...in
statement looks as follows:
for (variable in object) { statements }
Example
The following function takes as its argument an object and the object's name. It then iterates over all the object's properties and returns a string that lists the property names and their values.
function dump_props(obj, obj_name) { var result = ""; for (var i in obj) { result += obj_name + "." + i + " = " + obj[i] + "<br>"; } result += "<hr>"; return result; }
For an object car
with properties make
and model
, result
would be:
car.make = Ford car.model = Mustang
Arrays
Although it may be tempting to use this as a way to iterate over Array
elements, the for...in statement will return the name of your user-defined properties in addition to the numeric indexes. Thus it is better to use a traditional for
loop with a numeric index when iterating over arrays, because the for...in statement iterates over user-defined properties in addition to the array elements, if you modify the Array object, such as adding custom properties or methods.
for...of
statement
The for...of
statement creates a loop Iterating over iterable objects (including Array
, Map
, Set
, arguments
object and so on), invoking a custom iteration hook with statements to be executed for the value of each distinct property.
for (variable of object) { statement }
The following example shows the difference between a for...of
loop and a for...in
loop. While for...in
iterates over property names, for...of
iterates over property values:
let arr = [3, 5, 7]; arr.foo = "hello"; for (let i in arr) { console.log(i); // logs "0", "1", "2", "foo" } for (let i of arr) { console.log(i); // logs "3", "5", "7" }