while
Just
as the if
statement is the basic control statement
that allows JavaScript to make decisions, the
while
statement is the basic statement that allows
JavaScript to perform repetitive actions. It has the following
syntax:
while (expression
)statement
The while
statement works by first evaluating
expression
. If it is
false
, JavaScript moves on to the next statement
in the program. If it is true
, the
statement
that forms the body of the loop
is executed and expression
is evaluated
again. Again, if the value of expression
is false
, JavaScript moves on to the next
statement in the program; otherwise, it executes
statement
again. This cycle continues
until expression
evaluates to
false
, at which point the while
statement ends and JavaScript moves on. Note that you can create an
infinite loop with the syntax while(true)
.
Usually, you do
not want JavaScript to perform exactly the same operation over and
over again. In almost every loop, one or more variables change with
each iteration of the loop. Since the variables
change, the actions performed by executing
statement
may differ each time through the
loop. Furthermore, if the changing variable or variables are involved
in expression
, the value of the expression
may be different each time through the loop. This is
important -- otherwise, an expression that starts off
true
would never change and the loop would never
end! Here is an example while
loop:
var count = 0; while (count < 10) { document.write(count + "<br>"); count++; ...
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