for/in
The
for
keyword
is used in two ways in JavaScript. We’ve just seen how it is
used in the for loop. It is also used in the
for/in statement. This statement is a somewhat
different kind of loop with the following syntax:
for (variableinobject) statement
variable should be either the name of a
variable, a var statement declaring a variable, an
element of an array, or a property of an object (i.e., it should be
something suitable as the leftthand side of an assignment
expression). object is the name of an
object or an expression that evaluates to an object. As usual,
statement is the statement or statement
block that forms the body of the loop.
You
can loop through the elements of an array by simply incrementing an
index variable each time through a while or
for loop. The for/in statement
provides a way to loop through the properties of an object. The body
of the for/in loop is executed once for each
property of object. Before the body of the
loop is executed, the name of one of the object’s properties is
assigned to variable, as a string. Within
the body of the loop, you can use this variable to look up the value
of the object’s property with the []
operator. For example, the following for/in loop
prints the name and value of each property of an object:
for (var prop in my_object) {
document.write("name: " + prop + "; value: " + my_object[prop], "<br>");
}Note that the
variable
in the
for/in loop may be an arbitrary expression, as long as it evaluates to something ...