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---
layout: default
permalink: docs/iteration.html
---
# Iteration
Stylus allows you to iterate expressions via the `for/in` construct, taking the form of:
for <val-name> [, <key-name>] in <expression>
For example:
body
for num in 1 2 3
foo num
Yields:
body {
foo: 1;
foo: 2;
foo: 3;
}
The example below shows how to use the `<key-name>`:
body
fonts = Impact Arial sans-serif
for font, i in fonts
foo i font
Yielding:
body {
foo: 0 Impact;
foo: 1 Arial;
foo: 2 sans-serif;
}
And here's how you do a regular for loop
body
for num in (1..5)
foo num
Yields:
body {
foo: 1;
foo: 2;
foo: 3;
foo: 4;
foo: 5;
}
## Mixins
We can use iteration within mixins to produce powerful functionality. For example, we can apply expression pairs as properties using interpolation and iteration.
Below we define `apply()`, conditionally utilizing all the `arguments` so that comma-delimited _and_ expression lists are supported:
apply(props)
props = arguments if length(arguments) > 1
for prop in props
{prop[0]} prop[1]
body
apply(one 1, two 2, three 3)
body
list = (one 1) (two 2) (three 3)
apply(list)
## Functions
Stylus functions may also contain for-loops. Below are some example use-cases:
Sum:
sum(nums)
sum = 0
for n in nums
sum += n
sum(1 2 3)
// => 6
join:
join(delim, args)
buf = ''
for arg, index in args
if index
buf += delim + arg
else
buf += arg
join(', ', foo bar baz)
// => "foo, bar, baz"
## Postfix
Much like `if` / `unless` may be utilized post-statement, the same can be done with `for`. Below are the same examples as above utilizing the postfix syntax:
sum(nums)
sum = 0
sum += n for n in nums
join(delim, args)
buf = ''
buf += i ? delim + arg : arg for arg, i in args
We can also __return__ from within a loop, below is an example returning the
number when `n % 2 == 0` evaluates to __true__.
first-even(nums)
return n if n % 2 == 0 for n in nums
first-even(1 3 5 5 6 3 2)
// => 6
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