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48 lines
2.8 KiB
Markdown
48 lines
2.8 KiB
Markdown
## Loops and Comprehensions
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Most of the loops you’ll write in CoffeeScript will be **comprehensions** over arrays, objects, and ranges. Comprehensions replace (and compile into) **for** loops, with optional guard clauses and the value of the current array index. Unlike for loops, array comprehensions are expressions, and can be returned and assigned.
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```
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codeFor('array_comprehensions')
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```
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Comprehensions should be able to handle most places where you otherwise would use a loop, **each**/**forEach**, **map**, or **select**/**filter**, for example:<br>
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`shortNames = (name for name in list when name.length < 5)`<br>
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If you know the start and end of your loop, or would like to step through in fixed-size increments, you can use a range to specify the start and end of your comprehension.
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```
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codeFor('range_comprehensions', 'countdown')
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```
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Note how because we are assigning the value of the comprehensions to a variable in the example above, CoffeeScript is collecting the result of each iteration into an array. Sometimes functions end with loops that are intended to run only for their side-effects. Be careful that you’re not accidentally returning the results of the comprehension in these cases, by adding a meaningful return value — like `true` — or `null`, to the bottom of your function.
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To step through a range comprehension in fixed-size chunks, use `by`, for example:
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`evens = (x for x in [0..10] by 2)`
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If you don’t need the current iteration value you may omit it:
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`browser.closeCurrentTab() for [0...count]`
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Comprehensions can also be used to iterate over the keys and values in an object. Use `of` to signal comprehension over the properties of an object instead of the values in an array.
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```
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codeFor('object_comprehensions', 'ages.join(", ")')
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```
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If you would like to iterate over just the keys that are defined on the object itself, by adding a `hasOwnProperty` check to avoid properties that may be inherited from the prototype, use `for own key, value of object`.
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To iterate a generator function, use `from`. See [Generator Functions](#generator-iteration).
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The only low-level loop that CoffeeScript provides is the **while** loop. The main difference from JavaScript is that the **while** loop can be used as an expression, returning an array containing the result of each iteration through the loop.
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```
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codeFor('while', 'lyrics.join("\\n")')
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```
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For readability, the **until** keyword is equivalent to `while not`, and the **loop** keyword is equivalent to `while true`.
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When using a JavaScript loop to generate functions, it’s common to insert a closure wrapper in order to ensure that loop variables are closed over, and all the generated functions don’t just share the final values. CoffeeScript provides the `do` keyword, which immediately invokes a passed function, forwarding any arguments.
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```
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codeFor('do')
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```
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