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Map optimization : each boiler #1708

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Jun 27, 2014
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Map optimization : each boiler #1708

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19 changes: 13 additions & 6 deletions underscore.js
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Original file line number Diff line number Diff line change
Expand Up @@ -112,13 +112,20 @@
};

// Return the results of applying the iterator to each element.
_.map = _.collect = function(obj, iterator, context) {
var results = [];
if (obj == null) return results;
_.map = _.collect = function(obj, iterator, context) {
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It looks as though there's an extra space at the beginning of this line.

if (obj == null) return [];
iterator = lookupIterator(iterator, context);
_.each(obj, function(value, index, list) {
results.push(iterator(value, index, list));
});
var length = obj.length,
currentKey, keys;
if (length !== +length) {
keys = _.keys(obj);
length = keys.length;
}
var results = Array(length);
for (var index = 0; index < length; index++) {
currentKey = keys ? keys[index] : index;
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You shouldn't test keys in the loop. For optimum performance, we should pull the keys test out and have two loops: one that accesses keys[index] and one that uses index directly.

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You've brought this up with other parts of Underscore in the past & I believe at the time it was proven that the check was negligible. We can check again to validate but avoiding the second loop is a big win for avoiding code dup.

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Oh, I didn't realise. I'm not super opinionated either way. We'll stick with the simpler style.

results[index] = iterator(obj[currentKey], currentKey, obj);
}
return results;
};

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