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fix(perf): faster looseIdentical implementation #6364

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fix(perf): faster looseIdentical implementation #6364

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26 changes: 21 additions & 5 deletions modules/angular2/src/facade/lang.dart
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Original file line number Diff line number Diff line change
Expand Up @@ -235,10 +235,26 @@ class FunctionWrapper {

const _NAN_KEY = const Object();

// Dart can have identical(str1, str2) == false while str1 == str2. Moreover,
// after compiling with dart2js identical(str1, str2) might return true.
// (see dartbug.com/22496 for details).
bool looseIdentical(a, b) =>
// Dart VM implements `identical` as true reference identity. JavaScript does
// not have this. The closest we have in JS is `===`. However, for strings JS
// would actually compare the contents rather than references. `dart2js`
// compiles `identical` to `===` and therefore there is a discrepancy between
// Dart VM and `dart2js`. The implementation of `looseIdentical` attempts to
// bridge the gap between the two while retaining good performance
// characteristics. In JS we use simple `identical`, which compiles to `===`,
// and in Dart VM we emulate the semantics of `===` by special-casing strings.
// Note that the VM check is a compile-time constant. This allows `dart2js` to
// evaluate the conditional during compilation and inline the entire function.
//
// See: dartbug.com/22496, dartbug.com/25270
const _IS_DART_VM = !identical(1.0, 1); // a hack
bool looseIdentical(a, b) => _IS_DART_VM
? _looseIdentical(a, b)
: identical(a, b);

// This function is intentionally separated from `looseIdentical` to keep the
// number of AST nodes low enough for `dart2js` to inline the code.
bool _looseIdentical(a, b) =>
a is String && b is String ? a == b : identical(a, b);

// Dart compare map keys by equality and we can have NaN != NaN
Expand Down Expand Up @@ -334,4 +350,4 @@ var global = null;

dynamic evalExpression(String sourceUrl, String expr, String declarations, Map<String, String> vars) {
throw "Dart does not support evaluating expression during runtime!";
}
}