在Immutable.js Map（）数据结构中使用first（）方法

Question

The Map data structure in Immutable.js has a first() method.

As my understanding goes, Maps are unordered. How is it then the first() method always returns the same item ?

  it('should return the value of the fist item in data structure', ()=> {
    var x = Immutable.Map({a: 10, b: 20, c: 30});
    expect(x.first()).to.equal(10); //passess everytime

  });

Answer 1

It's just a byproduct of implementation that you're not allowed to rely on (and is probably at least partially browser-dependent, thanks to the object initializer you're using ¹ ). As it says in the documentation :

Iteration order of a Map is undefined, however is stable.

So the order is undefined, and could change from one dot rev to the next. But the same code, repeated, is going to have the same outcome. Computers are deterministic.

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¹ Until ES2015, JavaScript objects didn't officially have an order to their properties. As of ES2015, they do, but that order is only guaranteed to be followed by some operations. Notably, neither for-in nor Object.keys are required by the spec to follow the order. So since building the map will require iterating the object you're passing in, it's possible the result you see (which is undefined) will vary from JavaScript engine to JavaScript engine. That said, even before the spec, most engines handled properties at the same level in the object hierarchy in the same way (order of property creation, if we ignore property names that qualify as array indexes; and the properties of an object initializer are added to the object in source-code order [even in ES5]).