如何在ES6中获取Set的第一个元素(EcmaScript 2015)

Question

在ES6中,我们如何快速获得元素？

在MDN语法集中,我没有找到答案.

Answer 1

它们似乎没有公开List可以从instanced Object访问.这来自EcmaScript草案:

23.2.4设置实例的属性

Set实例是从Set原型继承属性的普通对象.设置实例也有[[SetData]]内部插槽.

[[SetData]]是Set所持有的值列表.

一个可能的解决方案(一个有点昂贵的解决方案)是获取迭代器然后调用next()第一个值:

var x = new Set();
x.add(1);
x.add({ a: 2 });
//get iterator:
var it = x.values();
//get first entry:
var first = it.next();
//get value out of the iterator entry:
var value = first.value;
console.log(value); //1

值得一提的是:

Set.prototype.values === Set.prototype.keys

Answer 2

有很多解决方案

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1. for...of环形
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const set = new Set();\nset.add(2);\nset.add(3);\n\n// return the first item of Set \xe2\x9c\x85\nfunction getFirstItemOfSet(set) {\n  for(let item of set) {\n    if(item) {\n       return item;\n    }   \n  }\n  return undefined;\n}\n\nconst first = getFirstItemOfSet(set);\nconsole.log(\'first item =\', first);

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2. destructuring assignment
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const set = new Set();\nset.add(2);\nset.add(3);\n\n// only get the first item \xe2\x9c\x85\nconst [first] = set;\nconsole.log(\'first item =\', first, typeof first);\n// first item = 2 number

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3. ...spread操作员
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const set = new Set();\nset.add(2);\nset.add(3);\n\n// convert Set to Array \xe2\x9c\x85\nconst first = [...set][0];\nconsole.log(\'first item =\', first);

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4. iterator&next()
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const set = new Set();\nset.add(2);\nset.add(3);\n\n// iterator \xe2\x9c\x85\nconst first = set.keys().next().value;\nconsole.log(`first item =`, first);\n\n// OR\nset.values().next().value;\n\n// OR\nset.entries().next().value[0];\n// OR\nset.entries().next().value[1];

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参考文献

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https://www.cnblogs.com/xgqfrms/p/16564519.html

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