Set

ES6 provides a new data structure, Set. It is similar to an array, but the values of the members are unique and there are no duplicate values.

The Set itself is a constructor used to generate the Set data structure.

Properties and methods of a Set instance

Instances of the Set structure have the following properties.

  • Set.prototype.constructorThe: constructor is the defaultSetFunction.
  • Set.prototype.sizeReturns theSetThe total number of members of an instance.

The methods of a Set instance fall into two broad categories: operation methods (for manipulating data) and traversal methods (for traversing members). The following four operations are introduced.

  • Set.prototype.add(value): adds a value and returns the Set structure itself.
  • Set.prototype.delete(value): Deletes a value and returns a Boolean value indicating whether the deletion was successful.
  • Set.prototype.has(value): Returns a Boolean value indicating whether the value isSetA member of.
  • Set.prototype.clear(): Clears all members with no return value.
let set = new Set();

set.add("abc");
set.add("abc");
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The code above adds members to the Set structure using the add() method, which shows that the Set structure does not add duplicate values.

Traversal operation

An instance of a Set structure has four traversal methods that can be used to traverse members.

  • Set.prototype.keys(): returns a traverser for key names
  • Set.prototype.values(): returns a traverser for key values
  • Set.prototype.entries(): returns a traverser for key-value pairs
  • Set.prototype.forEach(): Iterates through each member using the callback function
let set = new Set(['aaa', 'bbb']);

for (let item of set.keys()) {
  console.log(item);
}
//aaa
//bbb
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Application extension operators for traversal (…) Combined with the Set structure, you can remove duplicate members of an array.

let arr = [1, 2, 2, 3, 5, 5];
let unique = [...new Set(arr)];
// [1, 2, 3，5]
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Map

JavaScript objects are essentially collections of key-value pairs (Hash structures), but traditionally strings can only be used as keys. This puts a great limit on its use. The key of a map-like object can be of any type.

Let a = {"name":" zhang SAN "}; const myMap = new Map(); MyMap. Set (a, "death"); console.log(strMapToObj(myMap)); //{'[object object]': 'death penalty'}Copy the code

Instance properties and operation methods

  • sizeProperty returns the total number of members of the Map structure.
  • setMethod to set the key namekeyThe corresponding key value isvalue, and return to the entire Map structure. ifkeyIf there is already a value, the key value is updated, otherwise the key is generated.
  • getMethods readkeyCorresponding key value, if can’t findkeyTo return toundefined.
  • hasMethod returns a Boolean value indicating whether a key is in the current Map object.
  • deleteMethod to delete a key and returntrue. If the deletion fails, returnfalse.
  • clearMethod clears all members with no return value.
const myMap = new Map()
    .set('yes', true)
    .set('no', false);
console.log(strMapToObj(myMap));
// { yes: true, no: false }
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Traversal methods

The Map structure natively provides three traverser generating functions and one traversal method.

  • Map.prototype.keys(): returns a traverser for key names.
  • Map.prototype.values(): returns a traverser for key values.
  • Map.prototype.entries(): returns a traverser for all members.
  • Map.prototype.forEach(): Traverses all Map members.
Let a = {"name":" zhang SAN "}; const myMap = new Map(); MyMap. Set (a, "death"); for (let key of myMap.keys()) { console.log(key); //{name: 'zhang3'}}Copy the code

The most convenient way to convert a Map to an array is to use the extension operator (…). .

Let a = {"name":" zhang SAN "}; const myMap = new Map(); MyMap. Set (a, "death"); console.log([...myMap]);Copy the code

Map to object If all Map keys are strings, it can be converted to objects losslessly.

function strMapToObj(strMap) {
  let obj = {};
  for (let [k,v] of strMap) {
    obj[k] = v;
  }
  return obj;
}

const myMap = new Map()
  .set('yes', true)
  .set('no', false);
strMapToObj(myMap)
// { yes: true, no: false }
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If there is a non-string key name, it is converted to a string and used as the object’s key name.