21. Object oriented foundation "problems and solutions of object traversal"

y_ cj 2021-05-03 14:45:25
object oriented foundation problems solutions


1. Detect a member ( attribute / key ) Whether it belongs to this object , Or whether it belongs to the private property of the object

  • in: Check whether members belong to this object 「 characteristic : Whether it's private property , Or public property , As long as there is, the test result is true」

  • hasOwnProperty: Used to detect whether the current member is a private property of the object 「 characteristic : As long as it's private , The result is true, Even with this property , But it belongs to the public property , The result is also false」

function Fn() {
this.x = 10
this.y = 20
}
const f1 = new Fn()
console.log('x' in f1) // true
console.log(f1.hasOwnProperty('x')) // true
console.log('hasOwnProperty' in f1) // true
console.log(f1.hasOwnProperty('hasOwnProperty')) // false It's not a private property , It's a public property 
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  • Check whether this property is a public property of this object

    • Train of thought : It's a property , But it's not private yet , Then it must be public .「BUG: If a property is private , It's also public , The result is not accurate 」.
    function hasPubProperty(obj, attr) {
    return (attr in obj) && (!obj.hasOwnProperty(attr))
    }
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    • Train of thought two : The real idea is to test the properties on the prototype , Because all the attributes on the prototype are public .
    function hasPubProperty(obj, attr) {
    // Object.getPrototypeOf Get the prototype of the current object 
    let proto = Object.getPrototypeOf(obj)
    while(proto) {
    // Find the prototype chain in turn , Until I find Object.prototype until 
    if(proto.hasOwnProperty(attr)) {
    return true
    }
    proto = Object.getPrototypeOf(proto)
    }
    return false
    }
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    2. for in Ergodic problem

    • problem 1:

      • A lot of object Can't get Symbol Attribute
      const sy = Symbol()
      const obj = {
      name: 'zhufeng',
      [sy]: 100
      }
      for(let key in obj) {
      console.log(key) // Can't traverse Symbol Private property of 
      }
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      • solve : Just want to traverse private , contain Symbol Of
      // Object.keys Get an object that is not Symbol Private property of 
      // Object.getOwnPropertySymbols Get only Symbol Private property of 
      const keys = [
      ...Object.getOwnPropertyNames(obj),
      ...Object.getOwnPropertySymbols(obj)
      ]
      keys.forEach(key => {
      console.log(key, obj[key])
      })
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    • problem 2:

      • You can traverse to your own extended public properties 「 Built in public properties are not enumerable ( It can't be traversed )」
      const obj = {
      name: 'zhufeng'
      }
      obj.prototype.AAA = 100 // AAA yes obj The public nature of 
      for(let key in obj) {
      console.log(key) // The public property is also traversed 
      }
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      • solve : Can avoid traversing public
      for(let key in obj) {
      // Has traversed to the public , Then private has been traversed , End of cycle 
      if(!obj.hasOwnProperty(key)) break
      console.log(key)
      }
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    • problem 3:

      • Priority traversal of numeric properties , And from small to large ( It's not strictly written in the order of attributes )
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