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[Javascript] Advanced Flattening

JavaScript Advanced
2023-09-14 09:00:55 时间

Really the trick to flattening deeply nested structures is to just keep nesting map expressions until you have a variable or function argument, when is basically the same thing here, a variable bound to every single item you need to create your result.

Because there are three nested map expressions, I am going to end up with a three dimensional collection. The first step is figuring out how many levels deep your collection is. The next step is to flatten it n-1 times.

If your collection is three dimensional, you need two concat alls. It also matters where you put the concat all. I can't put the concat all here because I want you to notice that this expression by itself is just a one dimensional collection. We're just taking an array of closes, filtering it, and then mapping it not into another collection but just into an object.

var exchanges = [
  { 
    name: "NYSE",
    stocks: [
      { 
        symbol: "XFX", 
        closes: [
          { date: new Date(2014,11,24), price: 240.10 },
          { date: new Date(2014,11,23), price: 232.08 },
          { date: new Date(2014,11,22), price: 241.09 }
        ]
      },
      { 
        symbol: "TNZ", 
        closes: [
          { date: new Date(2014,11,24), price: 521.24 },
          { date: new Date(2014,11,23), price: 511.00 },
          { date: new Date(2014,11,22), price: 519.29 }     
        ]
      },
    ]
  },
  { 
    name: "TSX",
    stocks: [
      { 
        symbol: "JXJ", 
        closes: [
          { date: new Date(2014,11,24), price: 423.22 },
          { date: new Date(2014,11,23), price: 424.84 },
          { date: new Date(2014,11,22), price: 419.72 }
        ]
      },
      { 
        symbol: "NYN", 
        closes: [
          { date: new Date(2014,11,24), price: 16.82 },
          { date: new Date(2014,11,23), price: 16.12 },
          { date: new Date(2014,11,22), price: 15.77 }
        ]
      },
    ]
  }
];

Array.prototype.concatAll = function() {
  var results = [];
  
  this.forEach(function(subArray) {
    subArray.forEach(function(item) {
      results.push(item);
    });
  });
  
  return results;
};


// Want to output the closes which the date is 12/24
var christmasEveCloses = exchanges.
  map(function(exchange){
    return exchange.stocks
    .map(function(stock){
      return stock.closes.
       filter(function(close){
          return close.date.getMonth() === 11 &&
            close.date.getDate() === 24;
       })
       .map(function(close){
          return {
           symbol: stock.symbol,
           price: close.price
          }
      });
    }).concatAll();
  }).concatAll();
     
    
    
    
christmasEveCloses.forEach(function(christmasEveClose) {
  console.log(christmasEveClose);
});

 

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