How to iterate over an Object's specific keys in JavaScript?

Question

I have an object and I want to iterate over some specific keys of that object. How to achieve this?

Consider the snippet below:

How can I iterate over table1,table2 and table3 keys and not all of them?

var table_object = {
  table1: "hello world",
  table1_name: "greetings_english.html",
  table2: "hola",
  table2_name: "greetings_spanish.html",
  table3: "Bonjour",
  table3_name: "greetings_french.html"
};

Answer 1

You don't need fancy filters or regular expression to accomplish such a simple task! Ignore those misleading answers and start using the full power of JavaScript!
You should use the Object.defineProperty() method on your object and set to enumerable: false all the properties you don't want to iterate through. In this way you also decouple your naming convention from your logic. Let me show you:

// Defining iterable properties. This ones will be returned
// by Object.keys()

var table_object = {
  table1: "hello world",
  table2: "hola",
  table3: "Bonjour",
  // It works even if you declare them in advance
  // table1_name: "greetings_english.html",
  // table2_name: "greetings_spanish.html",
  // table3_name: "greetings_french.html",
};

// Declaring the not iterable properties. They can still be
// accessed, but they will not be iterated through

Object.defineProperty(table_object, "table1_name", {
  // delete the next line if you have already declared it
  value: "greetings_english.html", 
  enumerable: false
});

Object.defineProperty(table_object, "table2_name", {
  // delete the next line if you have already declared it
  value: "greetings_spanish.html",
  enumerable: false
});

Object.defineProperty(table_object, "table3_name", {
  // delete the next line if you have already declared it
  value: "greetings_french.html",
  enumerable: false
});

// Iterating through the object using for ... in, which iterates
// over the keys

for (var key in table_object) {
  console.log(table_object[key]);
}

The not enumerable properties can still be retrieved along with all the enumerable ones using Object.getOwnPropertyNames().
However, if you are planning to actually use this second method to iterate through all the properties when filtering is not needed anymore I've to give you some advice:

• enumerable can be set back to true, so if it a one-time change I'd highly suggest to revert it.

• If filtering is toogled back and forth more often than not, then you should check twice the structure of your object, since there may be more suitable options for your needs (such as arrays into the object).

Answer 2

I think a better way to do it in this case is making your object like this:

var table = {
    "hello world": "greetings_english.html",
    "hola": "greetings_spanish.html",
    "bonjour": "greetings_french.html"
};

for( var i in table ) {
    console.log( i );
    console.log( table[ i ] );
}

Or you could create two arrays:

var greetings = [
    "hello world",
    "hola",
    "bonjour"
];
var names = [
    "greetings_english.html",
    "greetings_spanish.html",
    "greetings_french.html"
];

for( var i = 0; i < greetings.length; i ++ ) {
    console.log( greetings[ i ] );
    console.log( names[ i ] );
}

You can make a table using this method

But in any case of your question:

var table = {
    table1: "hello world",
    table1_name: "greetings_english.html",
    table2: "hola",
    table2_name: "greetings_spanish.html",
    table3: "bonjour",
    table3_name: "greetings_french.html"
};

// Now there are three methods

console.log( "--- Method 1 ---" );
Object.keys( table )
      .filter( function( key ) {
          return /^table(\d+)$/.test( key );
      } )
      .forEach( function( key ) {
          console.log( key );
          console.log( table[ key ] );
          console.log( table[ key + "_name" ] );
      } );
      
console.log( "--- Method 2 ---" );
for ( var i in table ) {
    if ( /^table(\d+)$/.test( i ) ) {
        console.log( i );
        console.log( table[ i ] );
        console.log( table[ i + "_name" ] );
    }
}
      
console.log( "--- Method 3 ---" );
var keys = [
    "table1",
    "table2",
    "table3"
];
for ( var i = 0; i < keys.length; i ++ ) {
    console.log( keys[ i ] );
    console.log( table[ keys[ i ] ] );
    console.log( table[ keys[ i ] + "_name" ] );
}

Method 2 would be the best.

Answer 3

You have to specify which keys you have to iterate through:

var table_object = {
  table1: "hello world",
  table1_name: "greetings_english.html",
  table2: "hola",
  table2_name: "greetings_spanish.html",
  table3: "Bonjour",
  table3_name: "greetings_french.html"
};

var keysToIterateThrough = ["table1", "table2", "table3"];
keysToIterateThrough.forEach(key => {
    var value = table_object[key];
    console.log(`key: ${key} => value: ${value}`);
})

Answer 4

You could filter the keys and then iterate the rest.

var table_object = { table1: "hello world", table1_name: "greetings_english.html", table2: "hola", table2_name: "greetings_spanish.html", table3: "Bonjour", table3_name: "greetings_french.html" };

Object
    .keys(table_object)
    .filter(function (k) { return !/_/.test(k); })
    .forEach(function (k) { console.log(table_object[k]); });

Answer 5

You have to use Object.keys in order to find out all the object keys then apply a filter method in order to filter them.

var table_object = {
  table1: "hello world",
  table1_name: "greetings_english.html",
  table2: "hola",
  table2_name: "greetings_spanish.html",
  table3: "Bonjour",
  table3_name: "greetings_french.html"
};
var keysToIterate = ["table1", "table2", "table3_name"];
let values=Object.keys(table_object)
                .filter(a=>keysToIterate.includes(a))
                .map(a=> table_object[a]);
console.log(values);

Answer 6

Could try something like

var keys = ['table1', 'table2', 'table3']
Object.keys(table_object).forEach(function(key) {
    if (keys.indexOf(key) != -1) {
        var table_value = table_object[key]
    }
})

Answer 7

It may not be possible to iterate over limited keys, but you can have an array of keys which you want to iterate. Then loop over that array to get the corresponding value from the object

var objKeys = ['table1', 'table2', 'table3']

var table_object = {
  table1: "hello world",
  table1_name: "greetings_english.html",
  table2: "hola",
  table2_name: "greetings_spanish.html",
  table3: "Bonjour",
  table3_name: "greetings_french.html"
};

objKeys.forEach(function(item) {
  console.log(table_object[item])

})