Is there a way to verify the existence of an object property using a variable that stores the property name?

Question

Is there a way to verify the existence of an object property using a variable that stores the property name?

One of the tasks I'm currently working on involves verifying the presence of a specific object property using a variable that contains the name of the property in question.

var myObj;
myObj.prop = "exists";
var myProp = "p"+"r"+"o"+"p";

if(myObj.myProp){
    alert("yes, i have that property");
};

The result is undefined because it's attempting to locate myObj.myProp instead of myObj.prop, which is what I intend for it to inspect.

javascript object typeguards

Answer 1

Answer №1

var item = 'item';
if(myObject.hasKey(item)){
    alert("Yes, I possess that item");
}

Alternatively

var property = 'property';
if(property in myObject){
    alert("Yes, I have that property");
}

Or

if('key' in myObject){
    alert("Yes, I have that key");
}

Keep in mind that hasOwnProperty only looks for direct properties, while in includes inherited properties. For instance, 'constructor' in myObject is true, but myObject.hasKey('constructor') is false.

Answer 2

var item = 'item';
if(myObject.hasKey(item)){
    alert("Yes, I possess that item");
}

Alternatively

var property = 'property';
if(property in myObject){
    alert("Yes, I have that property");
}

Or

if('key' in myObject){
    alert("Yes, I have that key");
}

Keep in mind that hasOwnProperty only looks for direct properties, while in includes inherited properties. For instance, 'constructor' in myObject is true, but myObject.hasKey('constructor') is false.

Answer 3

Answer №2

If you want to check if an object has a property, you can utilize the hasOwnProperty method, but remember to enclose the property name in quotes:

if (myObj.hasOwnProperty('myProp')) {
    // perform some action
}

For more information on this method, visit: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwnProperty

Another approach is to use the in operator, also requiring the property name to be enclosed in quotes:

if ('myProp' in myObj) {
    // take necessary steps
}

For further details on the usage of the in operator, check out: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/in

Answer 4

If you want to check if an object has a property, you can utilize the hasOwnProperty method, but remember to enclose the property name in quotes:

if (myObj.hasOwnProperty('myProp')) {
    // perform some action
}

For more information on this method, visit: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwnProperty

Another approach is to use the in operator, also requiring the property name to be enclosed in quotes:

if ('myProp' in myObj) {
    // take necessary steps
}

For further details on the usage of the in operator, check out: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/in

Answer 5

Answer №3

Gratitude to all for the support and encouragement in eliminating the eval statement. It was necessary to use brackets for variables, not dot notation. This method is effective and results in clean, well-structured code.

The variables appChoice, underI, and underObstr are each important in this context.

if(typeof tData.tonicdata[appChoice][underI][underObstr] !== "undefined"){
    //write your code here
}

Answer 6

Gratitude to all for the support and encouragement in eliminating the eval statement. It was necessary to use brackets for variables, not dot notation. This method is effective and results in clean, well-structured code.

The variables appChoice, underI, and underObstr are each important in this context.

if(typeof tData.tonicdata[appChoice][underI][underObstr] !== "undefined"){
    //write your code here
}

Answer 7

Answer №4

When dealing with own property:

var assets = { value: 500 };
if(Object.prototype.hasOwnProperty.call(assets, "value")) 
{ 
   // This block will be executed
}

Note: It is recommended to use Object.prototype.hasOwnProperty instead of asset.hasOwnProperty(..), in case a custom hasOwnProperty is defined in the prototype chain (similar to the example below), like

var obj = {
      hasOwnProperty: function() {
        return false;
      },
      item: 'Custom object'
    };

// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwnProperty

If you need to consider inherited properties in the search, use the in operator: (make sure the right side of 'in' is an object, primitive values will cause errors, e.g. 'length' in 'house' will throw error, but 'length' in new String('house') won't)

const ninja = { hide: true };
const samurai = { attack: true };
const pirate = { sneak: true };
if ("hide" in ninja) 
    console.log("Ninja can hide");

if (!("attack" in ninja)) 
    console.log("Ninja cannot attack");

if (!("sneak" in ninja)) 
    console.log("Ninja cannot sneak");

Object.setPrototypeOf(ninja, samurai);

if ("attack" in ninja)
    console.log("Ninja can now attack");
if (!("sneak" in samurai))
    console.log("Samurai cannot sneak");

Object.setPrototypeOf(samurai, pirate);

if ("sneak" in samurai)
    console.log("Samurai can now sneak");
if ("sneak" in ninja)
    console.log("Ninja can also sneak");

// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/in

Note: Avoid using typeof and [ ] property accessor as shown in this code snippet which may not work as expected ...

var assets = { value: 500 };

assets.debt = undefined;

if("debt" in assets) // correct
{
    // This block will execute
}

if(typeof assets["debt"] !== "undefined") // incorrect
{
    // This block will not execute
}

Answer 8

When dealing with own property:

var assets = { value: 500 };
if(Object.prototype.hasOwnProperty.call(assets, "value")) 
{ 
   // This block will be executed
}

Note: It is recommended to use Object.prototype.hasOwnProperty instead of asset.hasOwnProperty(..), in case a custom hasOwnProperty is defined in the prototype chain (similar to the example below), like

var obj = {
      hasOwnProperty: function() {
        return false;
      },
      item: 'Custom object'
    };

// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwnProperty

If you need to consider inherited properties in the search, use the in operator: (make sure the right side of 'in' is an object, primitive values will cause errors, e.g. 'length' in 'house' will throw error, but 'length' in new String('house') won't)

const ninja = { hide: true };
const samurai = { attack: true };
const pirate = { sneak: true };
if ("hide" in ninja) 
    console.log("Ninja can hide");

if (!("attack" in ninja)) 
    console.log("Ninja cannot attack");

if (!("sneak" in ninja)) 
    console.log("Ninja cannot sneak");

Object.setPrototypeOf(ninja, samurai);

if ("attack" in ninja)
    console.log("Ninja can now attack");
if (!("sneak" in samurai))
    console.log("Samurai cannot sneak");

Object.setPrototypeOf(samurai, pirate);

if ("sneak" in samurai)
    console.log("Samurai can now sneak");
if ("sneak" in ninja)
    console.log("Ninja can also sneak");

// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/in

Note: Avoid using typeof and [ ] property accessor as shown in this code snippet which may not work as expected ...

var assets = { value: 500 };

assets.debt = undefined;

if("debt" in assets) // correct
{
    // This block will execute
}

if(typeof assets["debt"] !== "undefined") // incorrect
{
    // This block will not execute
}

Answer 9

Answer №5

To ensure a more secure method of checking if a property exists on an object, one can utilize an empty object or the object prototype to invoke hasOwnProperty()

var foo = {
  hasOwnProperty: function() {
    return false;
  },
  bar: 'Here be dragons'
};

foo.hasOwnProperty('bar'); // always returns false

// Utilize another Object's hasOwnProperty and invoke it with 'this' set to foo
({}).hasOwnProperty.call(foo, 'bar'); // true

// Alternatively, the hasOwnProperty property from the Object prototype can also be used for this purpose
Object.prototype.hasOwnProperty.call(foo, 'bar'); // true

Source: MDN Web Docs - Object.prototype.hasOwnProperty()

Answer 10

To ensure a more secure method of checking if a property exists on an object, one can utilize an empty object or the object prototype to invoke hasOwnProperty()

var foo = {
  hasOwnProperty: function() {
    return false;
  },
  bar: 'Here be dragons'
};

foo.hasOwnProperty('bar'); // always returns false

// Utilize another Object's hasOwnProperty and invoke it with 'this' set to foo
({}).hasOwnProperty.call(foo, 'bar'); // true

// Alternatively, the hasOwnProperty property from the Object prototype can also be used for this purpose
Object.prototype.hasOwnProperty.call(foo, 'bar'); // true

Source: MDN Web Docs - Object.prototype.hasOwnProperty()

Answer 11

Answer №6

there are much easier ways to solve this and I don't see a direct response to your question:

"it's searching for myObj.myProp but I want it to look for myObj.prop"

to get the value of a property from a variable, use bracket notation.
to verify if that property has a truthy value, utilize optional chaining
to return a boolean, employ double-not / bang-bang / (!!)
use the in operator when you are certain it is an object and only want to check if the property exists (true even if prop value is undefined). Or consider combining with nullish coalescing operator ?? to prevent errors.

var nothing = undefined;
var obj = {prop:"hello world"}
var myProp = "prop";

consolelog( 1,()=> obj.myProp); // no "myProp" in obj
consolelog( 2,()=> obj[myProp]); // bracket notation works
consolelog( 3,()=> nothing[myProp]); // throws error if not an object
consolelog( 4,()=> obj?.[myProp]); // excellent optional chaining ⭐️⭐️⭐️⭐️⭐️
consolelog( 5,()=> nothing?.[myProp]); // avoids throwing error with optional chaining
consolelog( 6,()=> nothing?.[nothing]); // safely handles error
consolelog( 7,()=> !!obj?.[myProp]); // double-not results in true
consolelog( 8,()=> !!nothing?.[myProp]); // false due to undefined
consolelog( 9,()=> myProp in obj); // 'in' operator works fine
consolelog(10,()=> myProp in nothing); // throws error if not an object
consolelog(11,()=> myProp in (nothing ?? {})); // secure from errors
consolelog(12,()=> myProp in {prop:undefined}); // true (prop key exists despite value being undefined)

// be cautious of 'hasOwnProperty' drawbacks
// can't identify inherited properties and 'hasOwnProperty' itself is inherited
// also verbose compared to other methods
consolelog(13,()=> obj.hasOwnProperty("hasOwnProperty")); // WARNING: yields false 
consolelog(14,()=> nothing.hasOwnProperty("hasOwnProperty")); // throws error with undefined
obj.hasOwnProperty = ()=>true; // potential overwrite risk
consolelog(15,()=> obj.hasOwnProperty(nothing)); // OVERWRITE DANGER: will always return true now
consolelog(16,()=> Object.prototype.hasOwnProperty.call(obj,"hasOwnProperty")); //😭 why?!
consolelog(17,()=> Object.hasOwn(obj,"hasOwnProperty")); //😭 why?! 

function consolelog(num,myTest){
    try{
    console.log(num,myTest());
    }
    catch(e){
    console.log(num,'throws',e.message);
    }
}

Answer 12

there are much easier ways to solve this and I don't see a direct response to your question:

"it's searching for myObj.myProp but I want it to look for myObj.prop"

to get the value of a property from a variable, use bracket notation.
to verify if that property has a truthy value, utilize optional chaining
to return a boolean, employ double-not / bang-bang / (!!)
use the in operator when you are certain it is an object and only want to check if the property exists (true even if prop value is undefined). Or consider combining with nullish coalescing operator ?? to prevent errors.

var nothing = undefined;
var obj = {prop:"hello world"}
var myProp = "prop";

consolelog( 1,()=> obj.myProp); // no "myProp" in obj
consolelog( 2,()=> obj[myProp]); // bracket notation works
consolelog( 3,()=> nothing[myProp]); // throws error if not an object
consolelog( 4,()=> obj?.[myProp]); // excellent optional chaining ⭐️⭐️⭐️⭐️⭐️
consolelog( 5,()=> nothing?.[myProp]); // avoids throwing error with optional chaining
consolelog( 6,()=> nothing?.[nothing]); // safely handles error
consolelog( 7,()=> !!obj?.[myProp]); // double-not results in true
consolelog( 8,()=> !!nothing?.[myProp]); // false due to undefined
consolelog( 9,()=> myProp in obj); // 'in' operator works fine
consolelog(10,()=> myProp in nothing); // throws error if not an object
consolelog(11,()=> myProp in (nothing ?? {})); // secure from errors
consolelog(12,()=> myProp in {prop:undefined}); // true (prop key exists despite value being undefined)

// be cautious of 'hasOwnProperty' drawbacks
// can't identify inherited properties and 'hasOwnProperty' itself is inherited
// also verbose compared to other methods
consolelog(13,()=> obj.hasOwnProperty("hasOwnProperty")); // WARNING: yields false 
consolelog(14,()=> nothing.hasOwnProperty("hasOwnProperty")); // throws error with undefined
obj.hasOwnProperty = ()=>true; // potential overwrite risk
consolelog(15,()=> obj.hasOwnProperty(nothing)); // OVERWRITE DANGER: will always return true now
consolelog(16,()=> Object.prototype.hasOwnProperty.call(obj,"hasOwnProperty")); //😭 why?!
consolelog(17,()=> Object.hasOwn(obj,"hasOwnProperty")); //😭 why?! 

function consolelog(num,myTest){
    try{
    console.log(num,myTest());
    }
    catch(e){
    console.log(num,'throws',e.message);
    }
}

Answer 13

Answer №7

One option is to utilize the hasOwnProperty() method in conjunction with the in operator.

Answer 14

One option is to utilize the hasOwnProperty() method in conjunction with the in operator.

Answer 15

Answer №8

Utilizing the recently introduced Object.hasOwn method provides an alternative to the existing Object.hasOwnProperty method.

This static method will return true if the specified object possesses the specified property as its own, or false if the property is inherited or does not exist on that object.

It's important to note that it is advisable to carefully review the Browser compatibility table before implementing this in production, as it is still considered an experimental technology and may not be fully supported by all browsers (although this is expected to change soon).

var myObj = {};
myObj.myProp = "exists";

if (Object.hasOwn(myObj, 'myProp')) {
  alert("yes, I have that property");
}

Additional information about Object.hasOwn - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwn

Browser compatibility for Object.hasOwn: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwn#browser_compatibility

Answer 16

Utilizing the recently introduced Object.hasOwn method provides an alternative to the existing Object.hasOwnProperty method.

This static method will return true if the specified object possesses the specified property as its own, or false if the property is inherited or does not exist on that object.

It's important to note that it is advisable to carefully review the Browser compatibility table before implementing this in production, as it is still considered an experimental technology and may not be fully supported by all browsers (although this is expected to change soon).

var myObj = {};
myObj.myProp = "exists";

if (Object.hasOwn(myObj, 'myProp')) {
  alert("yes, I have that property");
}

Additional information about Object.hasOwn - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwn

Browser compatibility for Object.hasOwn: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwn#browser_compatibility

Answer 17

Answer №9

Exploring the various methods to verify the existence of an object property.

const dog = { name: "Spot" }

if (dog.name) console.log("Yay 1"); // Output: Yay 1
if (dog.sex) console.log("Yay 2"); // No output.

if ("name" in dog) console.log("Yay 3"); // Output: Yay 3
if ("sex" in dog) console.log("Yay 4"); // No output.

if (dog.hasOwnProperty("name")) console.log("Yay 5"); // Output: Yay 5
if (dog.hasOwnProperty("sex")) console.log("Yay 6"); // No output, but displays undefined.

Answer 18

Exploring the various methods to verify the existence of an object property.

const dog = { name: "Spot" }

if (dog.name) console.log("Yay 1"); // Output: Yay 1
if (dog.sex) console.log("Yay 2"); // No output.

if ("name" in dog) console.log("Yay 3"); // Output: Yay 3
if ("sex" in dog) console.log("Yay 4"); // No output.

if (dog.hasOwnProperty("name")) console.log("Yay 5"); // Output: Yay 5
if (dog.hasOwnProperty("sex")) console.log("Yay 6"); // No output, but displays undefined.

Answer 19

Answer №10

Functioning properly.

if (typeof data?.d?.ping_interval != "null") {
}

Answer 20

Functioning properly.

if (typeof data?.d?.ping_interval != "null") {
}

Answer 21

Answer №11

The truthiness check using !! was missing in the answers I reviewed.

if (!!myVariable) // Perform some action

Answer 22

The truthiness check using !! was missing in the answers I reviewed.

if (!!myVariable) // Perform some action

Is there a way to verify the existence of an object property using a variable that stores the property name?

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

Answer №6

Answer №7

Answer №8

Answer №9

Answer №10

Answer №11

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