What is preventing me from assigning properties to an array element (a string) in JavaScript?

Question

What is preventing me from assigning properties to an array element (a string) in JavaScript?

let newArray = ['x', 'y', 'z'];
newArray[0].element = 'word';

alert(newArray[0].element);   
alert(newArray[0].element = 'term');
alert(newArray[0].element);

The outcome? undefined, 'term', and then undefined

What might be causing this unexpected behavior in the code?

javascript arrays string

Answer 1

Answer №1

Forget about the Array - your goal is to assign a property to a primitive:

A data that is not an object and does not have any methods. JavaScript has 5 primitive datatypes: string, number, boolean, null, undefined. With the exception of null and undefined, all primitives values have object equivalents which wrap around the primitive values, e.g. a String object wraps around a string primitive. All primitives are immutable.

If you really need to store extra information in a string using a property, consider using the object equivalent:

>>> var array = [new String('a'), new String('b'), new String('c')];
>>> array[0].property
undefined
>>> array[0].property = 'value'
"value"
>>> array[0].property
"value"

Be cautious if you go this route and later need to identify the value as a string:

>>> var a = ['s', new String('s')];
>>> a.map(function(s) { return typeof s; });
["string", "object"]
>>> a.map(function(s) { return s instanceof String });
[false, true]
>>> a.map(function(s) { return Object.prototype.toString.call(s) == '[object String]' });
[true, true]

Answer 2

Forget about the Array - your goal is to assign a property to a primitive:

A data that is not an object and does not have any methods. JavaScript has 5 primitive datatypes: string, number, boolean, null, undefined. With the exception of null and undefined, all primitives values have object equivalents which wrap around the primitive values, e.g. a String object wraps around a string primitive. All primitives are immutable.

If you really need to store extra information in a string using a property, consider using the object equivalent:

>>> var array = [new String('a'), new String('b'), new String('c')];
>>> array[0].property
undefined
>>> array[0].property = 'value'
"value"
>>> array[0].property
"value"

Be cautious if you go this route and later need to identify the value as a string:

>>> var a = ['s', new String('s')];
>>> a.map(function(s) { return typeof s; });
["string", "object"]
>>> a.map(function(s) { return s instanceof String });
[false, true]
>>> a.map(function(s) { return Object.prototype.toString.call(s) == '[object String]' });
[true, true]

Answer 3

Answer №2

The elements in your array are considered as string literals. In JavaScript, string literals may seem like objects with properties when you access them using the dot notation (e.g., array[0].property). However, in reality, JavaScript creates a temporary object for the string and assigns the property to that object.

This explains why the middle alert statement functions correctly while the others do not. To ensure all alerts work as expected, you can declare your array like this:

var array = [new String('a'), new String('b'), new String('c')];

By doing so, all three alerts will function properly.

Answer 4

The elements in your array are considered as string literals. In JavaScript, string literals may seem like objects with properties when you access them using the dot notation (e.g., array[0].property). However, in reality, JavaScript creates a temporary object for the string and assigns the property to that object.

This explains why the middle alert statement functions correctly while the others do not. To ensure all alerts work as expected, you can declare your array like this:

var array = [new String('a'), new String('b'), new String('c')];

By doing so, all three alerts will function properly.

Answer 5

Answer №3

The element array[0] contains a string, specifically the character 'a', which was assigned in the initial line of code. Keep in mind that in JavaScript, strings cannot have additional properties attached to them.

If you wish to include properties with your data, it is necessary to use an object instead. For instance, you can create a basic "empty" object using either new Object() or simply {}:

var array = [{}];
array[0].property = 'value';

alert(array[0].property);   
alert(array[0].property = 'value');
alert(array[0].property);

Answer 6

The element array[0] contains a string, specifically the character 'a', which was assigned in the initial line of code. Keep in mind that in JavaScript, strings cannot have additional properties attached to them.

If you wish to include properties with your data, it is necessary to use an object instead. For instance, you can create a basic "empty" object using either new Object() or simply {}:

var array = [{}];
array[0].property = 'value';

alert(array[0].property);   
alert(array[0].property = 'value');
alert(array[0].property);

Answer 7

Answer №4

In JavaScript, a unique quirk exists due to the performance restriction where primitive types like string, boolean, and number are immutable. This means that any property assignment made to these types will simply disappear. The exceptions are the single-inhabitant primitive types undefined and null, which will throw an exception when trying to assign a property. Despite being lightweight for the run-time as they are not real objects, this immutability feature is present in these primitive values.

However, there are wrapper types available for each primitive type: String, Boolean, and Number. These wrapper types are actual objects and can have custom properties assigned to them.

Although unconventional, it is possible to assign custom properties to wrapper types, for example:

var s = new String("foo");
s.bar = "hello"
alert(s.bar)

Nevertheless, using wrapper types introduces several peculiar behaviors. For instance, typeof "" will return "string", while typeof s will return "object". Additionally, "" instanceof String will be false, whereas s instanceof String will be true. Notably, calling new Boolean(false) surprisingly results in a truth-y value.

Happy coding!

*It's worth noting that this may disrupt libraries designed with assumptions such as typeof x === "string".

Answer 8

In JavaScript, a unique quirk exists due to the performance restriction where primitive types like string, boolean, and number are immutable. This means that any property assignment made to these types will simply disappear. The exceptions are the single-inhabitant primitive types undefined and null, which will throw an exception when trying to assign a property. Despite being lightweight for the run-time as they are not real objects, this immutability feature is present in these primitive values.

However, there are wrapper types available for each primitive type: String, Boolean, and Number. These wrapper types are actual objects and can have custom properties assigned to them.

Although unconventional, it is possible to assign custom properties to wrapper types, for example:

var s = new String("foo");
s.bar = "hello"
alert(s.bar)

Nevertheless, using wrapper types introduces several peculiar behaviors. For instance, typeof "" will return "string", while typeof s will return "object". Additionally, "" instanceof String will be false, whereas s instanceof String will be true. Notably, calling new Boolean(false) surprisingly results in a truth-y value.

Happy coding!

*It's worth noting that this may disrupt libraries designed with assumptions such as typeof x === "string".

Answer 9

Answer №5

Trying to assign a property to the string 'a' like 'a'.property = 'something' is not supported in any programming language.

No language allows you to do something similar to 12.property = 'something'.

Answer 10

Trying to assign a property to the string 'a' like 'a'.property = 'something' is not supported in any programming language.

No language allows you to do something similar to 12.property = 'something'.

Answer 11

Answer №6

JavaScript stores strings and simple types as values, not references.

An array containing objects will still function correctly.

Answer 12

JavaScript stores strings and simple types as values, not references.

An array containing objects will still function correctly.

What is preventing me from assigning properties to an array element (a string) in JavaScript?

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

Answer №6

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