JavaScript Data Types

In computer languages, data types are used to classify a certain type of data. JavaScript is an object-oriented programming language, which means that it is based on the concept of objects and their relationships with one another. It is also a dynamically-typed language, which means that the data type of a variable is determined at runtime rather than at compile-time.

This is significant because the data type you pick will influence the values you can assign to it and the actions you can take with it. To put it another way, 𝘂𝗻𝗱𝗲𝗿𝘀𝘁𝗮𝗻𝗱𝗶𝗻𝗴 𝘁𝗵𝗲 𝗱𝗮𝘁𝗮 𝘁𝘆𝗽𝗲 𝗼𝗳 𝗮𝗻𝘆 𝘃𝗮𝗿𝗶𝗮𝗯𝗹𝗲 𝗶𝘀 𝗰𝗿𝗶𝘁𝗶𝗰𝗮𝗹 𝘁𝗼 𝗽𝗲𝗿𝗳𝗼𝗿𝗺𝗶𝗻𝗴 𝗼𝗽𝗲𝗿𝗮𝘁𝗶𝗼𝗻𝘀 𝘄𝗶𝘁𝗵 𝘃𝗮𝗿𝗶𝗮𝗯𝗹𝗲𝘀 𝗶𝗻 𝗝𝗮𝘃𝗮𝗦𝗰𝗿𝗶𝗽𝘁.

Below are some tricky questions which generally asked during interview specific to JS data type:

𝗮.) 𝗪𝗵𝗮𝘁 𝗮𝗿𝗲 𝘁𝗵𝗲 𝗳𝗮𝗹𝘀𝘆 𝗮𝗻𝗱 𝘁𝗿𝘂𝘁𝗵𝘆 𝘃𝗮𝗹𝘂𝗲𝘀 𝗶𝗻 𝗝𝗮𝘃𝗮𝗦𝗰𝗿𝗶𝗽𝘁?

There are 6 values that are considered falsy in JavaScript:

• The keyword false
• The primitive value undefined
• The primitive value null
• The empty string (”, “”)
• The global property NaN
• A number or BigInt representing 0 (0, -0, 0.0, -0.0, 0n)

Every other value is considered truthy. It’s important to remember that this applies to all JavaScript values, even ones that might seem falsy, such as empty arrays ([]) or empty objects ({}).

𝗯.) 𝗘𝘅𝗽𝗹𝗮𝗶𝗻 𝗽𝗮𝘀𝘀𝗲𝗱 𝗯𝘆 𝘃𝗮𝗹𝘂𝗲 𝗮𝗻𝗱 𝗽𝗮𝘀𝘀𝗲𝗱 𝗯𝘆 𝗿𝗲𝗳𝗲𝗿𝗲𝗻𝗰𝗲 𝗢𝗥 𝗪𝗵𝗮𝘁 𝗶𝘀 𝘁𝗵𝗲 𝗱𝗶𝗳𝗳𝗲𝗿𝗲𝗻𝗰𝗲 𝗯𝗲𝘁𝘄𝗲𝗲𝗻 𝘀𝘁𝗼𝗿𝗲𝗱 𝗯𝘆 𝘃𝗮𝗹𝘂𝗲(𝗽𝗿𝗶𝗺𝗶𝘁𝗶𝘃𝗲) 𝗮𝗻𝗱 𝘀𝘁𝗼𝗿𝗲𝗱 𝗯𝘆 𝗿𝗲𝗳𝗲𝗿𝗲𝗻𝗰𝗲 (𝗻𝗼𝗻-𝗽𝗿𝗶𝗺𝗶𝘁𝗶𝘃𝗲)?

This can be tricky question to judge how much you are aware about value and reference type. Before that let’s first understand what is pass by value and pass by reference if someone is asking?

What is actually Pass by Value?
Pass by value in JavaScript means that a copy of the actual parameter’s value is made in memory i.e., a new memory allocation is done, and all the changes are made in that new value (i.e., copied value). The original value and the copied value are independent of each other as they both have a different space in memory i.e., on changing the value inside the function, the variable outside the function is not affected.

What is Pass by Reference?
Pass by Reference in JavaScript does not create a new space in the memory, instead, we pass the reference/address of the actual parameter, which means the function can access the original value of the variable. Thus, if we change the value of the variable inside the function, then the original value also gets changed.

Is JS pass by value or pass by reference?
Everything is passed by value, but if your value is an object, then the new scope gets a copy of the reference. If you modify the object by dereferencing, your modifications to the object persist outside of your scope. In JavaScript, there is no such thing as “pass by reference” for any variable. All variables and arguments have a value given to them, however the value of an object’s variable is a reference. As a result, if you supply an object and alter its members inside the method, those changes will remain outside of the function. This makes it appear as if it’s a pass-by-reference system. Primitive values like integer, string, and boolean are provided by value, but objects and arrays, as previously stated, are passed by reference.

𝗰.) 𝗻𝘂𝗹𝗹 𝗮𝗻𝗱 𝘂𝗻𝗱𝗲𝗳𝗶𝗻𝗲𝗱 𝗺𝗲𝗮𝗻 𝘁𝗵𝗲 𝘀𝗮𝗺𝗲 𝘁𝗵𝗶𝗻𝗴?

In simple words, undefined means a variable has been declared but has not yet been assigned a value. undefined is a type by itself (undefined). Unassigned variables are initialized by JavaScript with a default value of undefined.

If a variable is null then it means the variable has no value and that it was explicitly set to have no value by the programmer. A variable will never be null unless somewhere in the code a programmer set a variable to null.

The data type of undefined is undefined whereas that of null is object. Also, when used in arithmetic operations, undefined will result in NaN (not a number), whereas null will be converted to 0 behind the screens.

We can say that by setting a variable to undefined you are conveying the message that the variable no longer contains any useful information, while if the value is null then you are specifically saying the result of some action has no value.

𝗱.) 𝗪𝗵𝘆 𝗶𝘀 𝗻𝘂𝗹𝗹 𝗮𝗻 𝗼𝗯𝗷𝗲𝗰𝘁?

In JavaScript, typeof null is ‘object’, which incorrectly suggests that null is an object (it isn’t, it’s a primitive value). This is a bug and one that unfortunately can’t be fixed, because it would break existing codebase. null was a representation of the null pointer. However, there were no pointers in JavaScript like C. So null simply meant nothing or void and was represented by all 0’s. Hence all its 32 bits were 0’s. So whenever the JavaScript interpreter reads null, it considers the first 3 bits as type “object”. That is why typeof null returns “object”.
This may seem like a very obvious bug, but don’t forget that there was very little time to finish the first version of JavaScript. 😉

𝗲.) 𝗛𝗼𝘄 𝘄𝗼𝘂𝗹𝗱 𝘆𝗼𝘂 𝗰𝗼𝗺𝗽𝗮𝗿𝗲 𝘁𝘄𝗼 𝗼𝗯𝗷𝗲𝗰𝘁𝘀 𝗶𝗻 𝗝𝗮𝘃𝗮𝗦𝗰𝗿𝗶𝗽𝘁?

𝗳.) 𝗜𝗳 [] 𝗶𝘀 𝘁𝗿𝘂𝗲, 𝘁𝗵𝗲𝗻 []==𝘁𝗿𝘂𝗲 𝘀𝗵𝗼𝘂𝗹𝗱 𝗮𝗹𝘀𝗼 𝗯𝗲 𝘁𝗿𝘂𝗲. 𝗿𝗶𝗴𝗵𝘁?

𝗴.) 𝗪𝗵𝗮𝘁 𝘄𝗶𝗹𝗹 𝗯𝗲 𝘁𝗵𝗲 𝟵𝟮..𝘁𝗼𝗦𝘁𝗿𝗶𝗻𝗴()?

𝗵.) 𝗪𝗵𝗮𝘁 𝘄𝗶𝗹𝗹 𝗯𝗲 𝘁𝗵𝗲 𝘁𝘆𝗽𝗲𝗼𝗳(𝗡𝗮𝗡)?

𝗶.) 𝗗𝗼𝗲𝘀 𝗝𝗮𝘃𝗮𝗦𝗰𝗿𝗶𝗽𝘁 𝘀𝘂𝗽𝗽𝗼𝗿𝘁 𝗮𝘂𝘁𝗼𝗺𝗮𝘁𝗶𝗰 𝘁𝘆𝗽𝗲 𝗰𝗼𝗻𝘃𝗲𝗿𝘀𝗶𝗼𝗻?

𝗷.) 𝗪𝗵𝗲𝗿𝗲 𝗮𝗿𝗲 𝗿𝗲𝗳𝗲𝗿𝗲𝗻𝗰𝗲𝘀 (𝘄𝗵𝗶𝗰𝗵 𝗽𝗼𝗶𝗻𝘁 𝘁𝗼 𝗼𝗯𝗷𝗲𝗰𝘁𝘀 𝗮𝗻𝗱 𝗳𝘂𝗻𝗰𝘁𝗶𝗼𝗻𝘀) 𝘀𝘁𝗼𝗿𝗲𝗱? 𝗦𝘁𝗮𝗰𝗸 𝗼𝗿 𝗛𝗲𝗮𝗽?

𝗸.) 𝗗𝗶𝗳𝗳𝗲𝗿𝗲𝗻𝗰𝗲 𝗯𝗲𝘁𝘄𝗲𝗲𝗻 === 𝘃𝘀 == ?

There are many more questions…. where interviewer can ask indirectly about data type. Sometimes people are confused and telling Integer as a data type instead of Number during answer.

I am in process of collating more interviews questions (topic specific) which are basically asked during frontend developer interview and will publish soon on my LinkedIn post.

𝗣𝗹𝗲𝗮𝘀𝗲 𝗳𝗲𝗲𝗹 𝗳𝗿𝗲𝗲 𝘁𝗼 𝗮𝗱𝗱 𝗮𝗻𝘆 𝗾𝘂𝗲𝘀𝘁𝗶𝗼𝗻 𝘁𝗵𝗮𝘁 𝘆𝗼𝘂 𝗵𝗮𝘃𝗲 𝗲𝗻𝗰𝗼𝘂𝗻𝘁𝗲𝗿𝗲𝗱 𝗱𝘂𝗿𝗶𝗻𝗴 𝘆𝗼𝘂𝗿 𝗶𝗻𝘁𝗲𝗿𝘃𝗶𝗲𝘄 𝗶𝗻 𝗰𝗼𝗺𝗺𝗲𝗻𝘁 𝘀𝗲𝗰𝘁𝗶𝗼𝗻, 𝘁𝗵𝗶𝘀 𝘄𝗶𝗹𝗹 𝗵𝗲𝗹𝗽 𝗼𝘁𝗵𝗲𝗿 𝗳𝗿𝗼𝗻𝘁𝗲𝗻𝗱 𝗱𝗲𝘃𝗲𝗹𝗼𝗽𝗲𝗿 𝗷𝗼𝗯 𝗮𝘀𝗽𝗶𝗿𝗮𝗻𝘁𝘀.