- Chrome: V8
- Firefox: SpiderMonkey
- Microsoft Edge/ Internet Explorer: Chakra
Dynamic Typing: The interpreter figures out the type of the variable dynamically based on certain conditions.
Primitive Data Types: The primitive data types are the data types which have no methods attached to it i.e. some defined methods cannot be used with them and they are used in isolation. Though there are ways to use those methods by wrapping these primitive data type variables (covered in the next article). The following are the data types that comes under the primitive category:
- undefined:- If variable exists but is not defined the it is categorized under undefined.
- null:- If variable exists but is not explicitly set the it comes under null category.
- boolean:- The Boolean data type is to define if a variable is True or False.
- number:- number is the data type to define a number which can be integer, floating point, double. The only problem here is that we have to allocate a memory equivalent to a double variable every time we define a number.
- string:- This is used to define string values of a character.
- symbol:- This is a special data type which is new in ECMA Script 6. The data type “symbol” is a primitive data type having the quality that values of this type can be used to make object properties that are anonymous.
Creating objects: – There are 3 ways to create objects:
1. Using the Object() Constructor
2.Using Object literal
3. Directly specifying the values
Coercion is of two types:
- Explicit Coercion
Explicit coercion is the process by which we explicitly define a variable to a data type.
x = 42;
explicit = String(x);
// explicit is set to "42"chevron_right
- Implicit Coercion Implicit Coercion is the process by which the interpreter dynamically type casts the variable under certain conditions.
x = 42;
implicit = x +
// interpreter automatically sets implicit as "42"chevron_right
Variable lifetime: – The variable lifetime is from where they are declared until their function ends. If no function is defined then scope of the variable is global.
Hoisting: – Function definitions are hoisted but not variable declarations. This means that when a function is declared, it is usable from anywhere within your code.
Example of hoisting: –
The above function is executed successfully when called. But if we call the function first and then define it, then also it is successfully executed. This is called function hoisting.
However variable initialization are not hoisted.
The above output errors as undefined and as we know that undefined type is when the variable exists but is not defined. So the question is how does the variable exists even if it is declared and initialized later?
This is another type of hoisting just like function hoisting i.e. the variable ‘x’ is available to us previously.
- Creation Phase: Before executing the code, the engine reads through the entire file and will throw a syntactic error if one is found. While it does that, any function definitions will just be saved in memory. Any variable initialization will not be run but variable names will be declared.
- Execution Phase: The execution phase is the phase in which the code is run and hence the above variable hoisting example errors as undefined since in creation phase, the variable has been declared but not defined in the creation phase.
That’s it for this article. We’ll be covering some advanced topics in the next follow-up article.
This article is contributed by Anirudh Acharya. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to firstname.lastname@example.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
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