Interesting Facts About Javascript
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This presentation discusses fascinating aspects of JavaScript, particularly its dynamic typing and the distinction between types and values. It highlights the seven built-in types in JavaScript, the peculiar behavior of the 'typeof' keyword, and clarifies the differences between undefined and undeclared variables. Additionally, it explains the nature of arrays in JavaScript, emphasizing the importance of using the appropriate methods for manipulating array elements.
Interesting Facts About Javascript
- 1. Interesting Facts About Javascript --Manish Jangir--
- 2. Introduction Why I’ve named this presentation as “Interesting Facts About Javascript” because there is a whole lot of things we still do accidently in javascript without actually knowing that how it works. This presentation will literally break a lot of misconceptions about Javascript behaviour you might have put in your mind. So without wasting anymore time, let's quickly jump to the very basic javascript fundamental thing types.
- 3. Data Types In Javascript Many of you might have worked a little (may be in college time) in statically typed language like “C” and here you are working in Javascript as a dynamically typed language. So what is statically typed and dynamically typed language? In statically typed language, each variable you declare must have a type associated with it so that the compiler can allocate the proper memory for it. But in dynamically typed language, the variables don’t have any type to be associated to it. Did you ever declared a variable as integer, string or boolean?? So javascript is dynamically typed language. But why the hell we have some data types there in Javascript????
- 4. Data Types In Javascript Contd…. As we have seen in our Javascript code that both we and the javascript engine treat a value 30 differently than “30”. We consider 30 as a number but “30” as a string. What does it mean? The above thing means that the value may have a specific type associated to it. Value has a type or Variable??? Confused??? In javascript or any dynamically typed language, the variables are not associated with any type. Remember, the values have type not the variables. Means, var a = 5; means a is not type of “number” but the value in it is type of “number”.
- 5. Value Types In Javascript So now on, we will say it Value Types not Data Types. Basically there are 7 built- in types there in Javascript. 1. null 2. undefined 3. boolean 4. number 5. string 6. object 7. symbol (in ES6) All of the above types except object are primitive that means, they are passed by value not reference. Objects are non-primitive values which are passed through reference.
- 6. “typeof” Keyword In Javascript How do you check that what typeof value a variable is holding? The very useful keyword typeof does this for you. Whenever you check a value type, it will give you a string value out of the following 7 string values. In the above code, there are 8 lines but you can see that only 7 distinctly types are there. It means these are the seven types you will get in a typeof expression. Here are two strange things we can see: 1. typeof null returns object while it is not object. How null can be an object type value. 2. typeof function returns function while we have talked many times that functions are objects.
- 7. Why typeof null Is Object?? Nothing to be surprised here. It's a 20 years old BUG in Javascript. Yes its a bug that could not be fixed due to already having a large codebase on internet that is dependant on typeof null === “object” expression. Otherwise what we expect from the browser is typeof null should give us “null” as string value. If you want to check if variable has a null value type, check with the following ways. 1. if(variable === null) 2. if(!variable && typeof variable == “object”). Null is the only falsy value, which is false but still type of object.
- 8. “function” Type Value In Javascript, we have seen that functions are just objects which can have static properties on them. But they have been provided a specific type called “function”. Functions are kind of callable objects and have special place in Javascript environment.
- 9. Undefined VS. Undeclared - Most Confusing Variables which don’t have any value will have a specific hidden value called undefined. You can check it with typeof keyword like below: A lot of times, we see undefined and undeclared as same things but they don’t actually. An undefined variable is that which has been declared but not assigned any kind of value. Where an undeclared variable is that which has never been even declared in script. Here the most confusing part is that error message. Don’t you see “ReferenceError b is not defined” is exactly like “b is undefined”. But remember, undefined and is not defined are two completely different things in Javascript.
- 10. Undefined VS. Undeclared - Continued…. It could be very nice if the javascript engine could tell us that ReferenceError: b is not found if the variable is never declared. At Least it makes some sense. One more confusing part: Here you can see that if we put typeof on both the undefined and undeclared variables, it will return the type value “undefined”. It means, typeof has a special behaviour that gives us flexibility to check whether a variable is declared or not. It won’t throw error if the variable is not declared. The most common use of typeof is to check if we want to check any third party library global object like typeof jQuery !== ‘undefined’ etc. It won’t throw error even if you have not included jQuery in your script.
- 11. Arrays In Javascript How many of you have enough understanding of Javascript Arrays? Arrays are just a kind of container that hold any type of value. Whether it is string, number, boolean, object or another array (Multidimensional). You never need to define size of the array first. You can declare it and push values directly. For e.g.
- 12. Delete A Value From Array Never get trapped by the keyword delete. Though it will delete the value from array but will not behave exactly like you thought. For example: Using keyword delete will delete the value but will not actually re-index the array. What it will do is just make the slot empty. In the above example the slot at index 1 will actually have no value and that's why, it show undefined in log. So even deleting all the value from array with delete keyword, it will not update the length of the array. Instead use array.splice(1,1) which will remove the value from index 1 to 1 number of values.