Lately after ES6 released, many sources suggested that I use "const" and "let" instead of "var", and that I should stop using "var" in my JavaScript.
What I wonder is, if "var" has no advantage over "let" in all points of view, then why didn't they just fix var, or even deprecate "var" instead of letting them go along side each other?
Backwards compatibility.
You're right in saying there is no real advantage to using var over let - if you define them at the start of a function their meaning is basically identical.
You're right that there is no real reason to write new code using var (except maybe this, if relevant).
There are pages on the internet that are decades old though, and no one is going to rewrite them. There is nothing really to gain by removing var from the language. For languages like HTML and Javascript that are interpreted - backward compatability is absolutely mandatory.
That is also why they chose not to simply redefine var. Take the following example code;
// THIS IS AN EXAMPLE OF BAD CODE. DO NOT COPY AND PASTE THIS.
if (logic) {
var output = "true"
} else {
var output = "false"
}
console.log(output)
If var was changed to behave like let then the console.log would cause a reference error because of the scope difference.
I believe sometimes you need to redeclare a variable to write less code.
One example is this function that generates a unique id:
function makeUniqueId(takenIds) {
do {
var id = Number.parseInt(Math.random() * 10);
} while (takenIds.includes(id))
}
Which may be invoked like that
makeUniqueId([1,2,3,4,5,6,7])
Here I declare id variable simply inside do block and it get's "hoisted" to the function scope. That would cause an error if I used let, because while block wouldn't see the variable from the do block. Of course I could declate let before do..while, but that would create the same function scoped variable with extra line of code.
Another example is when you copypaste code to devtools console and every time variables get redeclared.
One more example. What if you want to keep your variable declarations close to their usages but still treat them as function globals? If you use let in this fashion, you'll get rather confusing expirience in dev tools (all those Block, Block scopes).
But var 'keeps' them together in one 'Local' list:
Everything has their own advantages and disadvantages using var const and let is dependent on their use cases.
var
Variable declarations are processed before the execution of the code.
The scope of a JavaScript variable declared with var is its current execution context.
The scope of a JavaScript variable declared outside the function is global.
let
The let statement allows you to create a variable with the scope limited to the block on which it is used.
const
const statement values can be assigned once and they cannot be reassigned. The scope of const statement works similar to let statements.
I hope you understand.
Related
I have heard that since JavaScript would hoist all the local variables to the front of the function, it is better if the programmer just hoist it himself or herself, so that everything is seen as how things would actually occur.
Example is:
var i, result = [];
which is to declare all local variables at the beginning of the function.
But I also saw in some classical code, such as in React JS's source code that it would do
for (var i = 0; i < ...; i++) { ... }
that is, declaring it when i is first used -- so that if this line is removed, the declaration will be removed together.
If we hoist it manually, there is a danger that when that loop is removed, the var i; is still left there. (although linting probably would catch it).
I also have seen weird look from interviewers when I hoists all the variables to the front of the function definition too.
Should we hoist all variables to the front? Or what if we hoist all variables except the temporary variables such as i and j?
Limiting the exposure of variables to the context they are useful in can be an extremely valuable auto-documentation strategy: It lets the next person who picks up the code know where variables are and are not relevant. So no, don't "hoist" because the JavaScript engine is internally doing it
I also have seen weird look from interviewers when I hoists all the variables to the front of the function definition too.
Should we hoist all variables to the front? Or what if we hoist all variables except the temporary variables such as i and j?
There are basically two schools of thought on this. One is to declare all of your variables in one place, usually at the top of whatever they are scoped to (which is the function in which they are defined, if you are using the var keyword). The other school of thought is to declare variables as closely as you can to where they are used. They both have valid arguments, and honestly is a matter of opinion and preference.
Before getting into let and ES6, I will first talk about the two arguments above.
Declaring at the top of the scope
The advantages of this are that you always know where your variable declarations are, and you are always aware of the parent scope. Immediately when looking at the function, you can determine what variables are used throughout, and you can trace where they are used from there.
The disadvantages of this are that, depending on your code structure, it may be 100 lines from where a variable is declared and where it is used, which can make debugging a bit of a challenge sometimes and requires you to carefully trace the variable you think you are using in the function, because it may not always be the one declared at the top, especially in the case of shadowing.
Declaring as close in proximity to where the variables are used
The advantages of this are that when trying to determine variable scoping and shadowing, it is very easy to determine what version of the variable you are working with (especially with nested functions). The other advantage is code-cleanup. If you remove a function or a loop and there is a variable declaration right above it, it is usually a pretty good reminder to remove that variable declaration as well because it will not be needed anymore. Obviously that's not always the case, but many times it is. When declaring at the top, variables can get lost in a sea of declarations and be left unused - yeah a linter can catch that, so it may be a moot point, but it's a point nonetheless.
The disadvantages of this are the exact opposite of the advantages of declaring at the top. If you are looking to see what variable names / identifiers are used in a given scope, you kind of have to go digging. CTRL + F is your friend, but it is faster to look at a list that is all in one place.
Now what about let??
Again, there are two schools of thought on this: one is "let is the new var" and the other is that "let simply allows us to do syntactically what we already were doing stylistically"
For example, take this code:
var result;
for (var i = 1; i <= 10; i++) {
result = 2 * i;
console.log(result);
}
Vs.
for (var i = 1; i <= 10; i++) {
var result = 2 * i;
console.log(result);
}
Now, from the compiler's perspective, they are identical. However, the second version (stylistically) is telling the reader of the code "The result variable is only being used inside this for loop", whereas the first version is ambiguous to the reader as to what may or may not be happening to the result variable later on in the code. These are stylistic and conventions that developers unspokenly adhere to in order to convey intent of the code. And that is the argument of the second school of thought - let simply allows us to do, syntactically, what we are already doing stylistically.
This might have been relevant a few years back.
Now, you should use ES5 or later, which introduced the let keyword, which solves the hoisting issue. let declared variables are block-scoped and will not be hoisted.
I guess sometimes it makes sense to use strict mode
"use strict";
which can be implemented for the whole file or for a function
function strictFunction() {
"use strict";
var y = 3.14; // This is ok
x = 2.414; // This will cause error
}
which can help writing more secure code by avoiding global variables. So technically it depends how your for loop is being use. Reference "use strict";
You can also use let instead or var as suggested by #RemcoGerlich to scope your variables
Can anyone tell me why use one var declaration for multiple variables and declare each variable on a newline consider is a good programming behavior?
// bad
var items = getItems();
var goSportsTeam = true;
var dragonball = 'z';
// good
var items = getItems(),
goSportsTeam = true,
dragonball = 'z';
It is not considered 'good' or 'bad'. It's a matter of preference.
The guy who built the code quality tool JSLint, Douglas Crockford likes it.
One 'advantage' it might have is that it avoids the possibility of variable hoisting. In JavaScript all var declarations move to the top of their scope automatically.
Here is why Crockford thinks the second option is better:
In languages with block scope, it is usually recommended that variables be declared at the site of first use. But because JavaScript does not have block scope, it is wiser to declare all of a function's variables at the top of the function. It is recommended that a single var statement be used per function. This can be declined with the vars option.
It's a preference, I wouldn't say good or bad. Yes JSLint complains about it, I don't really like how it complains about for loop variables being inline as well. The reason that it was put in JSLint was to prevent possible hoisting confusions.
Also in some cases declaring all of your variables at the top will lead to a slightly smaller file. Consider the following:
var a = 10;
a++;
var b = 20;
After Google Closure being run over it
var a=10;a++;var b=20;
As opposed to this if we pull b's declaration to the top.
var a=10,b;a++;b=20;
The main benefit (aside from style preference, I guess) is that it will prevent you from writing code that suffers from unintended variable hoisting consequences.
Take this example:
var foo = function(){alert('foo');}
function bar(){
foo();
var foo = function(){alert('foobar')};
foo();
}
bar();
By reading this code, the intent of bar appears to be as follows:
Call the outer foo function to alert the string 'foo'.
Create a local variable, foo.
Call the local foo function to alert the string 'foobar'.
In reality, what happens is this:
The local variable foo is hoisted to the top of the bar function.
foo now actually refers to the local variable instead of the outer variable of the same name. But since the local hasn't been assigned yet, its value is undefined. Hence, when you try to invoke foo, you'll get a TypeError.
Nothing. Because you threw an error. That's it. Your code broke.
The arguments for Crockfords preference have been well made and are valid. I am just beginning to return to the first format now however, as I believe that for experienced developers who understand variable hoisting and are not likely to fall foul to it, there are 2 advantages that I can think of:
When new variables are added to the list of definitions, diffing is made easier. This means you are likely to experience fewer merge conflicts (simple though they may be to resolve) and less cognitive load when analysing diffs. For similar reasons I have also been converted to dangling commas, which I would never have expected xD
As #ldsenow identified, it can make searching for variable definitions more simple. You will always be able to search for var <name> and get the result you want and nothing else.
Can somebody please explain me why the following code works?
function getLastName()
{
fullName.lastName = "World";
}
function writeName()
{
fullName = {};
fullName.firstName = "Hello";
getLastName();
document.write(fullName.firstName + " " + fullName.lastName);
}
writeName();
For some reason, getLastName() can reach local its enclosing method's local state. How can this work? And also should I utilize this feature of Javascript or it is considered as a bad practice? If it is a bad practice, could you please explain why?
You can see the actual code working here at http://jsbin.com/atituk/2/edit
You don't have any local variables, that would require using the var keyword. All your variables are global and can be accessed anywhere within window, which is not considered good practice at all.
You have not used the var keyword against fullName inside the writeName function, you are therefore taking it from the scope outside writeName. It continues up the chain until it reaches the outer most scope, at which point it creates a global.
Globals are, in general, bad practise as they are hard to keep track of and more likely to be overwritten by accident (e.g. in a race condition).
If you were using strict mode this would create an error instead of a global.
You are using all variables of yours as global variables. So all are recognized everywhere. In order to have a better understanding of variable scopes in Javascript have a look at this great example https://stackoverflow.com/a/500459/655316
I'm struggling with (what I believe to be) a scoping issue. Here's a sample of my code:
$(document).ready(function() {
var counter = 0;
function scrollTweets() {
counter ++;
// rest of code
}
...
)}; // end of document ready
When I look up the variable counter in Chrome's Javascript console it returns "ReferencedError". However, when I remove var from the code above and type counter into the console, it returns the value. Why is this?
I think understanding this simple concept would allow me to tackle issues that seem to pop up during development. Is it just a scoping issue on Chrome's part? Am I needlessly wrapping everything in the $(document).ready "function"?
The var locks the variable counter into whatever the lexical scope is -- which means its available in the current block, method, whatever, and can be attached to closed-in scopes (ie. closures), like you are doing with scrollTweets. So counter is only available in the ready callback and anything that has a closure around it, which is why you can't access it from your console.
When you take the var away, counter is effectively global, which is why you can access it in that case.
When you don't use var to set the scope of the variable, it automatically becomes a global variable, inside the global scope. That's why it is visible in the Chrome console.
As a note, I'm by no means implying you should make the variable global. In fact that's almost always a bad idea! Capturing the variable in the context of the scope where it's used is the right thing to do. If the Chrome console can't handle that you just need a better debugger. Firebug for Javascript does a wonderful job at handling scope - even clojures!
I am picking up maintenance of a project and reading code:
I see two methods of variable declaration. Can someone explain what the difference between the first and second line means?
To me, I am reading that in javascript, the var keyword is optional. in the first line, they have declared two new variables and initialized them. In the second line, they have declared two new varialbes but not have initialized them. Should I take anything more from this?
aURL = ""; msgNb = 1;
var mode, param, counter;
Unless all these variables are inside a function they're all globals, the first two are assignments which I would guess because they were previously declared, otherwise it may be shortened to
var aURL = '',
msgNb = 1,
mode,
param,
counter;
The unassigned ones have an undefined value by default.
You should always use the var keyword to keep the variable within the same function scope and not force it to become an implicit global, otherwise you could run into issues with duplicate variable naming and assignment.
If you're not using var then you're using (or creating) a variable from a "parent" scope, all the way up to being global if it doesn't find a local one at any scope.
This is not a "jquery" issue per say but rather a JavaScript issue. A variable without the "var" keyword has global scope, i.e., it is visible from all methods, objects, etc... A var is only visible within its specific scope.