CODEWITHSUNDEEP
javascript
LetMeSOThat4U
LetMeSOThat4U

Reputation: 6758

Should I really declare all vars in Javascript before using them?

Learning JS here, I run JSLint on this code:

/*jslint devel: true*/

function Animal(n) {
    "use strict";
    this.name = n;
    this.prop = "a";
}

var a = new Animal("duppa");

for (v in a) {
    if (a.hasOwnProperty(v)) {
        console.log("property:" + v);
    }
}

I get:

jslint:test2.js:11:6:'v' was used before it was defined.
jslint:test2.js:11:8:Cannot read property "kind" from undefined

jslint: ignored 0 errors.

It obviously complains that I did not declare v up front:

/*jslint devel: true*/

function Animal(n) {
    "use strict";
    this.name = n;
    this.prop = "a";
}

var a = new Animal("duppa");

var v;
for (v in a) {
    if (a.hasOwnProperty(v)) {
        console.log("property:" + v);
    }
}

This shuts JSLint up, but is it really necessary? In general I try to follow good conventions but is this one really necessary in JS? E.g. Python lives happily without such stuff (for x in range(10)... etc).

Upvotes: 1

Views: 219

Answers (5)

Farkhat Mikhalko
Farkhat Mikhalko

Reputation: 3645

There are 10 types of people in the world. Those who understand why you declare variables in javascript and those who have regular sex. (Just smile)

You must understand that every function have their own scope and you must use this scope. If you don't use declaration inside your function you change the global state, and it affects of course on many things.

So use var and don't create global variables !!!

Upvotes: 1

Joe Minichino
Joe Minichino

Reputation: 2773

Remember that JSLint doesn't complain because of some obscure academic reason but because it protects you from major problems your application may potentially suffer from.

You declare a variable to contain the variable in the current scope. It's a safeguard for you and your program as well making it more readable code (global vars appearing out of nowhere are always confusing).

Imagine if you had a global variable v in your application and then used the same nomenclature (v) for iteration in a function. Javascript will automatically assume that the global variable is being used and you'll find yourself with unwanted values in your global. Having said that, the less stuff you put in the global namespace the better.

Upvotes: 0

user229044
user229044

Reputation: 239301

Yes, you absolutely should declare the variable. Otherwise you're declaring v at global scope which is never good, but it's particularly bad for counting variables which are a single letter long like v.

Consider the case where two people get lazy about declaring a variable of the same name:

// Your code, which iterates over a nested array

var arr = [ [1, 2], [3, 4] ];

for (i = 0; i < arr.length; ++i) {
  AwesomeLibrary.doSomething(arr[i]);
}

// way down elsewhere, in awesome_library.js...

function doSomething(arr) {
  for (i = 0; i < arr.length; ++i) {
    // Now your `i` is clobbered, and we have a subtle but devastating bug
  }
}

This doesn't even require two lazy people: If you work with JavaScript long enough and refuse to declare your variables, you will eventually do this to yourself.

Upvotes: 1

user3042397
user3042397

Reputation: 1

As far as my understanding goes, if you use a variable without declaring it within a function scope, it gets declared in the global scope, which isnt a best practice. You will soon cloud your global scope with variables.

//This creates a global variable
function not_a_best_practice(){
    a=10;
}

//This creates a local variable
function not_bad(){
    var a=20;
}

This answer may throw more light on the discussion at hand: What is the scope of variables in JavaScript?

Upvotes: 0

CodeBird
CodeBird

Reputation: 3858

It is always a better practice to define your variables before using them. Here javascript's for loop requires you to define i, because it is in the for loop, use 

var v;
for( v in a)

Upvotes: 0

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