i can't assign a variable in if statement

Question

I've just started learning JavaScript and I have a question about the if statement: why when I assign a variable inside the if I don't need to add var before it and when I assign it outside the if , I must add var . So, mainly the question is: why I can't add var inside the if like this

if (var number = 5) {
   document.write("successful");
} else {
   document.write("failed");
}

and if i didn't add a var it assign the variable to the value like this

if (number = 5) {
    document.write("successful");
} else {
    document.write("failed");
}

I know it's a simple question but it got me confused.

Answer 1

Probably, what you are trying to do is this:

var number = 4;
if (number == 5){
    document.write("successful");
}
else{
    document.write("failed");
}

Answer 2

JavaScript does not support block level scoping and this means that declaring a variable inside of a block structure like a if or a for loop, does not restrict that variable to the statement.

Remember that you need to declare a variable before assigning it to something.

• Declaration of a variable

var x;

• Assignment of a variable

x = 'variable x is a string that was previously declared';

This means that in your first not syntactically correct example, you are trying to define a variable inside a statement that, instead, should check the truth of that variable against something else. And this seems actually what you are trying to do in your second example, except that you have to use a double equals == ( comparison ) or triple equals === ( strict equality or identity ).

At this point, you may want to understand the differences in using = , == and === .

• Using single equals you are declaring or assigning something to a variable

var a = 55;

• Using double equals you are comparing two different things (variables, numbers, strings, etc)

a == 55 /* In this case the output will be true if you have declared the previous variable */

• Using triple equals you are not only comparing the content of the variable but also the type of that variable

a === 55 /* Will return true as the content is the same and also the type (they are both numbers) */

a === '55' /* Will return false, as you are comparing a number against a string */

Said that, remember also that JavaScript has two variables scopes : global and local.

• Any variable declared outside of a function belongs to the global scope , and is therefore accessible from anywhere in your code.

var b = 'hello world';

function test() {
    console.log(b);
}

test();

/* Output will be:
hello world
*/

• Each function has its own scope, and any variable declared within that function is only accessible locally from that function and any nested functions.

var b = 'hello world';

function test() {
    var b = 'Wonderful World';
    console.log(b);
}

console.log(b);
test();
console.log(b);

/* Output will be:
hello world
Wonderful World
hello world
*/

Answer 3

It's because var means that you declare a variable. However you can't declare a variable in an if condition. You can just assign values to previously declared variables in the condition. It is Javascript's syntax.