Declaring variables in a javascript sequence expression

Question

I would like to know if there is a way to declare a local variable inside a sequence expression in Javascript. I want to declare the variable as a part of the sequence expression and not as a separate statement.

For example, I want to do something like this:

temp = "1", var a, ++i;

Thanks for the help guys!

Edit - I am trying to instrument Javascript (to find out potential DOM-based XSS)and the above code is just a snippet of the actual program. For example, I found a way to convert if statements to expressions using the ternary operator like:

if (a === 2) {a = 1} else {a = 3}

... is converted to a === 2 ? a = 1: a = 3; ? I wanted to know if var a = 2 can also be converted to an expression so that it can be added to a sequence expression.

As to why I am doing all this - I am replacing assignment statements in a JS program with a set of statements of my own. If I add in multiple statements in place of one single statement, it messes with the rest of the code. Therefore I am using a sequence expression to get around this.

For example, in a for loop like:

for (var i = 0; i < 2; i++) {}

I cant replace i = 0 with a bunch of semicolon separated statements. Thus I am trying to add in multiple statements with a sequence operator

Answer 1

Indeed you must begin with the var keyword followed by declarations and a semi-column. Then you can start your sequence.

If you really don't like it or do not want to use a semi-column (for whatever reason) you can try to make anonymous functions and use arguments as local variables :

(function(temp, a, i){ i=a, i++ })("1", 1)

So in a for loop you can do something like :

for (var i=function(){ /* any statement you want */; return 0 }(); i<10; i++) { /* ... */ }

Hope that helps.

Answer 2

Apperently you want the statement var a = this.b to be changed into something like this:

var a = this.b;
var lhs = a;
var rhs = this.b;
if(rhs == 1){
}; 
lhs = rhs;

To achieve this, you can write a separate function:

function xy(){
    var a = this.b;
    var lhs = a;
    var rhs = this.b;
    if(rhs == 1){
    }; 
    lhs = rhs;
    return a;
}

To make this work inside of a for-loop, in a single line, you can transform it into this:

for (var lhs, rhs, a = (function(){var a = this.b; lhs = a; rhs = this.b; if(rhs == 1){} lhs = rhs; return a;}).apply(this, []); a < 10; a++)

That should basically do it. The apply method guarantees that this inside the function is the same as this outside the function.