why (var a = b = 1;) is equals to (var a = 1; b = 1;) in javascript?

Question

When learning JavaScript I encountered a problem about hoisting that is if you run the code below, what you will get?

f1();
console.log(b);
console.log(a);

function f1() {
  var a = b = 1;
  console.log(a);
  console.log(b);
}

The answer is you get three 1 ,and an error which makes sense due to hoisting. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/var

Based on the result, I can infer that var a = b = 1; is equals to var a = 1; b = 1;

My problem is Why?

I tried to google it but it seems there is no explanation about the order that var and '=' are executed. I would appreciate it that you would show me how the var a = b = 1; is executed step by step.

Read here https://stackoverflow.com/questions/16975350/assign-multiple-variables-to-the-same-value-in-javascript — John Lobo, Mar 31 '22 at 16:29
`b = 1`, `a = b`, therefore `a` and `b` are both 1? You can see this with `a == 1` (`true`), `b == 1` (`true`), `a == b` (`true`) and `a === b` (`true`) — Tim Lewis, Mar 31 '22 at 16:29
It's not clear to me what the confusion is. `b = 1` is first, it sets a global `b`. `a = b` is next, and `a` is scoped to `f1`. — Dave Newton, Mar 31 '22 at 16:31
(Noting the error isn't because of hoisting, rather that there's no `a` in global scope.) — Dave Newton, Mar 31 '22 at 16:37
I got it, so the `b = 1` is first, the expression returns the value 1, and set a new var `a=1` . Problem solved. Thanks you guys. — LeopardCat Hiki, Mar 31 '22 at 16:46

David · Accepted Answer · 2022-03-31T16:41:48.643

Because assignment is an expression which evaluates to the assigned value. So this operation:

b = 1

not only assigns the value of 1 to b but also the entire expression evaluates to the value of 1. Since it's an expression which evaluates to a value, it can be used in place any time a value is needed.

This needs a value:

var a = (something)

So you can use any expression, for example:

var a = (b = 1)

And the parentheses are unnecessary since the assignment to a won't occur until the expression is evaluated and a value produced:

var a = b = 1

What wouldn't work is to both declare and assign as an expression. For example:

var a = (var b = 1)

Because the declaration is not an expression, it's a statement. Statements can be thought of as "a line of code" and can't be dropped in-place in the middle of other code.

You can test this in your browser's debugging tools by entering code directly onto the console. If you do this:

var a = 1

The console will print undefined, because the statement produced no value (even though it did assign a value to a). If you do this:

b = 1

The console will print 1 because the expression produced a value (as well as assigned a value to b).

Regarding the rest of the code being tested in the question, the key difference between a and b is that a is declared and thus has a specific scope, whereas b is not declared and thus defaults to a property on the window object which is accessible anywhere.

why (var a = b = 1;) is equals to (var a = 1; b = 1;) in javascript?

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