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How comma operator is used between the statements?

The description of comma operator says

You can use the comma operator when you want to include multiple expressions in a location that requires a single expression

And the syntax and parameters also says that its operands should be expressions.

Syntax

expr1, expr2, expr3…

Parameters

expr1, expr2, expr3…

Any expressions.

I want to know that why the following code doesnot throw error while it have a declaration let x = 3

let x = 3,
    y = 5
console.log(x,y)

If you put declaration in console.log() it throws error it means its not expression.

console.log(let x = 3,y=3)

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Answer

It’s only parsed as the comma operator when between expressions, not statements. let x = 3 is not an expression; it does not evaluate to any value. Trying to parse it as an expression does not work:

const result = let x = 3;
console.log(result);

The syntax let <variableName> = <expression>, <variableName2> = <expression2> is not invoking the comma operator – that syntax, when used around let, const, and var, simply initializes multiple variables at once, and said syntax simply happens to use commas, despite the fact that they’re not interpreted as the comma operator.

In contrast, with

let x = 3,
    y = 5
console.log(x,y)

You’re not invoking the comma operator here, either: you have a parameter list. To invoke the comma operator, you also need to be not immediately inside a parameter list, eg:

let x = 3,
    y = 5
console.log((x,y))

or

let x = 3,
    y = 5
const someVar = (x,y);
console.log(someVar);
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