Why is 'var' Necessary for Object Destructuring but Not Array Destructuring in JavaScript?

Object Destructuring: Why "var" is Essential for Object Assignments

When using object destructuring in JavaScript, it's crucial to use the "var" keyword when assigning to object properties. Without it, syntax errors can occur.

Object destructuring with an assignment (=) operator uses {...} operators, which can represent both blocks and expressions. When appearing at the beginning of a statement, {...} denotes a block, which is not assignable. However, when appearing later in a statement as an expression, it represents an object.

The "var" keyword disambiguates this issue. It forces {...} to be treated as an expression and assigns the destructured properties to variables declared before the assignment:

var {a, b} = {a: 1, b: 2}; // Works as expected

In contrast, omitting "var" results in a block interpretation, leading to a SyntaxError:

{a, b} = {a: 1, b: 2}; // SyntaxError: expected expression, got '='

Parentheses can also be used to group the assignment and achieve the correct behavior without "var":

( {a, b} = objectReturningFunction() ); // Equivalent to using "var"

However, why isn't "var" required for array destructuring, where [] operators are used instead of {}?

Array destructuring is inherently an expression. The [] operators always represent an array, not a block. Therefore, "var" is not necessary to ensure assignability in array destructuring.

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