Why Does JavaScript Throw a SyntaxError When Using Object Destructuring Without a Variable Declaration?

Object Destructuring Without Variable Declarations

In JavaScript, object destructuring can be used to extract properties from objects. However, using object destructuring without a preceding variable declaration (such as var, let, or const) can result in a SyntaxError.

This is because the {...} operator can have multiple meanings in JavaScript. When it appears at the beginning of a statement, it represents a block, which cannot be assigned to. But when it appears later in a statement as an expression, it represents an object.

The following example throws an error because {a, b} is interpreted as a block rather than an object:

{a, b} = {a: 1, b: 2};

To avoid this error, you can use parentheses to group the {a, b} expression:

( {a, b} = {a: 1, b: 2} );

Alternatively, you can use a variable declaration:

var {a, b} = {a: 1, b: 2};

Regarding the bonus question, array destructuring does not require a preceding variable declaration because square brackets are always interpreted as an expression, unlike curly braces. Therefore, the following example is valid:

[c, d] = [1, 2];

In the function example provided, the {a, b} expression is interpreted as a block because it appears at the beginning of a statement. As a result, a and b cannot be reassigned to new values within the block.

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