SpreadElement vs. Spread Syntax in JavaScript: What's the Difference?

What is the SpreadElement in ECMAScript Documentation? Is it Different from Spread Syntax at MDN?

The ECMAScript specification describes the SpreadElement as:

SpreadElement[Yield]:
...AssignmentExpression[In, ?Yield]

This raises the question of whether the SpreadElement is the same as the Spread syntax described at MDN documentation.

Spread Syntax at MDN

MDN describes Spread syntax as a construct that expands an iterable (e.g., an array expression, string) into individual elements in various contexts:

• Function calls: myFunction(...iterableObj)
• Array literals: [...iterableObj, 4, 5, 6]

Understanding the SpreadElement vs. Spread Syntax

The term "spread operator" encompasses various syntactic constructs that involve the ... notation. However, the SpreadElement and spread syntax have distinct meanings and use cases:

SpreadElement

• Expands an iterable into an array or object (e.g., var arr = [a, b, ...c]).
• Equivalent to [a,b].concat(c) for arrays.

Spread Syntax

• For function calls: Expands an iterable into a list of arguments (fun(a, b, ...c)).
• For array literals: Expands an iterable into individual elements ([...]).

Other Spread Constructs

In addition to SpreadElement and spread syntax, there are other spread-related constructs:

• Rest element: Collects remaining elements into an array during destructuring (e.g., [a, b, ...c] = arr).
• Rest parameter: Collects remaining arguments passed to a function as an array (e.g., function foo(a, b, ...c)).

Conclusion

While the term "spread operator" may be used informally to refer to various spread constructs, the SpreadElement in ECMAScript documentation and the Spread syntax at MDN documentation have specific and distinct meanings within the ECMAScript specification.

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