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The ... token in JavaScript represents two distinct contextual operations: Spread syntax and Rest parameters. Spread syntax expands an iterable or object into individual elements or properties, while Rest parameters condense multiple discrete elements or properties into a single array or object. The behavior is entirely determined by whether the operator is used in an evaluation/expression context (Spread) or a binding/assignment context (Rest).

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Spread Syntax (Expansion)

Spread syntax operates by unpacking values from a source. The underlying mechanism depends on the data type being spread. When applied to iterables (Arrays, Strings, Maps, Sets), the ... operator consumes the object’s Symbol.iterator method, sequentially yielding each value until the iterator is exhausted. 
// Function argument expansion
myFunction(...iterable);

// Array literal expansion
const newArray = [...iterable];
When applied to object literals, Spread syntax does not use the iterator protocol. Instead, it performs a shallow read of the source object’s enumerable own properties and defines them on the target object. It is functionally analogous to Object.assign(), though Spread defines new properties rather than invoking setters. 
// Object literal expansion
const newObject = { ...sourceObject };
Technical Characteristics of Spread:
  • Shallow Copying: Spread only copies values at the first level of depth. Nested objects or arrays are copied by reference, not by value.
  • Evaluation Order: In object literals, properties are evaluated from left to right. If a spread object contains a key that already exists in the target, the later value overwrites the earlier one.
  • Type Restrictions: Attempting to spread a non-iterable (like a standard object) into an array literal or function call will throw a TypeError.

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Rest Parameters (Condensation)

Rest syntax operates by packing remaining, unassigned values into a newly allocated structure. It is exclusively used in destructuring assignments and function parameter declarations. In function signatures, the Rest parameter collects an indefinite number of trailing arguments into a standard JavaScript Array. Unlike the legacy arguments object, a Rest parameter is a true Array instance, inheriting all Array.prototype methods. 
// Function parameter condensation
function fn(firstArg, ...restArgs) {
  // restArgs is an Array
}
In destructuring assignments, the Rest operator collects the remaining unmapped elements or properties from the right-hand side of the assignment into a new array or object. 
// Array destructuring
const [firstElement, ...remainingElements] = sourceArray;

// Object destructuring
const { specificKey, ...remainingProperties } = sourceObject;
Technical Characteristics of Rest:
  • Terminal Position Requirement: A Rest element must always be the final element in a destructuring pattern or function signature. Placing a comma or another variable after a Rest element results in a SyntaxError.
  • Exclusion of Prototype Chain: When used in object destructuring, the Rest operator only collects enumerable own properties that were not explicitly destructured. It ignores properties on the prototype chain.
  • Empty State: If there are no remaining elements or arguments to collect, the Rest parameter evaluates to an empty array [] or an empty object {}, rather than undefined.
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