**Spread Syntax vs Rest Parameter: What\'s the Difference in ES2015?**

Spread Syntax and Rest Parameter: Unraveling Their Differences in ES2015

Navigating the nuances of spread syntax and rest parameter in ES2015 can be a mind-boggling endeavor. In this guide, we'll dissect their contrasting roles in JavaScript's ever-evolving landscape.

Understanding Spread Syntax: From One to Many

Spread syntax (denoted by '...') allows us to expand an iterable (e.g., an array) into its individual elements. It operates on a single variable, breaking it into smaller parts:

<code class="js">var abc = ['a', 'b', 'c'];
var def = ['d', 'e', 'f'];
var alpha = [ ...abc, ...def ];
console.log(alpha); // alpha will output ['a', 'b', 'c', 'd', 'e', 'f']</code>

By using spread syntax with '...' before 'abc' and 'def', we're effectively flattening these arrays into a single, merged array.

Unveiling the Rest Parameter: From Many to One

In contrast, the rest parameter (also preceded by '...') captures multiple elements from a function's argument list and combines them into a single array. This technique is commonly employed when we want to handle an unknown number of arguments:

<code class="js">function sum(...args) {
  var sum = 0;
  for (var i = 0; i < args.length; i++) {
    sum += args[i];
  }
  return sum;
}

console.log(sum(1, 2, 3, 4, 5)); // sum will output 15</code>

Here, '...args' acts as a placeholder for all the arguments passed to the 'sum' function, which are then stored in the 'args' array.

Key Differences: Spread vs Rest

While both spread syntax and rest parameter utilize the '...' operator, they serve distinct purposes:

• Spread syntax expands an iterable into its individual elements.
• Rest parameter collapses multiple arguments into a single array.

In essence, spread syntax helps you break down a variable into its parts, while the rest parameter consolidates multiple variables into a cohesive unit.

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