为何JavaScript/Node.js文档将函数参数定义为嵌套数组式语法？

Great question! Let’s break this down step by step to make sense of that notation and why it’s used instead of the alternatives you mentioned.

What does value1[, value2[, ...[, valueN]]] actually mean? #

This is a standard notation borrowed from formal grammar specs (like BNF) that’s been used in programming docs for decades. Here’s the breakdown:

• Anything wrapped in square brackets [] is optional.
• The nested structure [, value2[, ...[, valueN]]] tells you the method accepts a variable number of arguments.

For Array.prototype.concat() specifically:

• You can pass just one argument: old_array.concat(10)
• Or two arguments: old_array.concat(10, "hello")
• Or as many as you want: old_array.concat(10, "hello", [1,2,3], true)
• Even no arguments work (the syntax shows value1 first just to start the sequence—concat() alone returns a shallow copy of the original array).

The ...[, valueN] shorthand simply means "this pattern repeats for any number of extra arguments."

Why not use ...values or valueList instead? #

There are practical, historical, and clarity-focused reasons this traditional notation is preferred:

1. Historical precedence #

Rest parameters (...values) are an ES6 (2015) feature, but methods like concat have existed since ES3 (1999). The nested bracket notation was already the standard way to document variable arguments long before ES6 syntax existed. It’s kept around for consistency with legacy docs and to avoid confusing developers who learned JS before ES6.

2. Clearer about argument structure #

concat doesn’t only accept arrays—it works with individual values, arrays, or a mix of both. The notation value1[, value2[, ...[, valueN]]] makes this explicit: you can pass single values one by one, or throw in arrays as separate arguments.

If docs used ...values, it might imply you need to pass a single rest parameter (like concat(...myArray)), but concat(1, 2, 3) is just as valid. The traditional notation covers both use cases without ambiguity.

3. valueList is too vague #

Writing valueList leaves developers guessing: Is that a single array argument? A comma-separated list of values? The bracket notation removes all confusion—it directly shows each subsequent argument is optional and independent.

4. Cross-language familiarity #

This bracket notation is used across countless programming languages (Python, Java, C#) to document optional and variable arguments. It’s a universal convention that developers from different backgrounds can recognize instantly, no JS-specific knowledge required.

内容的提问来源于stack exchange，提问作者Ravi Sevta