JavaScript文档语法参数含义解析及call方法语法方括号疑问

Let’s break down the syntax you mentioned: call([thisObj[, arg1[, arg2[, ..., argN]]]])

Every section wrapped in square brackets [] means that part is optional—you can choose to include it or leave it out. The nesting and commas are there to show how parameters relate to each other:

1. What the outer brackets mean #

[thisObj[, ...]] tells you the entire parameter list (starting with thisObj) is optional. You can call call() with no arguments at all, like myFunc.call().

2. Why the nested brackets? #

The nesting shows optional parameters are order-dependent—you can’t skip a middle parameter to pass a later one. For example:

• If you want to pass arg1, you have to pass thisObj first (you can’t write myFunc.call(, 'hello')—that’s invalid syntax)
• If you want to pass arg2, you need to pass both thisObj and arg1 first

The commas inside brackets mimic the actual syntax you’d use for multiple parameters. So [, arg1] means “if you’re adding arg1, put a comma after the previous parameter (which would be thisObj)”.

3. The ...argN part #

The ... before argN denotes variable-length arguments—it means you can pass as many extra arguments as you want after arg2, like myFunc.call(myThis, 'a', 'b', 'c', 'd').

Quick valid usage examples #

Here are all valid ways to use call() based on that syntax:

• No parameters: myFunc.call()
• Only thisObj: myFunc.call(myThisValue)
• thisObj + one argument: myFunc.call(myThisValue, 'firstArg')
• thisObj + multiple arguments: myFunc.call(myThisValue, 'arg1', 'arg2', 'arg3')

This bracket notation isn’t unique to JavaScript—you’ll see it in docs for Python, Java, and many other languages. It’s just a concise way to pack a lot of parameter rules into a single line.

内容的提问来源于stack exchange，提问作者Noushad Ali