JavaScript 函数在将 `this` 作为参数传递时会出错

Question

 function log(this) { console.log(this); }

It throws the error Unexpected token this . So why doesn't JavaScript accept this as an argument?

Answer 1

this is a reserved keyword so it can't be used as a variable name.

If you want to override the this value for a function, you can use call or apply .

 function log() { console.log(this); } log.apply({ custom: "this value" }); 

Answer 2

this has special meaning in the language. It's an identifier, but can only be defined automatically in specific situations, never explicitly by the developer.

Other identifiers that are automatically defined can still be defined by the developer, like arguments , undefined and window , though it should be avoided in most cases.

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To add clarity, in programming an identifier is a label used by a programmer to reference a value.

In JS, this is indeed a keyword, which, according to ECMAScript semantics, prevents it from being explicitly declared as an identifier. This does not mean that it isn't an identifier at all, which it clearly is since it will always let the programmer reference a value.

So something being a keyword does not mean that it isn't an identifier. It does mean though that in JS, you don't have the option of explicitly declaring an identifier with that name, though other languages do sometimes allow this.

Answer 3

Being this is a reserved keyword in JavaScript, you are not allowed that as a formal parameter to a function.

You can pass this as an argument to a function when you call or invoke that function but not as a formal parameter when you define your function.

 function log(that) { console.log(that); } log(this); // passed as an argument