为什么括号会导致对象变得未绑定? [英] Why do parentheses cause object to become unbound?

问题描述

当我用括号包围一个新的对象调用并立即对其调用一个方法时，Node(或一般来说只是v8)将引发"TypeError:this.getName不是函数"错误.如果我不将其包装在括号中，则不会抛出任何错误，并且 this 已正确绑定.

When I surround a new object invocation with parens and call a method on it immediately Node (or just v8 in general) will throw a "TypeError: this.getName is not a function" error. If I don't wrap it in parens no then no error will get thrown and this is properly bound.

function Greeter(name) {
  this.name = name;
}

Greeter.prototype.getName = function() {
  return this.name;
}

Greeter.prototype.helloWorld = function() {
  console.log(`Hello ${this.getName()}`);
}

// Throws, this in any class methods are bound to the global
(new Greeter('Olive')).helloWorld();
// Doesn't throw, this is bound to the object as expected
new Greeter('Olive').helloWorld();

parren的解释是什么，为什么'helloWorld'没有绑定?

What do the parens get interpreted as here, and why is 'helloWorld' unbound?

推荐答案

您所依赖的是自动分号插入，它无法按您期望的方式工作.

You are depending on automatic semi-colon insertion and it isn't working the way you expect.

此:

Greeter.prototype.helloWorld = function() {
  console.log(`Hello ${this.getName()}`);
}

// Throws, this in any class methods are bound to the global
(new Greeter('Olive')).helloWorld();

等效于:

let mygreeter = new Greeter('Olive');

let result_of_call = (function() {
  console.log(`Hello ${this.getName()}`);
}(mygreeter));

Greeter.prototype.helloWorld = result_of_call.helloWorld();

您需要在前一个表达式后加上分号，以防止(...)解释为将此函数作为带有参数的IIFE调用"

You need to put a semi-colon after the previous expression to prevent the (...) being interpreted as "Call this function as an IIFE with arguments"

Greeter.prototype.helloWorld = function() {
  console.log(`Hello ${this.getName()}`);
};

(new Greeter('Olive')).helloWorld();

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