JavaScript：为什么我的`new`需要括号？

Question

I wanted to instantiate a constructor that was returned by a function, but noticed that new was a little quirky about it:

  // This function returns a constructor function
function getConstructor(){ return function X(){this.x=true} }

getConstructor();       //=> function X(){this.x=true}
new getConstructor();  //=> function X(){this.x=true}

new (getConstructor());  //=> X {x: true}

Why are the parens needed?

Answer 1

In the first case new invokes getConstructor function as a "constructor" for an object. That function returns another function (you've set it explicitly) - that's why function X(){this.x=true} is the output.

In the second case parens make the new keyword invoke the function, that was returned from getConstructor execution.

For better understanding:

function getConstructor(){ return function X(){this.x=true} }

var func = getConstructor();       //=> function X(){this.x=true}
var instance = new func(); //=> X {x: true}

Answer 2

Because new operator has higher precedence than function call operator.

If you want the constructor function returned by getConstructor , you have to wrap it to have the function call execute first.

Check the Javascript Operator Precedence .

Answer 3

Without the parens, it looks like getConstructor is itself a constructor. Remember, new Something() attempts to create an object with the constructor Something (for example. new String() ). But you want your constructor to be the function returned to getConstructor(), so you need parens to make the call to getConstructor() parse as a lone function call rather than as the operand of new .