javascript类中的变量定义

Question

I am trying to understand a chrome extension which displays the xpath for any selected element in the html page. I came across this code I wasn't able to understand.

'use strict';

// Extension namespace.
var xh = xh || {};

xh.bind = function(object, method) {
  return function() {
    return method.apply(object, arguments);
  };
};
////////////////////////////////////////////////////////////////////////////////
// xh.Bar class definition

xh.Bar = function() {
  this.boundShowBar_ = xh.bind(this, this.showBar_);
  this.boundHandleRequest_ = xh.bind(this, this.handleRequest_);
  this.boundMouseMove_ = xh.bind(this, this.mouseMove_);
  this.boundKeyDown_ = xh.bind(this, this.keyDown_);

  chrome.extension.onMessage.addListener(this.boundHandleRequest_);

  this.barFrame_ = document.createElement('iframe');
  this.barFrame_.src = chrome.extension.getURL('bar.html');
  this.barFrame_.id = 'xh-bar';
  this.barFrame_.className = 'top';
  this.barFrame_.style.height = '0';

  // Temporarily make bar 'hidden' and add it to the DOM. Once the bar's html
  // has loaded, it will send us a message with its height, at which point we'll
  // set this.barHeightInPx_, remove it from the DOM, and make it 'visible'.
  // We'll add it back to the DOM on the first bar request.
  //this.barFrame_.style.visibility = 'hidden';
  document.body.appendChild(this.barFrame_);

  document.addEventListener('keydown', this.boundKeyDown_);
};

xh.Bar.prototype.active_ = false;
xh.Bar.prototype.barFrame_ = null;
xh.Bar.prototype.barHeightInPx_ = 0;
xh.Bar.prototype.currEl_ = null;
xh.Bar.prototype.boundHandleRequest_ = null;
xh.Bar.prototype.boundMouseMove_ = null;
xh.Bar.prototype.boundKeyDown_ = null;

What exactly is the bind function doing? The methods used while calling bind are not defined anywhere in the code.

Answer 1

 bind(thisArg : Object, [param1 : Object, [param2 : Object, [...]]]) : Function 

Returns a new function that, when called, will have this equal to thisArg, the first parameter equal to param1, the second parameter equal to param2, etc.

We use the Bind () method primarily to call a function with the this value set explicitly. It other words, bind () allows us to easily set which specific object will be bound to this when a function or method is invoked.