jquery - 为什么 jQuery 插件中是 this 而不是 $(this)

Question

The docs告诉我们:

Let's say we want to create a plugin that makes text within a set of retrieved elements green. All we have to do is add a function called greenify to $.fn and it will be available just like any other jQuery object method.

$.fn.greenify = function() {
    this.css( "color", "green" );
};


$( "a" ).greenify(); // Makes all the links green.

Notice that to use .css(), another method, we use this, not $( this ). This is because our greenify function is a part of the same object as .css().

我不明白最后一段。该函数向 this 传递什么？为什么不使用 $(this) 来引用 jQuery 对象？我们在 jQuery 中不是使用 $(el).css() 来正常设置 CSS 吗？那为什么不在插件中呢？

Answer 1

让我们尝试更深入地了解一下:

让我们尝试生成一个非常简化的版本库，例如 jQuery，并将其命名为 microM

(function(global) {
  //function analog jQuery
  var microM = function(context) { 
    return new microM.fn.init(context);
  }

  //init prototype
  microM.fn = microM.prototype = {
    version: '0.0.0.1',
    constructor: microM
  };

  //function for initialize context
  var init = microM.fn.init = function(context) {
    if (context instanceof microM) context = microM.extend([], context.context);

    this['context'] = [].concat(context);
    return this;
  };

  init.prototype = microM.fn;

  //add function extend to prototype and as static method
  microM.extend = microM.fn.extend = function() {
    if (arguments.length == 2) {
      var target = arguments[0],
        source = arguments[1];
    } else {
      var target = this,
        source = arguments[0];
    }
    for (var key in source) {
      target[key] = source[key];
    }

    return target;
  }

  //extend microM prototype with a few simple function
  microM.fn.extend({
    min: function() {
      return Math.min.apply(Math, this.context);
    },
    max: function() {
      return Math.max.apply(Math, this.context);
    },
    pow: function(exponent) {
      for (var i = 0, len = this.context.length; i < len; i++) {
        this.context[i] = Math.pow(this.context[i], exponent);
      }
      return this;
    },
    get: function() {
      return microM.extend([], this.context);
    },
    map: function(callback) {//a function that takes a callback
      var result = [];
      for (var i = 0, len = this.context.length; i < len; i++) {
        var callbackResult = callback.call(this.context[i], this.context[i], i);
        if (callbackResult instanceof microM) result = result.concat(callbackResult.get());
        else result = result.concat(callbackResult);
      }
      return microM(result);
    }
  });

  //looks a like jQuery :-)
  global.microM = microM;
})(window);

所以我们有一个最简单的库，看起来像 jQuery。现在我们要向其添加“插件”，例如函数 square。

就像在 jQuery 中一样，我们将其添加到原型(prototype)中，或者 fn 与我们案例中的原型(prototype)相同:

microM.fn.square = function() {
  return this.pow(2);
}

这里我们可以直接从 this 调用 pow，因为在这种情况下 this 是我们 microM 的实例，并且microM.prototype 中的所有功能均可直接使用；

但是当我们调用在回调中接受回调的 map 函数时，this 将是具体元素，例如 Number 原语，因为我们这样调用它

callback.call(this.context[i], this.context[i], i);

第一个参数在 call function 中- 是thisArg。

下面的代码片段可能可以澄清我困惑的解释:-)

(function(global) {
  var microM = function(context) {
    return new microM.fn.init(context);
  }

  microM.fn = microM.prototype = {
    version: '0.0.0.1',
    constructor: microM
  };

  var init = microM.fn.init = function(context) {
    if (context instanceof microM) context = microM.extend([], context.context);

    this['context'] = [].concat(context);
    return this;
  };

  init.prototype = microM.fn;

  microM.extend = microM.fn.extend = function() {
    if (arguments.length == 2) {
      var target = arguments[0],
        source = arguments[1];
    } else {
      var target = this,
        source = arguments[0];
    }
    for (var key in source) {
      target[key] = source[key];
    }

    return target;
  }

  microM.fn.extend({
    min: function() {
      return Math.min.apply(Math, this.context);
    },
    max: function() {
      return Math.max.apply(Math, this.context);
    },
    pow: function(exponent) {
      for (var i = 0, len = this.context.length; i < len; i++) {
        this.context[i] = Math.pow(this.context[i], exponent);
      }
      return this;
    },
    get: function() {
      return microM.extend([], this.context);
    },
    map: function(callback) {
      var result = [];
      for (var i = 0, len = this.context.length; i < len; i++) {
        var callbackResult = callback.call(this.context[i], this.context[i], i);
        if (callbackResult instanceof microM) result = result.concat(callbackResult.get());
        else result = result.concat(callbackResult);
      }
      return microM(result);
    }
  });

  global.microM = microM;
})(window);


microM.fn.printTo = function(id, descr) {
  document.getElementById(id).innerHTML += (descr ? descr + ": " : "") + JSON.stringify(this.get()) + '<br/>';
  return this;
}

microM.fn.square = function() {
  return this.pow(2);
}

var t = microM([2, 3, 4]).printTo('res', 'initial');
t.square().printTo('res', 'square')
  .map(function(el) {
    return microM(this + 10).square();
  }).printTo('res', 'mapped')
  .map(function(el) {
    return this instanceof Number;
  }).printTo('res', 'inside map: this instanceof Number');

<div id="res"></div>