Following steps are used to load the external Java Script library in lightning component.

1. Load your external java script library in static resource. Goto >> SetUp>> Custom Code >> Static Resources
2. Use <ltng:require> tag to load the Java Script library in lightnig component as follows.

   <!-- For one js library -->
   <!-- Here filename1 is static resource name -->
   
   <ltng:require scripts="{!$Resorce.filename1}"></ltng:require>
   
   
   <!-- For more than one js file -->
   <!-- Here filename1 and filename2 is static resource name -->
   <ltng:require scripts="{!join(',', 
                              $Resource.filename1, 
                              $Resource.filename2)}"></ltng:require>
   
   

In this example we print the list of fruits which is predefined in external JavaScript library.

1). Create a js file as follows:

/**
 * Webkul Software.
 *
 * @category  Webkul
 * @author    Webkul
 * @copyright Copyright (c) 2010-2016 Webkul Software Private Limited (https://webkul.com)
 * @license   https://store.webkul.com/license.html
 */



window._map = (function() {
    var fruits = ["Mango", "Apple", "Banana", "Graps", "Pineapple"];
    return {
        getFruits: function() {
            return fruits;
        }
    };
}());

2). Upload the external js file/library in static resource.  Goto >> SetUp>> Custom Code >> Static Resources
3). Create a lightning component and write following code.

<aura:component >
    <!-- 
    /**
    * Webkul Software.
    *
    * @category  Webkul
    * @author    Webkul
    * @copyright Copyright (c) 2010-2016 Webkul Software Private Limited (https://webkul.com)
    * @license   https://store.webkul.com/license.html
    */
    -->
    <ltng:require scripts="{!$Resource.fruits}"></ltng:require>
    <aura:attribute name="externalList" type="Array" ></aura:attribute>
    <div>
        <p> Click on button to show the name of fruits...</p>
        <lightning:button label="Show Fruits"
                          variant="brand"
                          onclick="{!c.getFruits}">
        </lightning:button>
        <aura:iteration items="{!v.externalList}" var="el">
            <p> {!el}</p>
        </aura:iteration>
    </div>
</aura:component>

4). Controller code.

({
	getFruits : function(component, event, helper) {
		var fruits = _map.getFruits(); // Get the list of fruits from external Java Script Library 
        console.log(fruits);
        component.set("v.externalList", fruits); 
	}
})