Added a test.

Improved the README

Author: Steve Thompson

README.md

@@ -1,29 +1,43 @@
-# errorIfIndexIsNegative(index): void
+# errorIfIndexNotValidAfterOffsetWasAdded(<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;index,<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;arrayLength<br>): void
 
-Triggers error if `index` is negative number, saying "value not found in array".  
-This function is intended to be called after you've called Array.indexOf(), or any  
-function that returns a negative number if the value being searched for in the array   
-was not found.  You then pass that resulting index to this function to make sure if  
-the value was found.
+Triggers error if `index` is not valid, assuming an offset was recently added to it.  
+For example, if you're writing a getIndex() function that gives you the option of adding  
+an offset to modify the index it returns, `errorIfIndexNotValidAfterOffsetWasAdded()`  
+might come in handy.  For more info, read the example below.
 
 ## Example
-```
-let arr = [1,2,3,4];
-let index = arr.indexOf(5);
-errorIfIndexIsNegative(index);
-// "Error:  value not found in array"
+```ts
+function getIndexSlightlyOff(value, array, offset){
+    let index = array.indexOf(value);
+    index += offset;
+    // Now check if index still valid:
+    errorIfIndexNotValidAfterOffsetWasAdded(index, array.length);
+    return index;
+}
+
+let arr = [1,2,3,4,5];
+let index = getIndexSlightlyOff(5, arr, 1);
+// Error: "After the offset was added to the index, the resulting 
+// index was too high for this array"
+
+let arr = [1,2,3,4,5];
+let index = getIndexSlightlyOff(2, arr, -2);
+// Error: "After the offset was added to the index, the resulting 
+// index was a negative number. This is not allowed."
 ```
 
 ## Installation
-`npm install  error-if-index-is-negative`
+`npm install  error-if-index-not-valid-after-offset-was-added`
 
 ## Loading
-```
+```ts
 // if using TypeScript:
-import { errorIfIndexIsNegative } from 'error-if-index-is-negative';
+import { errorIfIndexNotValidAfterOffsetWasAdded } 
+    from 'error-if-index-not-valid-after-offset-was-added';
 // if using ES5 JavaScript:
-var errorIfIndexIsNegative = 
-    require('error-if-index-is-negative').errorIfIndexIsNegative;
+var errorIfIndexNotValidAfterOffsetWasAdded = 
+    require('error-if-index-not-valid-after-offset-was-added')
+    .errorIfIndexNotValidAfterOffsetWasAdded;
 ```
 
 ## License

index.js

@@ -2,7 +2,7 @@
 Object.defineProperty(exports, "__esModule", { value: true });
 var index_is_valid_1 = require("@writetome51/index-is-valid");
 function errorIfIndexNotValidAfterOffsetWasAdded(index, arrayLength) {
-    var errorPrefix = "After the offset was added to the index of the passed value, the resulting index ";
+    var errorPrefix = "After the offset was added to the index, the resulting index ";
     if (index < 0) {
         throw new Error(errorPrefix + "was a negative number. This is not allowed.");
     }

index.ts

@@ -0,0 +1,22 @@
+"use strict";
+Object.defineProperty(exports, "__esModule", { value: true });
+/********************
+ function getIndexSlightlyOff(value, array, offset){
+    let index = array.indexOf(value);
+    index += offset;
+    errorIfIndexNotValidAfterOffsetWasAdded(index, array.length);
+    return index;
+}
+
+ //let arr = [1,2,3,4,5];
+ //let index = getIndexSlightlyOff(5, arr, 1);
+ // Error: "After the offset was added to the index, the resulting
+ // index was too high for this array"
+
+ let arr = [1,2,3,4,5];
+ let index = getIndexSlightlyOff(2, arr, -2);
+ // Error: "After the offset was added to the index, the resulting
+ // index was a negative number. This is not allowed."
+
+
+ ********************/