Adding TimingFunctions/IntervalTimer (#237)

* IntervalTimer added | DIRECTORY.md modified.

* IntervalTimer added | DIRECTORY.md modified.

* Fixed "extra ;" issue

Author: nandanvasudevan

DIRECTORY.md

@@ -113,6 +113,9 @@
   * [ReverseString](https://github.com/TheAlgorithms/Javascript/blob/master/String/ReverseString.js)
   * [ReverseWords](https://github.com/TheAlgorithms/Javascript/blob/master/String/ReverseWords.js)
 
+## Timing Functions
+  * [IntervalTimer](https://github.com/TheAlgorithms/Javascript/blob/master/TimingFunctions/IntervalTimer.js)
+
 ## Trees
   * [DepthFirstSearch](https://github.com/TheAlgorithms/Javascript/blob/master/Trees/DepthFirstSearch.js)
 

TimingFunctions/IntervalTimer.js

@@ -0,0 +1,93 @@
+/**
+ * @author Nandan V
+ * Sunday, 26 July 2020 8:33 AM
+ * @description Singleton class that handles the <b>timing of tests</b> and
+ *   specs. <br/> The class is singleton as <u>javascript does not support
+ *   multiple timer instances<u/>.
+ */
+class IntervalTimer {
+  /**
+   * @description Constructor for Timer.
+   * @param interval Sets the interval for running the timer.
+   * @param callBack The callback function to be executed.
+   * @return {IntervalTimer} If exists, the existing object.
+   */
+  constructor (interval = 10,
+    callBack = () => {}) {
+    this.prevInterval = 0
+    if (this.instance == null) {
+      this.interval = interval
+      this.callBack = callBack
+      this.instance = this
+    } else {
+      return this.instance
+    }
+  }
+
+  /**
+   * @description Starts the timer.
+   */
+  startTimer () {
+    this.timer = setInterval(this.callBack, this.interval)
+  }
+
+  /**
+   * @description Resets the timer.
+   * @return {number} Elapsed time in milliseconds.
+   */
+  resetTimer () {
+    clearInterval(this.timer)
+    this.callBack = () => {}
+    return this.getElapsedTime()
+  }
+
+  /**
+   * @return {number} Elapsed time in milliseconds since reset.
+   */
+  getElapsedTime (offset = 0) {
+    this.timeElapsed = this.timer - this.prevInterval
+    this.prevInterval = this.timer
+    return this.timeElapsed - offset
+  }
+
+  /**
+   * @return {number} Elapsed time since start.
+   */
+  getRunTime () {
+    return this.timer
+  }
+}
+
+/**
+ * @author Nandan V
+ * Saturday, 01 August 2020 8:33 AM
+ * @description Example usage
+ */
+const ExampleIntervalTimer = function () {
+  /**
+   * Create am object with default settings.
+   * @type {IntervalTimer} Used to get timing information.
+   */
+  const timer = new IntervalTimer()
+  timer.startTimer()
+
+  // ... Initialization code ...
+  // I generally use it for timing tests in Jasmine JS.
+
+  /**
+   * Gets the runtime till this point.
+   * Can be subtracted from ElapsedTime to offset timing of initialization code.
+   */
+  const initOffset = timer.getRunTime()
+
+  // ... A test ...
+  // The time taken to run the test.
+  console.log(timer.getElapsedTime(initOffset))
+
+  /**
+   * Returns the elapsed time and resets the timer to 0.
+   */
+  console.log(timer.resetTimer())
+}
+
+ExampleIntervalTimer()