initial commit

README.md

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+# Time Since
+
+A demo of a function creating another function
+
+## Preliminary Code
+```ts
+function sleep(milliseconds: number) {
+    const timeWhenStarted = Date.now()
+    let timeNow = Date.now()
+    while (timeNow < timeWhenStarted + milliseconds) {
+        timeNow = Date.now()
+    }
+}
+```
+
+## First Version
+
+```typescript
+const timeSinceInitializer1 = () => {
+    const initialTime = Date.now()
+    const timeSince = () => {
+        const now = Date.now()
+        const diff = now - initialTime
+        const message = 'It has been ' + diff + ' ms since initialization'
+        console.log(message)
+    }
+    return timeSince
+}
+```
+
+The function `timeSinceInitializer1` sets the `initialTime` variable to the time when the *initializer* function was executed. Then it creates a `timeSince` function and returns that function.
+
+Notice that the `timeSince` function is not executed, only created, so the code inside that function does not get run yet. When the `timeSince` gets executed, it sets the `now` variable to the time when the `timeSince` function is executed. Then it calculates the difference between these two times, and logs the information.
+
+Usage:
+```ts
+const timeSince1 = timeSinceInitializer1()
+sleep(1000)
+timeSince1() // prints "It has been 1000 ms since initialization" 
+```
+
+## Second Version
+
+```typescript
+const timeSinceInitializer2 = (includeInitialTime: boolean) => {
+    const initialTime = Date.now()
+    const timeSince = () => {
+        const now = Date.now()
+        const diff = now - initialTime
+        let message = 'It has been ' + diff + ' ms since initialization'
+        if (includeInitialTime) {
+            const initialTimeString = new Date(initialTime).toISOString()
+            message = message + ` (${initialTimeString})`
+        }
+        console.log(message)
+    }
+    return timeSince
+}
+```
+
+In the second version we include a parameter to the initializer function. It is a boolean parameter. If it is set to true, the message in the `timeSince` function will include the initial time. If not, the message will be as it was in the first version.
+
+The noteworthy thing here is that by using a parameter in the *initializer* function, we can create different kinds of result functions. When they have been created, the behaviour can not be changed.
+
+Usage:
+```ts
+const timeSince = timeSinceInitializer2(true)
+sleep(1000)
+timeSince() // prints "It has been 1000 ms since initialization (2023-11-24T09:09:34.295Z)" 
+```
+
+## Third Version
+
+```ts
+const timeSinceInitializer3 = (includeInitialTime) => {
+    const initialTime = Date.now()
+    const timeSince = (messagePart1 = 'It has been ', messagePart2 = ' ms since initialization') => {
+        const now = Date.now()
+        const diff = now - initialTime
+        let message = messagePart1 + diff + messagePart2
+        if (includeInitialTime) {
+            const initialTimeString = new Date(initialTime).toISOString()
+            message = message + ` (${initialTimeString})`
+        }
+        console.log(message)
+    }
+    return timeSince
+}
+```
+
+In the third version we add even more complexity by having parameters in both the initializer function and the resulting function. 
+
+The parameter in the initializer function dictates if the initial time is shown. This behaviour can not be changed when the function has been created. 
+
+The parameters in the timeSince function tell what the printed message will be. This allows us to control the behaviour of the resulting function after creation.
+
+Usage:
+```ts
+const timeSince3 = timeSinceInitializer3(true)
+sleep(1000)
+timeSince3('Holy Cow! It has been ', ' ms since the initialization!')
+// prints "Holy Cow! It has been 1001 ms since the initialization! (2023-11-24T09:13:54.369Z)"
+```

src/time_since.ts

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+/**
+ * Stops program progress for given amount of milliseconds.
+ * 
+ * NOTICE: This is NOT a good way to do this!
+ * It is simple for a demonstration, but should not be used
+ * in actual programs. 
+ */
+function sleep(milliseconds: number) {
+    const timeWhenStarted = Date.now()
+    let timeNow = Date.now()
+    while (timeNow < timeWhenStarted + milliseconds) {
+        timeNow = Date.now()
+    }
+}
+
+// First version
+const timeSinceInitializer1 = () => {
+    const initialTime = Date.now()
+    const timeSince = () => {
+        const now = Date.now()
+        const diff = now - initialTime
+        const message = 'It has been ' + diff + ' ms since initialization'
+        console.log(message)
+    }
+    return timeSince
+}
+
+const timeSince1 = timeSinceInitializer1()
+sleep(1000)
+timeSince1()
+
+
+// Second version
+const timeSinceInitializer2 = (includeInitialTime: boolean) => {
+    const initialTime = Date.now()
+    const timeSince = () => {
+        const now = Date.now()
+        const diff = now - initialTime
+        let message = 'It has been ' + diff + ' ms since initialization'
+        if (includeInitialTime) {
+            const initialTimeString = new Date(initialTime).toISOString()
+            message = message + ` (${initialTimeString})`
+        }
+        console.log(message)
+    }
+    return timeSince
+}
+
+const timeSince2 = timeSinceInitializer2(true)
+sleep(1000)
+timeSince2()
+
+// Third version
+const timeSinceInitializer3 = (includeInitialTime: boolean) => {
+    const initialTime = Date.now()
+    const timeSince = (messagePart1 = 'It has been ', messagePart2 = ' ms since initialization') => {
+        const now = Date.now()
+        const diff = now - initialTime
+        let message = messagePart1 + diff + messagePart2
+        if (includeInitialTime) {
+            const initialTimeString = new Date(initialTime).toISOString()
+            message = message + ` (${initialTimeString})`
+        }
+        console.log(message)
+    }
+    return timeSince
+}
+
+
+const timeSince3 = timeSinceInitializer3(true)
+sleep(1000)
+timeSince3('Holy Cow! It has been ', ' ms since the initialization!')