moar

Author: Basarat Ali Syed

SUMMARY.md

@@ -3,7 +3,9 @@
 * [Getting Started](docs/getting-started.md)
  * [Why TypeScript](docs/why-typescript.md)
 * [JavaScript](docs/javascript/recap.md)
+ * [Equality](docs/javascript/equality.md)
  * [Null vs. Undefined](docs/javascript/null-undefined.md)
+ * [this](docs/javascript/this.md)
  * [Closure](docs/javascript/closure.md)
 * [Future JavaScript Now](docs/future-javascript.md)
  * [Classes](docs/classes.md)

docs/javascript/equality.md

@@ -0,0 +1,29 @@
+## Equality
+
+One thing to be careful about in JavaScript is the difference between `==` and `===`. As JavaScript tries to
+be resilient against programming errors `==` tries to do type coercion between two variables e.g. converts a
+string to a number so that you can compare with a number as shown below:
+
+```js
+console.log(5 == "5"); // true , TS Error
+console.log(5 === "5"); // false , TS Error
+```
+
+However the choices JavaScript makes are not always ideal. For example in the below example the first statement is false
+because `""` and `"0"` are both strings and are clearly not equal. However in the second case both `0` and the
+empty string (`""`) are falsy (i.e. behave like `false`) and are therefore equal with respect to `==`. Both statements
+are false when you use `===`.
+
+```js
+console.log("" == "0"); // false
+console.log(0 == ""); // true
+
+console.log("" === "0"); // false
+console.log(0 === ""); // false
+```
+
+> Note that `string == number` and `string === number` are both compile time errors in TypeScript, so you don't normally need to worry about this.
+
+Similar to `==` vs. `===`, there is `!=` vs. `!==`
+
+So ProTip: Always use `===` and `!==` except for null checks, which we cover next.