Add ESLint rules and documentation for conditional best practices

Author: andreasnicolaou

.gitignore

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+node_modules

.npmignore

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+.idea
+/docs
+.prettierrc
+.github

.prettierrc.js

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+module.exports = {
+  tabWidth: 2,
+  semi: true,
+  singleQuote: true,
+  printWidth: 120,
+  arrowParens: 'always',
+  proseWrap: 'preserve',
+  trailingComma: 'es5',
+};

docs/no-constant-conditionals.md

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+# no-constant-conditionals
+
+
+## Description
+
+This rule disallows conditionals that always evaluate to true or false.
+
+## Options
+This rule does not accept any options.
+
+## Severity
+- Type: Problem
+- Recommended: Yes
+
+## Examples
+
+### **Invalid** 👎
+
+```js
+if (true) { sayHi(); }
+```
+
+```js
+if (false) { sayHi(); }
+```
+
+### **Valid** 👍
+
+```js
+const name = "andreas"; const x = "hello"; if (x === name) { sayHi(); }
+```

docs/no-duplicated-conditions.md

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+# no-duplicated-conditions
+
+
+## Description
+
+This rule disallows duplicate conditions in if-else chains.
+
+## Options
+This rule does not accept any options.
+
+## Severity
+- Type: Problem
+- Recommended: Yes
+
+## Examples
+
+### **Invalid** 👎
+
+```js
+if (a) {} else if (a) {}
+```
+
+### **Valid** 👍
+
+```js
+if (a) {} else if (b) {}
+```

docs/no-excessive-nested-conditionals.md

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+# no-excessive-nested-conditionals
+
+
+## Description
+
+This rule disallows excessive nesting of conditionals in order to improve code readability.
+
+## Options
+
+```json
+{
+  "rules": {
+    "@andreasnicolaou/conditional-best-practices/no-excessive-nested-conditionals": ["warn", { "max": 2 }]
+  }
+}
+```
+
+- max (optional): The maximum depth of nested conditionals. The default value is 3.
+
+## Severity
+- Type: Suggestion
+- Recommended: Yes
+
+## Examples
+
+### **Invalid** 👎
+
+```js
+if (a) { 
+    if (b) { 
+        if (c) { 
+            if (d) { doSomething(); } 
+        } 
+    } 
+} else if (b) {} else if (c) {} else if (d) {};
+```
+
+```js
+if (a) { 
+    if (a) { 
+        if (b) { 
+            if (c) { 
+                if (d) { doSomething(); } 
+            } 
+        } 
+    } 
+} else if (b) {} else if (c) {} else if (d) {};
+```
+
+### **Valid** 👍
+
+```js
+if (a) { if (b) { if (c) { doSomething(); } } }
+```
+
+```js
+if (a) {} else if (b) {} else if (c) {} else if (d) {}
+```

docs/no-long-else-if-chains.md

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+# no-long-else-if-chains
+
+
+## Description
+
+This rule limits the number of consecutive else-if statements.
+
+## Options
+
+```json
+{
+  "rules": {
+    "@andreasnicolaou/conditional-best-practices/no-long-else-if-chains": 
+    ["warn", { "max": 2 }]
+  }
+}
+```
+
+- max (optional): The maximum number of consecutive else-if statements. The default value is 3.
+
+## Severity
+- Type: Suggestion
+- Recommended: Yes
+
+## Examples
+
+### **Invalid** 👎
+
+```js
+if (a) {}
+else if (b) {}
+else if (c) {}
+else if (d) {}
+else if (e) {} 
+```
+
+```js
+if (a) {} 
+else if (b) {} 
+else if (c) {} 
+else if (d) {} 
+else if (e) {}
+else if (f) {}
+```
+
+### **Valid** 👍
+
+```js
+if (a) {}
+```
+
+```js
+if (a) {} else if (b) {}
+```
+
+```js
+if (a) {} else if (b) {} else if (c) {}
+```

docs/no-nested-ternary-operators.md

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+# no-nested-ternary-operators
+
+
+## Description
+
+This rule disallows nested ternary operators.
+
+## Options
+This rule does not accept any options.
+
+## Severity
+- Type: Problem
+- Recommended: Yes
+
+## Examples
+
+### **Invalid** 👎
+
+```js
+const result = condition1 ? "a" : condition2 ? "b" : "c";
+```
+
+```js
+const result = condition1 ? (condition2 ? "a" : "b") : "c";
+```
+
+```js
+const result = condition1 ? (condition2 ? "a" : "b") : (condition3 ? "x" : "y");
+```
+
+### **Valid** 👍
+
+```js
+const result = condition ? "a" : "b";
+```

docs/no-useless-ternary.md

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+# no-useless-ternary
+
+
+## Description
+
+This rule disallows unnecessary ternary expressions. 
+
+## Options
+This rule does not accept any options.
+
+## Severity
+- Type: Suggestion
+- Recommended: Yes
+
+## Examples
+
+### **Invalid** 👎
+
+```js
+const isActive = condition2 ? true : false;
+```
+
+```js
+const isInactive = condition3 ? false : true;
+```
+
+### **Valid** 👍
+
+```js
+const isEnabled = condition1 ? "yes" : "no";
+```

docs/prefer-early-return.md

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+# prefer-early-return
+
+
+## Description
+
+This rule encourages the use of early returns instead of deeply nested if-else blocks.
+
+## Options
+This rule does not accept any options.
+
+## Severity
+- Type: Problem
+- Recommended: Yes
+
+## Examples
+
+### **Invalid** 👎
+
+```js
+function check(x) {
+    if (x) { helloWorld(); } 
+    else { return; }
+}
+```
+
+### **Valid** 👍
+
+```js
+function check(x) { if (!x) return; helloWorld(); }
+```