🧑‍💻 Compendium

JavaScript

Fundamentals

• Variable names can contain _ and $.
• NaN**0 is 1.
• NaN is not equal to itself. Use isNaN() to check for NaN.
• typeof null is object (bug).
• typeof alert is function.
• In numeric conversion undefined becomes NaN and null becomes 0.
• In boolean conversion 0, '', null, undefined and NaN, becomes false and other values become true.
• Unary plus operator can be used for numeric conversion.
• The comma operator allows us to evaluate several expressions. Each of them is evaluated but only the result of the last one is returned.
• When comparing values of different types, JavaScript converts the values to numbers.
• Use strict equality check === and !== as they disallow type coercion.
• null and undefined are equal to each other but not to any other value.
• A double NOT !! can be used to convert a value to boolean type.
• The OR || operator finds the first truthy value.
• The AND && operator finds the first falsy value.
• The nullish coaleascing ?? operator finds the first defined(neither null nor undefined) value.
• Labels for break and continue
• switch uses strict equality check.
• 5 Differences Between Arrow and Regular Functions

Object

• 
  let obj = new Object(); // "object constructor" syntax
  let obj = {}; // "object literal" syntax
  obj.prop = 5;
  obj[hello + "world"] = 7; // computed property
  delete obj.prop; // to remove a property
  
  // property value shorthand
  let user = {
    name,  // same as name:name
    age: 30
  };
  
  // iterate over an object
  for (let key in obj) {
      // ...
  }

• Object property names can be any string or symbol.
• To check for existence for key in object use in operator.
• In an object integer properties are sorted, others appear in creation order.
• For shallow copying use Object.assign().
• For deep copying use JSON.parse(JSON.stringify(obj)) or structuredClone(obj) (comparison).
• Gentle Explanation of "this" in JavaScript
• Constructor and operator new.

  
  function Obj(name) {
    this.name = name;
  }
  
  let obj = new Obj("Abhinav");

• Inside a function, we can check whether it was called with new or without it, using a special new.target property.
  It is undefined for regular calls and equals the function if called with new.
• If return is called with an object, then the object is returned instead of this. If return is called with a primitive, it's ignored.
• The optional chaining ?. is a safe way to access nested object properties, even if an intermediate property doesn't exist.
  The optional chaining ?. syntax has three forms:
  
  1. obj?.prop - returns obj.prop if obj exists, otherwise undefined.
  2. obj?.[prop] - returns obj[prop] if obj exists, otherwise undefined.
  3. obj.method?.() - calls obj.method() if obj.method exists, otherwise returns undefined.
• Symbol type
• Object to primitive conversion